Volume 51, Issue 5-6

A comparison of high-resolution NMR methods for quadrupolar nuclei is presented. The samples studied are pure and modified boron oxide glasses, and the boron NMR spectra are recorded using three different experiments capable of high resolution: Double Rotation, Dynamic Angle Spinning, and Multiple-Quantum Magic Angle Spinning. Resolution of the similar ring and nonring boron sites in these glasses, in the presence of disorder, provides a realistic study of the strengths and limitations of these experiments. Conclusions about the relative applicability of these experiments in different situations are presented.

In this context a ‘‘dimension’’ can be of a spatial or of a spectroscopic nature. In the last two decades, multidimensional nuclear magnetic resonance and imaging have proven to be most useful tools for the investigation of materials. It turned out that some of these measuring principles can be applied to zero-field NQR as well. The purpose of this presentation is to elucidate two-dimensional exchange spectroscopy and imaging procedures suitable for NQR and to outline potential applications.

Two-dimensional zero-field nutation NQR spectroscopy has been used to determine the full quadrupolar tensor of spin - 3/2 nuclei in serveral molecular crystals containing the 3 5 Cl and 7 5 As nuclei. The problems of reconstructing 2D-nutation NQR spectra using conventional methods and the advantages of using implementation of the maximum entropy method (MEM) are analyzed. It is shown that the replacement of conventional Fourier transform by an alternative data processing by MEM in 2D NQR spectroscopy leads to sensitivity improvement, reduction of instrumental artefacts and truncation errors, shortened data acquisition times and suppression of noise, while at the same time increasing the resolution. The effects of off-resonance irradiation in nutation experiments are demonstrated both experimentally and theoretically. It is shown that off-resonance nutation spectroscopy is a useful extension of the conventional on-resonance experiments, thus facilitating the determination of asymmetry parameters in multiple spectrum. The theoretical description of the off-resonance effects in 2D nutation NQR spectroscopy is given, and general exact formulas for the asymmetry parameter are obtained. In off-resonance conditions, the resolution of the nutation NQR spectrum decreases with the spectrometer offset. However, an enhanced resolution can be achieved by using the maximum entropy method in 2D-data reconstruction.

Transitions between the Zeeman levels of spin 3/2 systems have been detected in low magnetic fields (ca. 25 mT) via the dynamic solid effect. Transitions are detected at frequencies v P ± v Q , where v Q corresponds to the Zeeman splitting of the ± 1/2 and/or the ± 3/2 levels in the small applied field, and v P is the proton NMR frequency. Using this approach, the line shape for polycrystalline samples can be recorded with a good signal-to-noise ratio despite the low frequencies involved. Computer calculated line shapes are also presented, which show that the asymmetry parameter can be obtained from the experimental spectra.

A combination of a new pulse sequence allowing fast data acquisition in the rotating frame version of NQR (ρ-NQRI) together with a method for slice selection, is reported. The procedure allows us to record the magnetization evolution during its motion in the rotating frame. At the same time a zero-crossing external magnetic field gradient is applied in order to select a determined slice of the object to be imaged. The experiments reported are the first steps toward a fast tridimensional ρ-NQRI; even more, as the spectroscopic information is preserved during the spatial encoding procedure, it could be considered as a 3D spatially resolved NQR spectroscopy technique.

The response of a quadrupolar spin system in zero applied magnetic field to a long rf pulse, for both single crystal and polycrystalline samples possessing broad Lorentzian-shaped resonance lines has been studied. The dependencies of magnetization on frequency offset, linewidth and power are investigated both theoretically and experimentally. The problem of the effective field direction in both single crystal and polycrystalline samples is also discussed. For a polycrystalline cuprous oxide (Cu 2 O) sample it is observed that the magnetization after a long pulse in on-resonance condition does not become zero for time t ≫ T 2 , in agreement with theoretical results. It has also been shown that the magnetization increases with increase in the width of the resonance line as well as with the decrease in the excitation power.

The method of two-dimensional nutation NQR spectroscopy, introduced by Harbison in 1989, has been successfully used for determining of the asymmetry parameter of the EFG tensor for spin-3/2 nuclei in both powdered and monocrystal samples in the absence of an external magnetic field. The inconvenience inherent in the method, however, is the long time required, because data acquisition must be repeated for various RF pulse lengths. We discuss a method to reduce the time of the nutation experiment by using a sequence of identical short RF pulses of length r w and distance τ. It is shown that for an NQR frequency ω 0 , frequency offset Δω, and pulse parameters satisfying the relation ω 0 τ + Δωt w = 2πk (k = 1, 2, 3,..., n) a synchronism of pulse action takes place and the nutation interferogram can be measured "stroboscopically" between the pulses. The maximum time saving factor that can be obtained as compared to the conventional nutation experiment is of the order of the number of pulses used in a pulse train. The method has been successfully applied for determination of the asymmetry parameter for one of the two 3 5 Cl sites in polycrystalline 2,4,6-trichloro-1,3,5-triazine (cyanuric chloride).

Mössbauer spectroscopy is a useful probe for investigating nuclear quadrupole interactions. It enables the magnitude, sign and asymmetry of the electric quadrupole coupling constant to be determined. It is especially important for its ability to measure these parameters for excited nuclear states. The Mössbauer effect is used to measure excited state quadrupole moments in materials whose electric field gradient (EFG) is known. More commonly however, it is used to determine the EFG from which atomic bonding and electronic structure are determined. The technique also allows temperature dependent orientation of sublattice magnetization to be measured using the direction of the electric quadrupole coupling as a reference.

Bulk glasses of Te 3 _ z Se z -(CI or Br) 2+y , the composition spanning the glass forming range, were characterized by differential scanning calorimetry, 125 Te absorption, and 129 I emission Mössbauer spectroscopy. The results reveal that the stoichiometric glasses Te 3 (Cl or Br) 2 consists of c-Te 3 Cl 2 -like chain fragments about 1.5 nm long and terminated by one-fold coordinated halogen atoms. The 125 Te electric hyperfine structure results reveal that Te replacement by Se (z ≠ 0) leads to preferential occupancy at those Te sites in the chains that are two-fold coordinated, and in a rather striking fashion demonstrate that the average length of the chain fragments remains independent of Se concentration. Halogen atoms in the glasses act as chain terminators, and also bond to Te-sites in the chains to produce four-fold coordinated sites. The low glass transition temperatures (T g ~ 70 -90°C), the low average coordination number of the proposed chain-fragments, along with the hyperfine structure results, suggest that the extensive glass forming tendency in the chalcohalides derives from the floppiness of the chain-fragments.

This review deals with studies of the variations of quadrupole splitting, electronic structure and stereochemistry of iron associated with qualitative changes of biomolecules. The possibility to determine various iron containing species resulting from the destruction of biomolecules using Mössbauer parameters is shown. A small change of iron stereochemistry leads to a small change of the iron electronic structure which could be detected by small changes of quadrupole splitting. It is expected that quadrupole splitting of iron gives new information for biomedical research on a molecular level.

Pulsed excimer laser irradiation effects (λ = 308 nm, τ = 10 ns, w = 3 J/cm 2 , N = 2 laser pulses/spot, repetition rate 1 Hz) on the magnetic properties and phase composition of thermally annealed Fe 66 Co 18 B 15 Si 1 metallic glass have been studied by transmission and conversion electron Möss-bauer spectroscopy (CEMS). Radiation-driven surface modifications were examined by scanning electron microscopy (SEM). Excimer laser induced amorphization was observed in partially crystallized Fe 66 Co 18 B 15 Si 1 samples (T A = 648 K, t A = 1 hour) and was found to be accompanied by the development of laser-induced magnetic anisotropy. In totally crystallized Fe 66 Co 18 B 15 Si 1 specimens (T A = 723 K, = 1 hour), the effect of excimer laser induced amorphization exhibited phase selectivity with respect to the crystalline components of the alloy system. When compared to bulk data, a different behavior of the surface magnetic texture and relative abundance of alloy phases was observed. The laser-induced phase transformation of thermally annealed Fe 66 Co 18 B 15 Si 1 specimens was shown to consist of partial amorphization and surface oxidation of the irradiated material.

This paper gives an elementary introduction to time differential perturbed angular correlation of γ-rays emitted from excited nuclei with particular reference to nuclear quadrupole interactions. We start with a comparison to other techniques such as nuclear quadrupole resonance and the Mössbauer effect, quote all necessary formulae, describe a typical spectrometer, list suitable isotopes, and finally give some examples for modern applications.

Perturbed-angular-correlation (PAC) spectroscopy was used to measure in ceramic samples of PbTiO 3 and BaTiO 3 the temperature dependence of the Ti-site electric-field gradients (EFGs) at temperatures very close to the ferroelectric-to-paraelectric transition temperatures T c . The samples were doped with small amounts of Hf that carried the 181 Hf - 181 Ta probe radioactivity. A high-frequency nuclear quadrupole interaction that decreases very little as the temperature approaches T c , characterizes the PbTiO 3 transition. The tetragonal and cubic phases for PbTiO 3 appear to coexist over a temperature interval of 8 ± 1 K, and the transition shows a thermal hysteresis of about 4 K. In contrast, a lower-frequency interaction that decreases rapidly as temperature approaches T c , characterizes the BaTiO 3 , transition. Both phases of BaTiO 3 appear to coexist over an interval of about 2 K, and the thermal hysteresis is about 1 K. At temperatures above T c , both PbTiO 3 and BaTiO 3 show weak, non-vanishing Ti-site EFGs. Although, for BaTiO 3 , this effect limits the accuracy with which critical effects can be measured, we estimate a power-law exponent ß = 0.21 ± 0.05, which most likely is somewhat lower in magnitude than the actual critical exponent. For the explanation of our observations we assume the existence of a distribution of T c -values. This distribution would arise because the crystals could have spatially non-uniform distributions of nucleation sites, which for PbTiO 3 and BaTiO 3 could be point defects.

Using Bayes’ formula, the parameters in a given model fitting data from perturbed angular correlation of γ-rays (PAC) experiments are all integrated out, giving as the result the total proba-bility for the model. Experimental examples are given from PAC for models containing one and two nuclear quadrupole interactions, respectively. It is shown how the Bayesian formulation leads to a quantification of the probability of the correctness of the models. Furthermore, this method leads to a transparent and intuitively appealing criterion for model selection. Examples are given using PAC measurements on two proteins: liver alcohol dehydrogenase and azurin.

In order to study the interaction of cysteine containing peptide chains with Hg(II) the nuclear quadrupole interaction (NQI) of 199m Hg in the Hg complex of the oligopeptide Alanine-Alanine-Cysteine-Alanine-Alanine (AACAA) was determined by time differential perturbed angular correla-tion. Different 199m Hg-NQI’s for free and resin-bound AACAA were obtained. Furthermore, the 199m Hg-NQI’s are influenced by the Hg:AACAA stoichiometry.

The effect of mutations in the vicinity of the putative electron transfer path on the metal center of the electron transfer protein plastocyanin (spinacea) is investigated by monitoring the nuclear quadrupole interaction of 111m Cd in Cd-derivatives of the protein via time differential perturbed angular correlation. The spectra for the wild type protein and the mutants were rather similar. All spectra exhibit a peculiar line profile which points towards a small scale intramolecular flexibility of the metal center.

The hyperfine quadrupole interaction of 181 Ta and 111 Cd probes in HfAg and HfiAg com-pounds was studied at 24 to 1100 K with the time -differential perturbed angular correlation (TDPAC) technique. The EFGs measured in HfAg on 181 Ta and 111 Cd are 1.388(5)x10 18 Vcm -2 and 0.340(5)x10 18 Vcm -2 , respectively. For both probe atoms a linear decrease of the EFG with temperature was observed with slopes 1.11(2)x10 -4 K -1 and 1.73(5)x10 -4 K -1 for 181 Ta and 111 Cd, respectively. In Hf 2 Ag, 0.469(2)x10 18 Vcm -2 was determined on 181 Ta nuclei and 0.428(5)x10 18 Vcm -2 on 111 Cd. Also a linear EFG temperature dependence with the large slope of 2.39(6)x10 -4 K -1 was observed for 111 Cd probes in Hf 2 Ag, while the EFG measured on 181 Ta nuclei in this compound exhibits a very weak temperature variation which can be fitted neither with a linear T nor a T 3/2 dependence. In all cases the electronic contributions to the EFG are very small indicating that these transition element systems do not obey the universal correlation between the electronic and the ionic EFG observed in numerous other systems not belonging to the transition metal series.

We present ab initio Hartree-Fock calculations of electric field gradients, which are related to experimental determinations of nuclear quadrupole coupling constants. The nucleus of special interest is 17 O but other nuclei in the molecules, especially 14 N, 2 H and 33 S are included. The calculations were performed (a) as single molecules either at the SCF and MP2 correlated levels, (b) as clusters of molecules about a test site, or (c) as lattice calculations computed in the unit cell environment. The basis sets are triple zeta valence plus polarisation in (a). Because of limitations in exponent range in (c), the bases in both (b) and (c), where comparison was sought, are DZ at the SCF level.

We present ab initio Hartree-Fock lattice calculations on a series of heterocyclic and other compounds. Some of these have previously been studied by cluster calculations with the same DZ basis sets at the SCF level. The electric field gradients at the nitrogen centres, are related to NQR experimental determinations of nuclear quadrupole coupling constants. The compounds studied include imidazole and pyrazole, 4-nitroimidazole, 4-nitropyrazole, pyrazine, and tetramethylpyrazine, pyridinium chloride, and N-methylpyridinium iodide, and pyridine-N-oxide.

The nuclear quadrupole coupling constants from microwave spectroscopy (MW) and quadrupole resonance (NQR) for amides and thioamides are discussed in relation to Hartree-Fock calculations with and without Moller-Plesset correlation effects. The view that the larger dipole moments from thioamides than the corresponding amides is a function of enhanced resonance in the former is discussed and (in effect) confirmed by the present procedures. The principal mechamism seems to be the push/pull π/σ effects of the N atom with respect to the CO and CS groups, with S being a better σ-donor than O; however, the effect is still present with formamidine where no electronegativity effects are important, so the overall effect is the 2,1,1 π-electron contribution to the allylic system from N, C, O(S). The use of localised MO’s and NO’s is described, and the centroid positions are discussed in relation to the polarity of the bonds. The LMO’s largely truncate the contributions to each NQCC to the three attached bonds (or 2 bonds + a lone pair orbital at O or S), as is used in the Townes-Dailey procedures. More distant LMO’s generally contribute < 0.05 a. u. to the EFG, simplifying the analysis. The effects of O( or S)-protonation of urea and thiourea is discussed.

We present ab initio Hartree-Fock lattice calculations on adenine, guanine and hypoxanthine, and some pyrimidines, including cytosine and uracil derivatives. The electric field gradients at the nitrogen centres are related to NQR experimental determinations of nuclear quadrupole coupling constants. The calculations were performed as lattice calculations in the unit cell environment, with 6-31G or double zeta basis sets at the SCF level. The present analysis strongly suggests that χ zz at N 3 in cytosine, N 3 in guanine are both positive, and approximately tangential to the ring at that centre. In contrast, N 7 in guanine is like most other azine-type N centres, with a largely radial direction for χ zz . The 3-protonated cytosine ring has χ zz as the local π-direction.

The splittings of nuclear energy levels caused by the electric field gradients acting on the quadrupole moments of nuclei in the neighbourhood of atomic defects in cubic metals may serve as ‘‘fingerprints’’ providing us with a unique characterization of these defects. In favourable cases the NQDOR technique (n̲uclear q̲uadrupole d̲ouble r̲esonance) permits sensitive measurements of these splittings with good resolution. The present paper outlines the status of the ab-initio calculation of electric field gradients with emphasis on the theoretical basis (density functional theory with local density approximation) and on the techniques required for handling the specific problems associated with defects. Recent work by the supercell approach on atomic defects in Al and Cu, making use either of the full-potential linearized augmented-plane-wave method or of the ab-initio pseudopotential method, are reported and compared with experiments. The excellent agreement between experiment and theory for the field gradients acting on the nearest-neighbour nuclei of monovacancies in Al demonstrates the reliability and the potential of the theory.

The paper investigates the Dynamic Solid Effect (DSE) in Nuclear Quadrupole Double Reso-nance (NQDOR) on a system (dilute alloys of CuPd with 8, 42, 210, or 1000 at.ppm Pd) chosen for its simplicity and the possibility to test the theoretical concepts on which the experimental tech-niques (in particular the so-called Berthier-Minier technique for exhibiting the DSE) are based. NQDOR allows to observe the transitions between the Cu nuclear energy levels split by the quadrupolar interaction with the electric field gradients generated by nearby Pd atoms even in dilute alloys, in which the fraction of Cu nuclei experiencing a given field gradient is very small. The DSE permits transitions at frequencies corresponding to the sums or differences of quadrupolar level splittings at neighbouring nuclei and thus gives access to information on the spatial correlation of nuclei accessible to NQDOR studies. The DSE information is shown to be in full accord with the conclusions drawn earlier, on the basis of line-intensity arguments, on the assignment of quadrupo-lar transitions to the first four shells of Cu nuclei surrounding isolated Pd atoms but, in addition, allows to identify the low-frequency NQDOR lines associated with Cu nuclei in the fifth and sixth shells.

The electron density distribution of the nickel compounds BaNiO 2 , BaNiO 3 and CaNiN has been investigated experimentally by 61 Ni Mössbauer spectroscopy and theoretically by band structure calculations using the FP-LMTO (Full Potential Linear Muffin-Tin Orbital) method. For all compounds good agreement is found between the experimental and theoretical values of the electric field gradient q exp. and q theor. at the nickel site. BaNiO 2 contains nickel in a square-planar coordination forming puckered chains of edge-sharing NiO 4 squares. |q| at nickel is large: q exp = - 15.7(1.5) • 10 21 Vm -2 and q theor. = - 15.59 • 10 21 Vm -2 . The principal axis z is perpendicular to the NiO 4 squares. The crystal structure of BaNiO 3 contains face-sharing chains of NiO 6 octahedra. In BaNiO 3 q(Ni) is small: q exp. = ± 3.6(2.0) • 10 21 Vm -2 and q theor. = - 1.86 • 10 21 Vm -2 . Because of the small broadening of the Mössbauer resonance line the sign of q could not be determined experimentally. The nitridoniccolate CaNiN contains infinite linear chains 1 ∞ [NiN 2/2 ] which run perpendicular to the c axis. Unexpectedly, |q(Ni)| in CaNiN is small: q exp. = 0.0 (2.0) • 10 21 Vm -2 and q theor. = - 3.05 • 10 21 Vm -2 . Again the sign of q(Ni) could not be determined experimentally. It is found theoretically that the small value of q(Ni) is caused by severe cancellation between σ and π contributions.

A first-principles method for the computation of electric field gradients (EFG) is illustrated for various borides. It is based on energy band calculations using the full-potential linearized aug-mented plane wave (LAPW) method within density functional theory. From the self-consistent charge density distribution the EFG is obtained without further approximations by numerically solving Poisson's equation. The dependence of the EFG on structure, chemical composition or substitution is demonstrated for the diborides MB 2 (with M = Ti, V, Cr, Zr, Nb, Mo, and Ta), the hexaborides (CaB 6 , SrB 6 and BaB 6 ) and boron carbide which is closely related to α-boron.

A molecular conformation study on N-Chloromethylenimine and its 1:1 dimeric form with water has been carried out using the ab-initio method at 6-31 G and 6-31 G* basis set. We consider the two most stable conformers of the N-chloromethylenimine -water binary mixture involving double hydrogen bonds. In all cases the proton affinity has been calculated. Each system considered in this work has the nuclear quadrupole interactions of the nitrogen and chlorine resonant nuclei which have been calculated and compared. It is found that the nuclear quadrupole coupling constant for the nitrogen nucleus increases in the hydrogen bonded complexes and decreases for the chlorine nucleus compared to the monomeric values. The influence of proton affinity is reflected in the nuclear quadrupole coupling constant.

We use the linear muffin-tin orbital formalism, in the atomic sphere approximation, to investigate the trends of the electric field gradient (EFG) at the nucleus for the non-equivalent sites in Zr 2 T (T = Fe, Co and Ni) intermetallic compounds. As all those compounds crystallize in the same C16 crystallographic structure, they offer a rare opportunity to investigate electronic structure effects coming from transition metals on the EFG at Zr site. Those results are compared with EFG values obtained from quadrupole coupling constant measurements performed with the time differential perturbed angular correlation (TDPAC) technique, using the 191 Ta probe.

The modified Townes-Daily equation is shown to be better than the original one for frequencies of compounds having N-Cl bonds. Of the three semi-empirical methods AM 1, MNDO, PM 3, the first is preferable. The asymmetry parameters for the same compounds have been evaluated. The 35 Cl NQR frequencies of two tautomers of N-chlorobenzotriazole are compared.

The results of ab initio and MNDO calculations of the Cl 2 C = CHOCH 3 , XCOCl (X = CH 3 , OCH 3 and COCl), 4-ClC 6 H 4 CH 2 Cl, (CNCl) 3 and PCl 5 with total optimization of their geometry are presented. The ab initio calculations were executed using Hartree-Fock theory and the split valence basis set 6-31G* (RHF/6-31 G*//RHF/6-31G*). Using the calculated p-orbital populations of the CI atoms in these molecules the 35 Cl NQR frequencies and asymmetry parameters of the EFG at the 35 Cl nuclei have been determined. When the populations of the less diffuse components of orbitals in the split valence basis set are used the calculated and experimenal ν and η values are in good agreement. Linear correlations between these calculated and corresponding experimental ν and η values are obtained. The causes of the nonconformity of the earlier calculated ν and η values and their experimental ones are analysed.

The F-tensor coefficients are useful in evaluating the stress-induced electric field gradients in cubic crystals. These coefficients are derived for the non-Coulombic inter-atomic potentials Gr -n and Ge -br . Interesting differences are pointed out between these results and those for Coulombic potentials.

Electric Field Gradient (EFG) calculations have been carried out for Antimony based clusters with 20 and with 22 valence electrons. It is shown that EFGs are very sensitive to structural changes of the clusters under study. This feature can be used to demonstrate the validity of the Zintl-Klemm-Busmann principle for these systems through EFG determinations.

Time Differential Perturbed Angular Distribution (TDPAD) measurements of the nuclear quadrupole hyperfine parameters for 19 F* implanted into amorphous, polycrystalline and crystalline silicon and germanium are reported and reviewed. Two signals are observed in the crystalline materials (≈ 35 and 23 MHz in silicon, ≈ 33 and 27 MHz in germanium) while only one is detected in the amorphous and polycrystalline samples (≈ 22 MHz in silicon, ≈ 27 in germanium). Impurity sites in these materials were modeled using a Hartree-Fock cluster procedure. The Intrabond, Antibond, and Substitutional sites in the bulk were studied in both silicon and germanium. The ATOP and Intrabond Surface sites were also studied in silicon and the results extended to germa-nium. Lattice relaxation effects were incorporated by employing a geometry optimization method to obtain minimum energy configurations for the clusters modelling each site. The electronic wave functions were obtained for each optimized cluster by applying Unresctricted Hartree-Fock theory, and these wave functions were used to calculate the nuclear quadrupole hyperfine parameters at the site of the fluorine nucleus. Comparison of the theoretical hyperfine parameters to the experimental values indicates that 19 F* located in the Intrabond and Intrabond surface sites could readily explain the higher frequency signal that has been observed. 19 F* in the Antibond and the surface ATOP sites yield hyperfine parameters consistent with the low frequency signal observed in the crystalline materials and the single signal observed in the amorphous (or polycrystalline) materials. Examina-tion of these two sites, in view of other available experimental evidence including the temperature dependence of the TDPAD signals, leads to the conclusion that the lower frequency signal is due to 19 F* implants which have come to rest at the site of dangling bonds in the bulk. These dangling bonds are created as a result of damage generated in the individual collision cascades during the implantation process.

The Quadrupole Coupling Constant e 2 qQ and Asymmetry Parameter η of fluorine for fluorine-substitute methane compounds are calculated using the Hartree-Fock Roothaan procedure. Our results are compared with experimental data from measurements by the Time Differential Perturbed Angular Distribution (TDPAD) technique using excited 19 F* (spin 5/2) nuclei. The theoretical e 2 qQ’s for the 19 F* nuclei in the fluoromethanes are in good agreement with experimental results. For CH 2 F 2 and CHClF 2 molecules, where finite η are expected from symmetry considerations, our results for η are small, 0.12 and 0.05 respectively, in agreement with experimental observation. Besides the 19 F* coupling constants associated with the C-F bonds in fluoromethanes, additional interesting NQI parameters, close to those in solid hydrogen fluoride, are observed in the TDPAD measurements. It is demonstrated through investigations of the total energies and electric field gradients that these additional NQI parameters for the fluoromethanes can be explained by a HF* molecule hydrogen-bonded through the hydrogen to a fluorine atom in the host molecular systems. This complexing of an ionic molecule to the host molecules in organic solids containing strongly electronegative atoms is expected to be a general feature in both implantation and conventional techniques.

125 Te absorption and 129 I emission Mössbauer spectroscopies have proved to be the methods of choice to probe chalcogen chemical order in condensed matter using nuclear quadrupole interactions. In chalcogenide glasses, the technique has been unusually rewarding in decoding both the elements of local atomic structures and those of the medium-range atomic structures.

High-resolution solid state NMR spectroscopy of quadrupolar nuclei (preferentially 27 Al) has been employed to study the microstructure of glasses. Importance has been attached to the extraction of the fundamental NMR parameters like chemical shift, quadrupole coupling constant etc., from the spectra, first of all by applying satellite transition spectroscopy. Exemplary results are represented for ternary oxide glass systems such as K 2 O-Al 2 O 3 -SiO 2 , B 2 O 3 -Al 2 O 3 -P 2 O 5 , Na 2 O-B 2 O 3 -SiO 2 and Na 2 O-B 2 O 3 -Al 2 O 3 .

Nuclear magnetic resonance (NMR) of 51 V and 209 Bi, and electron paramagnetic resonance (EPR) of Mn 2+ and Fe 3+ impurities in ferroelastic BiVO 4 single crystals are reviewed. The nuclear quadrupole parameters and principal axes of the two host nuclei, 51 V and 209 Bi, and the second-or-der zero field splitting tensor of the isoelectronic 3 d 5 ions, Mn 2+ and Fe 3+ , in BiVO 4 are determined for the first time. Temperature dependence studies on 51 V NMR and Mn 2+ EPR confirmed that this crystal undergoes a second order phase transition. The substitutional sites of Mn 2+ and Fe 3+ in BiVO 4 are Bi 3+ and V 5+ , respectively.

Nuclear quadrupole resonance (NQR) has provided a valuable tool for investigating the local structural order in semiconducting glasses. Measurements of NQR lineshapes and spin-lattice relaxation times help elucidate the local structural order and the unusual vibrational properties, respectively, of these glasses. In addition, NQR lineshape measurements on samples that are made under conditions that produce solids whose structures are far from equilibrium, such as fast-evap-orated films or rapidly-drawn fibers, provide detailed information on the changes in the local structural order from that which occurs in the well-annealed amorphous semiconductors.

The NQR of a quadrupolar probe nucleus is often used to investigate the effect of substituent in molecules. The inductive effect, based on a partial charge migration along the molecular skeleton is the only one present in saturated aliphatics, the conjugative effect appearing in conjugated molecules, especially aromatics. As the stepwise charge migration mechanism, formerly used to explain the inductive effect, is now believed obsolete, we have wanted to reexamined the case of chlorine substituted acetic acids and salts. The data in literature was extended by observing reso-nances and determining NQR frequencies in several acids and salts. The present analysis of the salification of mono-, di-and tri-chloroacetic acids, which is equivalent to a deprotonation or the substitution of the acid hydrogen by a negative unit charge, shows that a model based on the polarization of the chlorine atom(s) by the carboxyle group is consistent with experimental results: the polarization energy appears to be proportional to the NQR frequency shifts; experimental data show a correlation between the NQR frequency shifts accompanying salification and the variations of the intrinsic acidity measured in the gas phase; this, in turn shows that there is a proportionality between the polarization energy and the variations in the acid free enthalpy of dissociation. From the comparison between fluorine, chlorine, bromine and iodine, it also appears that an alternative mechanism, the polarization of the carboxyl group by the halogen, would be important only in the case of the fluoroacetic acid.

A linear relationship exists in oxide glasses between glass transition temperature (T g ) and quadrupole splitting (Δ) of Fe 3+ . DTA and Mössbauer measurements of silicate and tungstate glasses yielded large slopes of 680 and 260 °C/(mm s -1 ) from the slope of the straight lines when Fe 3+ occupied, as network former (NWF), distorted tetrahedral Si 4+ and distorted octahedral W 6+ sites, respectively. This linear relationship, named a ‘‘T g - Δ rule’’, suggests that T g is in proportion to the distortion of NWF-oxygen polyhedra and a higher activation energy is required for the fragments of glass to have a cooperative movement in the supercooled liquid state. The ‘‘T g - Δ rule’’ was also valid in the case of potassium silicate glasses heat treated at the temperature close to T g , indicating a distortion of SiO 4 and FeO 4 tetrahedra. Substitution of Fe 3+ for individual NWF in silicate and tungstate glasses was confirmed from the Fourier transform infrared (FT-IR) spectra because only the absorption bands due to Si and W were respectively observed without the bands due to Fe.

(2) The crystal structures at room temperature of the N-chloro-N-(2,6-dichlorophenyl)-chloroacetamides 2,6-Cl 2 C-H 3 NClCOCCl 3 (1) and 2,6-Cl 2 C 6 H 3 NClCOCH 2 Cl have been determined. (1): orthorhombic, Pbca. Z = 8, a = 2051.7(8) pm, b = 1134.8(4) pm, c = 1089.0(4) pm. (2): orthorhombic, Pbca, Z = S, a = 2078.7 (4) pm, b= 1487.9(4) pm, c = 694.0 (2) pm. The 35 Cl NQR spectrum of (1) was studied in the range 77≤T/K≤370. At 77 K, a sextet spectrum is observed (ν in MHz): v 1 = ν (Cl (N) = 56.327; ν 2 = 40.636, ν 3 = 40.317, ν 4 = 40.294; ν 5 = 36.532, ν 6 = 36.182. The lines ν 2 , ν 3 , and ν 4 are assigned to the CCl 3 group because of their position in the frequency scale. Also, due to librational motions, this triplet part of the spectrum fades out at T f ≈ 320 K. There is no problem in assigning in both compounds, on the basis of the known 35 Cl NQR frequency scale, the line ν 1 to the CI atom attached to the nitrogen atom and the lines ν 5 and ν 6 to the ring chlorines (Cl (2) and Cl (6) ). The crystal structures and NQR spectra of (1) and (2) are discussed together with literature data for 2,6-Cl 2 C 6 H 3 NHCOR, C 6 H 5 NClCOR (where R = CCl 3 , CClH 2 ) and other substituted acetamides.

We report the 63 Cu NQR spectra of seven two-and three-coordinated complexes with the siloxyl ligand. R 3 SiO - . The frequencies are compared to those of other two- and three-coordinated species, and it is demonstrated that the partial field gradient of the R 3 SiO - group is very similar to that of Cl - .

The σ-and π-electron effects of alkyl/phenyl substituents on the Sn-Hal bonds in penta- and hexacoordinate tin(IV) chloro and iodo adducts are discussed based on their 35 Cl and 127 I NQR spectra. Unexpectedly high 127 I EFG asymmetry parameters were found in most dialkyl(phenyl)-substituted adducts of thin(VI) iodides.

The electric field effect on NQR in ferroelectric materials, 93 Nb in LiNbO 3 and 14 N in NaNO 2 and SC(NH 2 ) 2 , has been investigated at 77 K. In these crystals with single domain, only the line shift due to the external electric field was observed. In the case of NaNO 2 powder and a crystal with multi-domains, line broadening was observed in the external electric field. These phenomena can be explained with the fact that the direction of spontaneous polarization in a domain is related to the direction of the applied electric field. The rate of the NQR line-shift due to the electric field is remarkably smaller in mostly ionic crystals, such as LiNbO 3 and NaNO 2 , than in a molecular crystal such as SC(NH 2 ) 2 . This is due to the strong ionic bonding in ionic crystals. Also, the difference of the Stark shift'between NaNO 2 and SC(NH 2 ) 2 is discussed in terms of the local electric field and polarizability at the resonant nuclear site.

This is a review of investigations on iodo-benzoic acid, iodohippuric acid, and iodophenol derivatives by NQR, NMR, and emission Mössbauer spectroscopy. The influence of intermolecular, intramolecular, and bifurcated O-H … I and O-H … O bonds on the properties of iodine-carbon bonds is demonstrated. Ionized and non-ionized forms of tyrosine differ by the probability of H-bond formation. The simultaneous use of 127 I NQR and emission Mössbauer 125 I spectroscopy open the possibility to determine the types of tellurium-organic substances formed in the process of K-decay of 125 I.

The spin-lattice relaxation time T 1 of 23 Na-NMR in a dehydrated Na-X zeolite has been measured from 20 to 300 K. The recovery curve is not single-exponential at all measured temperatures and T 1 -1 increases with the square of temperature around room temperature. The results are analyzed by assuming non-equivalent sites and by applying the theory of the Raman process based on covalency.

NMR experiments are reported for the copper chalcogenide spinel CuV 2 S 4 , which has incommensurate lattice distortions at low temperatures in spite of the cubic symmetry at high temperature. A broad spin-echo spectrum of 51 V has been observed below 90 K at 17 MHz. but it shows a remarkable frequency dependence where it becomes sharper with decreasing frequency. Possible coexistence of charge and spin density waves is suggested at low temperatures in this material.

A very unusual magnetic behaviour of α - Bi 2 O 3 was observed in a wide range of temperatures and magnetic fields. A longitudinal magnetoelectric effect was found at 4.2 K. The existence of an antiferroelectrically ordered subsystem of electric dipoles in α - Bi 2 O 3 is proposed. The mechanism of spin-orbit coupling is regarded as the physical origin for the occurrence of the ordered magnetic moments in α - Bi 2 O 3.

The peculiarities of the structure and the IR and NQR spectra of acid iodates and pyroiodates which contain the I 2 O 5 group are reported. Based in this, the results for several iodates that contain organic cations are considered.

Cs 2 SbCl 6 is known as a typical mixed-valence compound. It crystallizes into a tetragonal space group I4 1 /amd and contains two different complex anions, Sb(III)Cl 3- 6 and Sb(V)Cl - 6 . The dark blue color of this compound has been considered to originate from a charge transfer between the above two anions. In order to study the electronic structure of these complex anions and the existence of charge transfer between them we measured the 121 Sb NMR spectrum and carried out molecular orbital calculations on the electronic states of these anions. The 121 Sb NMR spectrum consists of two peaks at 0 and 30 kHz which can be assigned to the central transition of 121 Sb in Sb(V)Cl - 6 and Sb(III)Cl 3- 6 , respectively. The line shape analyses of the spectra led to nuclear quadrupole coupling constants of nearly zero for Sb(V)Cl - 6 and 4.9 ± 0.5 MHz for Sb(III)Cl 3- 6 at room temperature. The quadrupole coupling constant of 121 Sb(III) decreases steadily on heating. The calculations of the electronic ground state energies of both anions were calculated by the MS-Xα molecular orbital method. The calculated charge-transfer band from the A 1g state of Sb(III)Cl 3- 6 to the A 1g state of Sb(V)Cl - 6 appears at 610 nm and can account for the experimental electronic spectrum, the calculated quadrupole coupling constant in Sb(III)Cl 3- 6 however is far larger than the experimental one. The contribution of the charge-transferred state to the ground state is negligible and so the temperature dependence of the quadrupole coupling constant of 121 Sb(III) is attributed to an anisotropic thermal expansion of the compound.

Twenty-five inclusion complexes formed by bis(N-alkylimidazolidine-2-thione)cuprous halides with the small, chlorine-containing guest molecules CCl 4 , CHCl 3 , CH 2 Cl 2 , CH 3 CCl 3 or C 2 H 2 Cl 2 , have been prepared. Their 63 Cu and 35 Cl NQR spectra have been measured over the temperature range 77-300 K. For the corresponding bromides the 79 Br and/or 81 Br spectra have also been investigated. The studies reveal the number and symmetry of different inclusion site. The onset of rapid reorientation of the guest molecules is indicated by the fading out of their NQR signals between 77 and 300 K, whereas the resonances from the host molecules usually persist throughout this temperature range. However, as evidenced by discontinuities in the temperature-dependence of the NQR frequencies of the hosts, sixteen of the complexes show one or more phase-changes in this temperature-range.

A series of tribromogermanate(II) complexes RGeBr 3 (R = CH 3 NH 3 , (CH 3 ) 2 NH 2 , (CH 3 ) 4 N, C 2 H 5 NH 3 , (C 2 H 5 ) 4 N) have been synthesized and characterized by 81 Br NQR, AC conductivity, DTA, and X-rax diffraction. These measurements revealed the presence of successive phase transitions in these complexes. The crystal structures of Phases I and II in (NH 3 ) 4 NGeBr 3 were derived from the Rietveld analysis of powder X-ray diffraction. The electric conductivity was considerably high in CH 3 NH 3 GeBr 3 and (NH 3 ) 4 NGeBr 3 . The conduction mechanism is discussed on the basis of 81 Br NQR spin-lattice relaxation times and crystal structures.

The 1 H NMR spin-lattice relaxation time T 1 , 35 Cl NQR frequency ν Q , and 35 Cl NQR spin-lattice relaxation time T 1Q of [Zn(H 2 O) 6 ] [SnCl 6 ] (zinc hexachlorostannate(IV) hexahydrate) have been measured at temperatures between 77 and 350 K. The NQR spin echo signal with ν Q = 15.689 MHz at 77 K showed a positive temperature coefficient attributable to O-H-Cl type H-bonds in the crystal. ν Q at 77 K is strongly correlated with the electronegativity χ M of the metal M in the series of stannates [M(H 2 O) 6 ] [SnCl 6 ] (M = Mg, Ca, Mn, Co, Ni, Zn). A T 1 minimum observed for the Zn salt is ascribed to 180° flips of water molecules with an activation energy of 20 kJ mol -1 . The motion is influenced by repulsive forces among the water molecules within a cation rather than by attractive forces between the H-bonded H and CI atoms. T 1Q proved to be mainly governed by lattice vibrations, weakly modulated by the fluctuating electric field gradient caused by the 180° flip motions.

The temperature dependence of NQR frequencies of nuclei in molecules undergoing quasi-har-monic rotational oscillations is calculated. Following the model proposed by Bayer, the instanta-neous orientation of the electric field gradient principal axes is assumed to change from its average orientation as a result of rotational oscillations, while the EFG magnitude remains constant. The eigenstates and energy eigenvalues of a rotator in a potential of the form V(θ)= V 2 θ 2 - K 4 θ 4 are calculated by standard perturbation methods, followed by the evaluation of statistical averages of square angular amplitudes. For molecular crystals with more than one molecule in a unit cell the angular displacements are expressed in terms of normal coordinates, and the same procedure is followed. The final expression for the change in the NQR frequency consists of two terms, the first one being related to the harmonic part of the potential and the second one to the quartic term. At the absolute zero it yields a nonzero contribution to ⟨θ 2 ⟩, and for high temperatures it reproduces the nonlinear character of ν Q (T) observed experimentally. The model has been tested with measure-ments on solid Cl 2 performed by Nakamura and Chihara, combined with libration mode frequencies measured in Raman spectroscopy by Cahill and Leroi and analysed according to our anharmonic libration picture. The fitting of our equation to experimental data supports Cahill and Leroi's hypothesis about the existence of an undetected libration mode in Cl 2 Raman spectra of frequency near 60 cm - 1

K 2 SeCl 6 undergoes structural phase transitions of first order at 35 K, 65 K, and 79 K. The temperature dependence of the 35 Cl NQR spin-lattice relaxation time suggests that the phase transitions at 65 K and 79 K are displacive ones associated with a resonant soft mode. The presence of a precursor cluster is suggested by the temperature dependence of the signal intensity above 79 K.

Results are presented of a theroretical consideration of the nuclear quadrupole resonance and spin-lattice relaxation for a sample containing nuclei of two different types coupled by strong heteronuclear dipole-dipole interactions and influenced by an external continuous or pulsed ra-diofrequency magnetic field acting only on the nuclei of one sort with spin I > 1/2. A kinetic equation is obtained from which the time dependence of the magnetization of the sample is derived. The kinetic coefficients are calculated as a function of the concentration and distribution of the nuclei of both sorts.

The temperature dependences of the 35 Cl NQR frequency (ν), spin-lattice relaxation time (T 1 ), and spin-spin relaxation time (T 2 ) have been studied in 2,4,6-trinitrochlorobenzene (picryl chloride) from 77 K up to the melting point (354 K). The T 1 (T) curve exhibits a pronounced composite mimimum near 300 K which gives evidence for the reorientations of the two ortho-NO 2 groups around their two-fold symmetry axes with the activation energies of 27.4 kJ mol - 1 and 31.2 kJ mol - 1 . These values can be related to the ortho-NO 2 groups having the twist angles of 33° and 81°, respectively (the crystal structure of picryl chloride is known). The T 2 (T) dependence exhibits interesting features, too: a deep minimum about 140 K and a new rapid decrease above 270 K.

The temperature dependence of the average lattice mode frequency of a molecule undergoing librational motion in a quasi-harmonic potential has been calculated with the purpose of finding an explicit relationship between the observed shift of Raman spectral lines with temperature and the anharmonic term in the rotational potential. Calculations were carried out both for uncoupled and coupled oscillators. The equations obtained with this model provide good fittings for solid Cl 2 and benzene data. This result can be applied to the analysis of the temperature dependence of NQR frequencies in molecular crystals.

39 K, 87, 85 Rb, 133 Cs, 205 T 1 , and 1, 2 H NMR spin-lattice relaxation times T 1 and 14 N NQR spin-lattice relaxation times T 1Q were determined for R 2 Pb[Cu(NO 2 ) 6 ] (R = K, Rb, Tl, Cs, and NH 4 ). T 1 of 39 K and 87 Rb showed very short values in the incommensurate phase as compared with those in the other phases. When the commensurate-incommensurate phase transition point is approached from below, 14 N T 1Q of the R = K, Rb, Tl, and NH 4 compounds showed rapid decrease. On the other hand, that of the R = Cs compound began to decrease first after passing beyond the corresponding transition point. The difference of the T 1Q behavior may be ascribed to the difference of the condensed phonon mode in the incommensurate phase.

The variation of NMR spectra and magnetization with temperature in (R 1 - x R´ x ) 2 Co 14 B (R = Y or Gd, R´ = Tb. Dy or Ho 0 ≤ x ≤ 1) have been measured. These compounds undergo spin reorientation at a temperature T SR when Y and Gd are substituted by Tb, Dy and Ho, and the temperature T SR increases with the fraction x . Phase diagrams of spin arrangement in these systems have been obtained. The results are discussed from the view point of competition between the crystalline electric field and the Co-R exchange interaction.

The spin-lattice and spin-spin relaxation times T 1 and T 2 of 35 Cl NQR were measured for the high (I) and low (II) temperature phases of 4-chlorobenzyl alcohol in the range 50 < T/K < 338 (transition and melting points: 236 and 343.5 K, respectively). In phase II the T 1 behavior in the range 50 < T/K < 90 can be explained by lattice vibrations and that in the range 90 < T/K< 190by reorientation with an activation energy of ca. 3.8 kJ/mol. In phase I, the T 1 behavior at T > 260 K is explainable by another reorientation with E a ≈ 4.7 kJ/mol. The T 1 , vs. T curves revealed that in both phases the crystal dynamics is anomalous in the vicinity of the transition temperature. Lattice defects accumulated in phases I and II by the phase transition were found to be responsible for the extremely short inverse line width parameters (T 2 *).

Chlorine NQR studies of solid nitrosyl chloride NOCl at temperatures below 200 K were undertaken. They indicate an ionic character of the N-Cl chemical bond and confirm the phase transition near 140 K. Thermal hysteresis of the transition temperature was observed.

The second order phase transition at 253 K of pyridinium tetrabromoantimonate(III), C 5 H 5 NHSbBr 4 , has been characterized by means of X-ray diffraction and 2 H NMR. As was suggested from the 81 Br NQR spectra, the Rietveld refinements of the X-ray diffraction patterns at 297 K and 116 K confirm the structural change of the anion from SbBr - 4 to SbBr 3 • Br - . The line shape analysis of the 2 H NMR using the deuterated analog revealed that the librational amplitude of the pyridinium ring about the axis normal to the plane increased with temperature and its rms amplitude reaching ca. 25° at T c . The librational motion may induce the phase transition breaking the hydrogen bonds of the type N-H … Br. However, nearest-neighbor 2π/6 jump or reorientation such as seen in benzene were not observed even at 350 K.

The structural stability of Rb 2 Zn(Cl 1 - x Br x ) 4 (x = 1% and 3%) is investigated using the 35 Cl quadrupole resonance frequency and spin lattice relaxation time (T 1Q ) in the paraelectric (PE) phase. The PE to incommensurate (IC) phase transition temperature T 1 , observed at 302 K in the pure compound, is lowered to 300 K for x = 1 % and to 293 K for x = 3%. These effects are smaller than in the earlier reported case of 3% Cs substitution in Rb 2 ZnCl 4 , which reduces T 1 by 25 K. While bigger cation substitution affects only the barrier for anion dynamics about directions perpendicular to the a-axis (direction of low temperature IC modulation wave), bigger anion substitution is found to affect the barrier for anion dynamics about the a-axis as well. The present study also indicates that the cusp like dip in T 1Q observed while approaching T 1 from above (characterizing the soft mode condensation associated with the structural phase transition) is essentially unaffected by anion impurity substitution, in contrast to the case where bigger cation substitution tends to smear the transitional effects. These results seem to suggest the dominant role played by cations in stabilizing the PE phase of these A 2 BX 4 systems, in comparison to anions.

By 35 Cl NQR the temperature dependences of the resonance frequencies and spin-lattice relaxation times for the 35 Cl nuclei have been studied in solid CCl 3 COHal (Hal = F, CI, Br, I). A linear correlation between the average 35 Cl NQR frequencies of the CCl 3 groups and the electronegativities of the halogen atoms in the COHal groups has been discovered. The temperature-dependent NQR data reveal thermoactivated reorientations of the CCl 3 COHal molecules as a whole in the crystal lattice. The activation energies of this motion are found to lie in the range 31 - 37 kJ mol - 1

NQR and DTA revealed phase transitions in [(CH 3 ) 4 N] 2 HgBr 4 and [(CH 3 ) 4 N] 2 HgI 4 at 272 K and 264 K, respectively. The NQR resonance lines faded out with increasing temperature. From preliminary measurements of 81 Br NQR spin-lattice relaxation times and 199 Hg NMR a reorientational motion of HgBr 4 ions around one of their pseudo C 3 axes in the room temperature phase of [(CH 3 ) 4 N] 2 HgBr 4 is suggested.

The temperature dependence of the 15 N and 133 Cs NMR spin-lattice relaxation times, the 15 N spin-spin relaxation time, and the 15 N and 133 Cs spectra of CsNO 2 was observed in the plastic phase (209.2 < T < 673 K (m. p.)) and the low-temperature phase (Phase II). In Phase II we found the NO - 2 180°-flip, which could be attributed to the anomalous increase of the heat capacity curve, and determined the activation energy of this motion to be 8.7-11.7 kJ mol -1 . The 15 N and 133 Cs spectra in this phase are inconsistent with the reported crystal structure R3̅m and can be explained by lower crystal symmetry. In the plastic phase we detected a new anionic motion with 11 kJ mol -1 , an isotropic NO - 2 reorientation with 8.5-9 kJ mol -1 , and ionic self-diffusion with 47 kJ mol -1 . The presence of ionic self-diffusion was confirmed by measuring the electrical conductivity.

The temperature dependence of 14 N NQR, measured at 77 to 300 K, is in better agreement with the model proposed by Blinc et al. than the Bayer theory. The nuclear quadrupole coupling constant and asymmetry parameter show a discontinuity at about 160 K and 230 K, respectively. The linewidths are broadened at these temperatures up to 22 kHz and 18 kHz, respectively. These results indicate that KNO 2 undergoes phase transitions at these two temperatures.

The temperature variations of the 63 Cu, 65 Cu and 51 V NMR spectra and 51 V spin-lattice relaxation rate 1/T 1 have been measured in a thiospinel CuV 2 S 4 . Drastic changes in the spectra and relaxation rates were observed at the charge density wave (CDW) transition temperature. With decreasing temperature T, 1/T 1 decreases exponentially due to the CDW transition (75 K < T< 90 K). The energy gap made by the CDW formation at 90 K is estimated to be 0.052 eV from an Arrhenius plot of 1/T 1 .

C(CH 2 Cl) 3 (CH 3 ), melting point T m = 291.3 K, shows in the DTA/DSC experiment a phase tran-sition from an orientationally disordered plastic phase I into an ordered phase II at T I ⃗ II = 235.9 K. From 77 K up to T II ⃗ I the 35 Cl NQR spectrum is a triplet with (ν in MHz, T=77 K) ν 1 = 34.213, ν 2 = 34.183, ν 3 = 33.786. The 35 Cl NQR fade-out temperature T f = 247 K coincides with T II ⃗ I found from the DSC experiment. The plastic phase of 2-chloromethyl-2-methyl-l,3-propanediol is cubic bcc, Im3m, Z = 2 and the lattice constant increases linearly 755 pm at 235 K to 768 pm at 283 K. In the heating cycle we found (ΔH in kJ/mol): ΔH II ⃗ I = 12.0, ΔS m /R = 1.0. The compound belongs to the group of plastic molecules formed by ellipsoid like distorted tetrahedra and is derived by substitution from methane. The crystal structure of 2,2-bis(bromomethyl)-1,3-propanediol was determined. DTA-DSC show that an orientationally disordered plastic phase does not exist. At room temperature C(CH 2 Br) 2 (CH 2 OH) 2 crystallizes monoclinic, space group Cc, Z = 4, a = 628.2 pm, 6 = 2015.5 pm, c = 659.6 pm, β = 94.27°. The molecules interact by intermolecular hydrogen bonds between the OH-groups and by van der Waals forces Br…Br.

The paper discusses some NQR/NMR studies performed on Y-Ba-Cu-O superconductors at the University of Zürich. In particular, we review studies performed in Y 2 Ba 4 Cu 7 O 15 by measuring various planar Cu NQR/NMR parameters: the spin-lattice relaxation time, the Knight shift and the indirect component of the Gaussian contribution to the spin-spin relaxation time. The temperature dependence of these parameters reveals a coupling between adjacent planes of a double plane. The existence of the inter-plane coupling has independently been confirmed by performing NQR Spin-Echo Double Resonance (SEDOR) experiments. The appearance of a spin gap seems to be the consequence of inter-plane coupling.

We have carried out Ru NMR and NQR measurements in superconductor CeRu 2 having T c = 6.1 K in the temperature range of 1.3-30 K. From the 99 Ru NMR in 10.2 T above H c2 , T 1 T = const was found in the measured-T range, and the value of T 1 TK 2 is almost the same as that in the uncorrelated Korringa relation, suggesting that there does not exist the strong spin fluctuation of Ru d-spins. The property of the superconducting state was investigated by using Ru NQR. We observed the Hebel-Slichter peak just below T c and the exponential decrease in 1/T 1 indicating that this superconductivity has an ordinary s-wave character. The effect of Al substitution for Ru was also studied by NMR and NQR.

Cu(1) NQR spectra of the CuO chain site in Y 1 _ x Pr x Ba 2 Cu 3 O 7 (x = 0.1 and 0.2), and the 63 Cu(1) NQR spin-lattice relaxation rates 1/T 1 in Y 0.9 Pr 0.1 Ba 2 Cu 3 O 7 have been measured. The half widths are about three times wider than those of the pure YBa 2 Cu 3 O 7 . The peak frequencies are lowered by the Pr doping. This seems to indicate a decrease of on-site holes at the Cu(1) site. The relaxation rate 1/T 1 is found to obey the same temperature dependence, 1/T 1 = aT + bT 3 as reported for pure YBa 2 Cu 3 O 7 .

The 63 Cu nuclear transverse relaxation rates have been measured in the normal state on both square and pyramidal CuO 2 planes in HgBa 2 Ca 2 Cu 3 O 8 + δ with T c = 133 K. The Gaussian component of the spin-echo decay rate, 63 (1/T 2G ), for both sites increases with decreasing temperature, followed by a peak around T* ~ 150 K, indicating that the spin correlation becomes stronger with decreasing temperature. Also, it is found that the magnitude of 63 (1/T 2G ) for a square site is larger than that for a pyramidal one, suggesting that the spin correlation in the square plane is stronger than that in the pyramidal one.