Volume 47, Issue 1-2

abstract The paper discusses some NQR/NMR studies performed on Y-Ba-Cu-O superconductors at the University of Zürich. It is shown how the resonance methods can yield information on electronic properties of quite different character. The review deals with: (1) the temperature and pressure dependence of Cu NQR frequencies in YBa 2 Cu 4 O 8 (1-2-4); (2) an NQR study of the properties of Ca substituted 1-2-4; (3) the temperature dependence of the Cu Knight shifts in 1-2-4; (4) the NMR of plane Cu 2 in the paramagnetic state of YBa 2 Cu 3 O 6 ; (5) the dependence on temperature, pressure and orientation of the Cu spin-lattice relaxation in 1-2-4.

Recent NQR and NMR experiments on La 2-x Sr x CuO 4 , YBa 2 Cu 3 O 7 , Tl 2 Ba 2 CuO 6+y , and other compounds are reviewed. The antiferromagnetic spin fluctuations decrease in the order of La, Y, T1 compounds. In the normal state the T-dependence of (T 1 T) -1 is considered to follow the Curie- Weiss law at high temperature and the (Ti 7") = const law at low temperature. The T dependence of 1/T1 below T c suggests strongly d-wave pairing. Correlations of the NQR frequency of 63 Cu and 17 O with the antiferromagnetic spin fluctuations and the superconducting transition temperature are discussed.

The oxygen diffusion in YBa 2 Cu 3 O 7-δ was studied between 300 K and 1273 K in flowing oxygen by monitoring the 133 Ba(EC) 133 Cs nuclear quadrupole interaction by time differential perturbed angular correlation. The data are analyzed in a model of two different nearest neighbour oxygen configurations and yield hopping rates, the energy barrier and the difference in free enthalpy between both states. In addition, we studied the reduced system YBa 2 Cu 3 O 6 +ε .

NQR-spectra and nuclear relaxation of 63 Cu in ceramic samples of the Y - Ba - Cu -O system with various oxygen content have been compared with those in iodinated YBa 2 Cu 3 O 6.1 .

The Cu-NQR lines of various high-T c superconductors at the Cu(2) site appear between 15 and 40 MHz. There exists an experimental correlation showing that the NQR frequency increases with decreasing T c . By comparison, it can be concluded from these results that a formal valence state of the Cu ion around 2 + is favourable for superconductivity. All lines exhibit a strong broadening compared to CuO 2 crystals, which is attributed to lattice modulations within the CuO 2 layers. This effect also influences the temperature dependence of the linewidth in YBa 2 Cu 3 O 7-δ .

Pure nuclear quadrupole resonance (NQR) spectroscopy at low frequencies (down to 275 kHz) has been used to investigate a number of borate, aluminate, and vanadate crystalline compounds and borate glasses. The values of the quadrupole coupling constants (Q cc ) and asymmetry parameters (η) are obtained with error limits that are one or two orders of magnitude less than those for values obtained from NMR spectra. New sites, not resolved in NMR spectra, have been detected for boron in borate glasses.

Simulations of solid-state 29 Si NMR spectra in nitrogen-containing silicon ceramics and glasses are described. The range of SiN x O 4-x tetrahedra is covered, with emphasis on the experimental conditions needed to observe the effect of the incompletely averaged 29 Si, 14 N dipolar coupling. A simple equation is discussed concerning its use in the interpretation of these 29 Si spectra, including the computation of 14 N quadrupole coupling constants with its absolute sign.

The electric field gradients (EFG) in aluminium due to a monovacancy and the interstitial muon are evaluated. The valence effect EFG q v is calculated using perturbed electron density δn(r)values obtained from density functional theory in an analytic expression which is valid at all distances from the impurity. The size effect EFG q s is evaluated using a new oscillatory form for the near neighbour (nn) displacements. The numerical values of q s are computed using fractional nn displacements available in the literature. For the total EFG good agreement with experiment is obtained without the use of any parameter.

A review is given of pulsed NQR spin-lattice relaxation measurements on the rhombohedral semimetals, arsenic (As) and antimony (Sb). The nuclear spin-lattice relaxation time T 1 has been measured in the temperature range 100 mK-600 K. The data follow the Korringa relation at low temperatures, but deviations from this relation are found as the temperature is increased. The results are quantitatively explained by considering interactions of the nuclei with the carriers at the Fermi surface. Above the Debye temperature (θ D ) the data may be understood by including a two phonon process in the analysis. The relaxation data for As and Sb are found to scale with θ D . T 1 has been measured for As at 295 K in the pressure range 0 - 10 kbar. The results are discussed in terms of changes in the Fermi surface parameters.

The 127 NQR spectra of cesium enneaiododimetallates Cs 3 M 2 I 9 , M = Dy, Ho, Er, Yb, Y and Tb, Sm, Nd, Pr have been studied as a function of temperature. The compounds are characterized by X-ray powder diffraction, too. The first group of compounds, M = Dy • • • Y, belongs to the family of Cs 3 Cr 2 Cl 9 -type enneahalogenodimetallates (III). The crystal structure of the second group, M = Tb ...Pr , is less symmetric, as in the 127 I NQR the presence of two crystallographically independent iodines shows up. Nuclear quadrupole coupling constants for the terminal and bridging halogens have been calculated from the NQR frequencies. The dependence of the NQR frequencies and of the crystal structure from the radius of the RE3⊕ -ion is discussed. The NQR spectra reveal phase transitions of the investigated enneaiododimetallates (III), for which Tlr also depends on r(RE3⊕). Positive temperature coefficients of the 127 I have been found and the influence of π-bond character is considered.

35 Cl, 81 Br, 121 Sb, and 209 Bi NQR of the title compounds was observed. According to the results of the temperature dependences of NQR frequencies and the DTA measurements, phase transitions take place in [N(CH 3 ) 4 ] 3 Bi 2 Br 9 (T tr =183K), [N(CH 3 ) 4 ] 3 Bi 2 Cl 9 (T tr = 155K), and (CH 3 NH 3 ) 3 Bi 2 Cl 3 (T tr = 200 and 249 K). 2 D NMR spectra for partially deuterated (CH 3 ND 3 ) 3 Bi,Br 9 showed that the phase transitions in this compound are related to the motion of the methylammonium cations. Single-crystal X-ray work at room temperature shows that the space group for [N(CH 3 ) 4 ] 3 Sb 2 Cl 9 is P6 3 /mmc with a = 925.1 pm, c = 2173.4 pm, Z = 2. For (CH 3 NH 3 ) 3 Sb 2 Br 9 the space group is P3ml with a = 818.8 pm, c = 992.7 pm, Z = 1; in both cases the cations show dynamical disorder. The Rietveld analysis of the powder X-ray diffraction for (CH 3 NH 3 ) 3 Bi 2 Br 9 reveals the space group P3ml with a = 821.0, c - 1000.4 pm, Z = 1 at room temperature; the compound is isomorphous with (CH 3 NH 3 ) 3 Sb 2 Br 9 . The crystal symmetries of the low-temperature phases of (CH 3 NH 3 ) 3 Bi 2 Br 9 and [N(CH 3 ) 4 ] 3 Bi 2 Br 9 were deduced from the results of the NQR spectroscopy

The crystal structure of (Ph 3 P) 2 AgOC 6 H 2 Cl 3 (I) is reported along with the syntheses and 35 Cl and 63 Cu NQR spectra of I and several related silver (I) and copper (I) 2,6-di- and 2,4,6-trichlorophenolates containing phosphines, phosphites, and pyridine as co-ligands. I crystallizes in space group P2 1 /cc with a= 16.692(4) Å, b= 17.942 (4) A, c = 12.857 (3) Å, β = 97.60 (1)°, F = 3816.68 Å 3 , and Z = 4. The final R (F) = 0.0475 and R (W) = 0.0396. Ag is coordinated in a trigonal planar geometry by the P atoms of the two triphenylphosphine ligands and the O atom of the chlorophenolate; Ag is then capped by one ortho-chlorine of the trichlorophenolate ligand at a distance of 3.160 (2) Å. In the 35 Cl NQR spectrum of this compound the two ortho-chlorines of the trichlorophenolate ligand have a large frequency difference of 1.500 MHz, indicating that one ortho-chlorine is coordinated to the silver; 35 Cl NQR spectra of related complexes are also presented and discussed. The 35 Cl NQR frequency differences of coordinated and non-coordinated ortho-chlorines in metal chlorophenolates correlate well with the metal-chlorine distances but not with the metal-chlorine-carbon bond angles. A different correlation is found for the silver complexes of dichloroalkanes; possible reasons for this are discussed.

The nuclear quadrupole interaction of 199m Hg in Hg-cysteine and Hg-tert-butyl mercaptide was determined at room temperature by perturbed angular correlations. The dramatic difference in the interaction strength between both compounds is a result of the two-fold and four-fold sulfur coordination, respectively. Moreover, we compared the preparation of Hg-cysteine using a Hg: cysteine ratio of 1:2 and using carrier-free 199m Hg from ISOLDE/CERN and obtained identical results.

The 111m Cd nuclear quadrupole interaction in CdCl 2 and CdCl 2 • H 2 O was determined in the temperature range from 77 K to 783 K and 363 K, respectively, by time differential perturbed angular correlation. In addition, both the crystallization of the monohydrate from solution and its dehydration were monitored in situ.

14 N NQR spectra of 1,10-phenanthroline and of dicyano bis(1,10-phenanthroline)iron(lll) were obtained at room temperature. An analysis based on the shifts of the NQR frequencies and in the topology of the charge distribution in a model (pyridine) showed that the complex formation produces a strong decrease in the N valence shell nonbonded maximum and an increase in the bonded density along each of the N - H bond directions. The NQR spectrum showed that the complex formation produces only small changes in the charge distribution of the cyanide groups.

The 81 Br and 127 I NQR frequencies in (CH 3 NH 3 ) 2 HgBr 4 and (CH 3 NH 3 ) 2 HgI 4 , respectively, have been recorded from above 77 K to around room temperature. The resonance lines exist in a wide range of frequencies. An unusual temperature dependence of the resonance frequencies has been observed. Small 1 H - 2 D isotope effects have been observed between (CH 3 ND 3 ) 2 HgBr 4 and (CH 3 NH 3 ) 2 HgBr 4 . Both negative and positive frequency differences between the deuterated and the nondeuterated compound have been observed at 77 K. The relatively large asymmetry parameters in the iodine compound decrease with increasing temperature, indicating the existence of H-bonds. The present results are interpreted qualitatively by considering the effects of H-bonding, N - H • • • X, between the halogen atoms and the CH3NH3 ions which may undergo large molecular motions. It is suggested that not only frequency-lowering but also frequency-rising effects may be produced, depending on the direction of the H-bond, i.e., the angle between the Hg -X bond and the X • • • H bond.

The temperature-dependence of the 63 Cu and 127 I NQR spectra of bis(2-picoline) Copper(I) Iodide reveals the existence of a phase change at 250 K. An X-ray crystallographic study of the low-temperature phase reveals that the iodine-bridged dimeric structure, which was observed at room-temperature, is retained in the low temperature phase but that the bond-lengths and, particularly, the bond-angles of the central four-membered ring are considerably modified.

The temperature dependent 7 Li-solid-state-NMR spectra of the two compounds [Li(N,N,N',N'- tetramethylethylenediamine)]ClO 4 and [Li(N,N,N\N tetramethylethylenediamine) 2 ]Al(CH 3 ) 4 are presented. Both compounds were investigated in the temperature range 160 K ≦ T ≦ 360 K. Above room temperature, the spectra for the tetramethylaluminate show the typical line shape for a first-order quadrupolar-disturbed central transition with an axially symmetric asymmetry parameter and a slightly positive temperature dependence of the quadrupole coupling constant. The mean temperature coefficient ά = + 9 • 10 -4 K -1 . The quadrupole coupling constant is in the range of 36-39 kHz. In the Temperature range 240 K ≦ T ≦ 275 K the quadrupolar splitting cannot be resolved. Below 240 K the quadrupole coupling constant is about 100 kHz. Contrarily, in the Perchlorate the quadrupole coupling constant is 75 kHz and is temperature independent. With increasing temperature the satellites lower in intensity and the central transition undergoes motional narrowing

The results of an NQR study on the MCl 4 complexes (M = Si, Ge, Sn) with organic heteroatom ligands (including those containing several coordination centers) are summarized: chloroanhydrides of unsaturated, aromatic and heteroaromatic carboxylic acids; alkylarylethers, ketones, aryldichlorophosphates, nitrogen-containing compounds, etc. The octahedral or trigonal-bipyramidal structure of these complexes has been established, and the factors influencing the formation and structure of such complexes (component ratio, character of ligand and M atom) have been analyzed. The structure of several complexes may vary with time or depend on the crystallization condition of the system. Some of these results indicate coordination isomerism. The quantitative ratios between the 35 Cl NQR frequency shifts of ligand and axial Sn-Cl bonds in trigonal-bipyramidal RCOCl • SnCl 4 complexes relative to the initial component frequencies have been found

63, 65 Cu NQR frequencies, together with their temperature dependence, are reported for several complexes formed between cuprous chloride or bromide and heterocyclic thiones. For bromides the 79,81 Br resonances have also been detected. These results indicate that, with one exception, the copper atom is tricoordinated by two sulphur atoms and one halogen. Several of these complexes show phase changes in the range 77-300 K.

A single 79 Br NQR line showing a frequency of 19.594 MHz at room temperature has been observed in the crystals of trans-[CoBr 2 (en) 2 ] [ H 5 O 2 ] Br 2 and assigned to the Br - ions which are not coordinated to the central Co(III) atom. The electric field gradient (EFG) at the Br - nuclei arises from O-H • • • Br - hydrogen bond formation between the Br - ions and the terminal O - H hydrogen atoms in [ H 5 O 2 ] + ions. The induced EFG is greater for the present bromine complex than that for the isostructural chlorine complex. A point charge model calculation explains well the relative magnitude of the EFG in the two crystals by introducing Sternheimer's antishielding factors for the halogen ions.

The results of NQR studies on electron density redistribution due to formation of adducts of various types of metal halides are reviewed and their consistency with X-ray data is discussed

The 35 Cl and 79 Br NQR spectra of halogen-containing organic compounds of Si, Ge, Sn and Te, in which these atoms may be tetra- or pentacoordinated, have been compared to the results of their X-ray diffraction analysis. The NQR technique was also used to study a large number of such compounds for which X-ray investigations were found to be impossible. It has been shown that the NQR technique may be considered as a rapid method for the determination of the electronic and spatial structure of these compounds. The conditions for pentacoordination of atoms of IVA group elements have been also formulated.

The 35 Cl NQR spectra of organyl(chloroalkyl) sulfides ClCH 2 S(CH 2 ) n CH 3 , n = 0-11, Cl(CH 2 ) m SR, m = 1 - 3 and alkyl(chloromethyl selenides ClCH 2 SeR (R = C 2 H 5 , C 3 H 7 ) have been obtained. In going from m = 1 to m = 2, the electron density on the CI atom in the series Cl(CH 2 ) m YR increases when Y = S and decreases when Y = O.

The EFG asymmetry parameters at 35 Cl nuclei have been measured for polycrystalline compounds of series Cl 2 C = CHX (X = OR, OCOR) and Cl 2 C = C(Cl)OCOR at 77 K. For the Cl trans-atoms in molecules Cl 2 C = CHX, the q values are equal or close to zero. For the Cl cis-atom of these compounds and for all Cl atoms of compounds Cl 2 C = C(Cl)OCOR, η ~ 6 - 9%. It was shown that the deviation from the axial symmetry of electron distribution of Cl atoms in these compounds is due to the influence not of the geminal, but of a vicinal atom or group directly through the field. The expected p, π -conjugation between the unshared electron pair of halogen and the Ti-electron system of double bond is absent in the molecules studied.

The 81 Br and 35 Cl NQR spectra of anhydrous Group 1, tetraalkylammonium, and thallium(I) 2,6-dichlorophenolates, 2,4,6-trichlorophenolates, and 4-bromo-2,6-dichlorophenolates were searched for evidence of solid-state cation-organochlorine interactions that might, for example, be (in part) responsible for the difference in the thermal decomposition reactions of these salts (to give the supertoxic environmental pollutants, the polychlorinated dibenzodioxins) versus those of the corresponding Group 11 chlorophenolate complexes, which do not decompose to dioxins. For cations ranging in size from tetraethylammonium to potassium, the total range of 35 Cl NQR frequencies in 2,6-dichlorophenolate and 2,4,6-trichlorophenolate salts is only 1.401 MHz, which, although larger than the 0.7 MHz range likely in molecular chlorocarbons, is smaller than in individual chlorophenolates of copper(I) and silver(I) (up to 2.0 MHz). In sodium and thallium(I) 2,4,6-trichlorophenolates substantially large frequency shifts (ca. 2.0 MHz) occur, to higher frequencies for the para-chlorines and lower frequencies for the ortho-chlorines (Na only). These disappear even in closely related compounds and show a pattern that seems unrelated to the decomposition products of the chlorophenolates.

The temperature dependence of 35 Cl NQR frequencies and the phase transition behaviour of chloroacetanilides (N-[2,6-dichlorophenyl]-2-chloroacetamide, -2,2-dichloroacetamide, -2,2,2-trichloroacetamide) were investigated. The crystal structure determination of N-[2,6-dichlorophenyl]- 2-chloroacetamide leads to the following: a = 1893.8 pm, b = 1110.7 pm, c = 472.1 pm, space group P2 1 2 1 2 1 = D 2 4 with Z = 4 molecules per unit cell. The arrangement of the molecules and their geometry is comparable to the high temperature phase of the acetyl compound N-[2,6-dichlorophenyl]- acetamide. For N-[2,6-diclorophenyl]-2,2,2-trichloroacetamide it was found: a = 1016.6 pm, b = 1194.3 pm, c = 1006.7 pm, ß= 101.79°, space group P2 1 /c = C 5 2h , Z = 4. The structure is similar to the low temperature phase of N-[2,6-dichlorophenyl]-acetamide. Parallelism between the temperature dependence of the 35C1 NQR lines of the CCl 3 group and the X-ray diffraction results concerning the different behaviour of the chlorine atoms was observed. The structures of the compounds show intermolecular hydrogen bonding of the N - H • • • O - C type. The phenyl group and the HNCO function are nearly planar. A bleaching out of several 35 Cl NQR lines at a temperature far below the melting point of the substances was observed. The different types of chlorine atoms (aromatic, chloromethyl) can be distinguished by their temperature coefficients of the 35 Cl NQR frequencies. All the resonances found show normal "Bayer" temperature behaviour. N-[2,6-dichlorophenyl]-2,2-diehloroacetamide shows several solid phases. One stable low temperature phase and an instable high temperature phase (at room temperature) were observed. The different phases were detected by means of 35 Cl NQR spectroscopy and thermal analysis

The 79 Br and the 127 I NQR spectrum of tert-butylammonium bromide and iodide, respectively, was studied as a function of temperature. No phase transition was observed for the bromide in the temperature range covered. The 79 Br NQR frequency is 9.056 MHz at 113 K and increases with increasing temperature (10.040 MHz at 293 K). The NQR spectrum reveals a phase transition of ( CH 3 ) 3 CNH ⊕ 3 I ⊖ around 360 K with pronounced hysteresis. The nuclear quadrupole coupling constant of 127 I is 69.335 MHz with an asymmetry parameter of 0.154 for the high temperature phase at T = 370 K. For the low temperature phase the corresponding values are 67.136 MHz and 0.081 at 300 K. The crystal structure of tert-butylammonium bromide was studied at room temperature. The compound crystallizes orthorhombic, space group D 152h Pbca, with 4 formula units in the unit cell, a = 1832.1 pm, b = 918.5 pm, c = 899.8 pm. The crystal structure and hydrogen bond system are discussed in connection with the NQR results.

The two line 35 Cl NQR spectrum of 4,5-dichloroimidazole was measured in the temperature range 77≦ T/K ≦ 389. The temperature dependence of the NQR frequencies conforms with the Bayer model and no phase transition is indicated in the curves v ( 35 Cl)= f(T). Also the temperature coefficients of the 35 Cl NQR frequencies are "normal". At 77 K the 35 Cl NQR frequencies are 37.409 MHz and 36.172 MHz and at 389 K 35.758 MHz and 34.565 MHz. The compound crystallizes at room temperature with the tetragonal space group D 4 4- P4 1 2 1 2, Z = 8 molecules per unit cell; at 295 K : a = 684.2(5) pm, c = 2414.0(20) pm. The relations between the crystal structure and the NQR spectrum are discussed.

The electric field gradient (EFG) in hexagonal Agl at the iodine site is studied theoretically by three different procedures, namely the point charge model, the Hartree-Fock cluster approach, and the augmented plane wave band structure procedure. A comparison is made for the electronic charge distribution and the sizes of the various contributions to the EFG obtained by the different procedures. From the point charge model and the Hartree-Fock cluster approach one gets almost the same result for the EFG, q = +0.743 • 1021 V/m 2 and +0.816 • 1021 V/m 2 respectively. Only a rather preliminary result of q = +0.393 • 1021 V/m2 is found by the band structure calculation to be compared with the experimental value of q exp = ±0.415 • 1021 V/m 2 . The confidence limits of the theoretical results are discussed, including possible ways to bridge the gap between the theoretical results by different methods and experiment. Both the cluster and the band structure calculations are shown to support an ionic model for hexagonal Agl with some overlap between the charge distributions on neighboring ions.

The available values of the nuclear quadrupole moments for the elements H - Ca are reviewed and related to recent advances in numerical methods in quantum chemistry. For Li the quantum chemical and nuclear Coulomb scattering values now agree. For Mg, also the muonic result agrees with them. For Na and Al, the new, accurate atomic values deviate from the earlier muonic values

A first principles method for the computation of electric field gradients (EFGs) is illustrated for various solids. This scheme is based on self-consistent energy band-structure calculations by the full potential linearized augmented plan wave (FLAPW) method which provides the electronic charge density including all polarization effects. By numerically solving Poisson's equation we obtain the Coulomb potential in a form which allows to compute the EFG directly. Our method is demonstrated for insulators (Cu 2 O), metals (hcp-Zn), supercondutors (YBa 2 Cu 3 O 7 ) and molecular crystals (Cl 2 , Br 2 , I 2 ).

The ab initio calculation of 33 S nuclear quadrupole coupling constants (NQCC) for a range of S-containing compounds with S 2 , S 4 and S 6 bonding types is described. All of the calculations used a triple zeta valence + polarisation basis set (TZVP) of gaussian type orbitals; all of the molecules were studied at the TZVP equilibrium geometry. The electric field gradients (EFG) calculated were correlated with the experimental NQCC obtained by either microwave spectroscopy (MW), nuclear quadrupole resonance (NQR) or NMR relaxation methods; although the experimental data cover a wide diversity of chemical types over a long period of time, the slope of the relationship between the EFG (q ii ) and the NQCC (χ ii ) yields a value for the 33 S atomic quadrupole moment of - 0.064 barn, very close to recent calculations with a large atomic basis set, and to experimental data. The relationship between the EFG tensor components and the internal molecular structure features is discussed for a diverse series of molecules.

14 N NQR spectra of [Co(NH 3 ) 6 ] • 3Cl, [Co(NH 3 ) 5 CO 3 ] • NO 3 , [Zn(NH 3 ) 4 ] • 2Cl, [Zn(NH 3 ) 4 ] •(BF 4 ) 2 , and [Ag(NH 3 ) 4 ] • NO 3 were obtained at 77 K. The results, analyzed by means of the topology of the charge distribution obtained from ab-initio MO calculations of free and of a model of coordinated NH 3 , showed that bonding to the metal-ion produces a strong decrease (Co ≫ Zn ≈Ag) in the N nonbonded density ("lone pair") and an increase in the bonded maxima found in the N - H bond direction of the N valence shell.

Milli - Kelvin NQR experiments are often essential for the study of pseudo one-dimensional materials. A brief overview of the special technical consideration for carrying out NQR measurements in a dilution refrigerator is given. Recent halide quadrupole resonance experiments on the pseudo one-dimensional XY crystals PrCl 3 and PrBr 3 are reviewed including the measurement and interpretation of frequencies, and spin-lattice relaxation times.

The electric field effect on 14 N NQR in ferroelectric thiourea [SC(NH 2 ) 2 ] single crystals has been investigated at 77 K up to a field of 10 kV/cm. The measured Stark shifts are, in units of kHz • cm/ kV, dv 1 + / dE + = 0.87±0.10, dv 1 + dE_ = 1.17±0.20, dv 2 + /dE + = 0.80±0.10, and dv 2 + /dE_ = 0.90 ±0.15. These values are on the order of 102 times those found in other ferroelectric crystals such as sodium nitrite (NaNO 2 ) . The remarkable difference between these two Stark shifts is discussed by taking into account the polarizability and the local electric field at the 14 N sites in these crystals

The temperature dependence of 35 Cl NQR frequencies in bis(4-chlorophenyl)sulfone was measured in the temperature range of 4.2-297 K. Below 115 K four NQR lines due to four inequivalent chlorines exist in the commensurate phase, contrary to the single NQR line reported by Corbero et al. It follows that the commensurate-incommensurate transition is of the first-order, and the incommensurate-normal transition at 150 K is quasi-continuous. It is also suggested that the successive phase transitions are triggered by the small angle rotation of the molecules about their C 2 -axes.

High temperature-resolution NQR measurements in an ultra pure Rb 2 ZnCl 4 crystal and a mixed [Rb 1-x (NH 4 ) x ] 2 ZnCl 4 crystal with a controlled amount of impurities (x = 0.01) verify the existence of a temperature region where the characteristic incommensurate line splitting is averaged out. The results show that for the system with high purity the averaging of the incommensurate splitting is due to large thermal fluctuations. In contrast, for the mixed crystal the added impurities seem to play the predominant role.

The 35 Cl NQR spectra of several chlorodifluoroacetates were studied as a function of temperature, including the acid ClF 2 CCOOH. The cations were: Ammonium, guanidinium, paramethylanilinium. Also some acid salts M⊕ClF 2 CCOO⊖ • n - ClF 2 CCOOH ( n > l ) were studied by 35 Cl NQR. The bleaching temperatures of the NQR signals were determined. In the para-methylanilinium salt and in the guanidinium salt a phase transition has been observed. The crystal structure of guanidinium chlorodifluoroacetate has been determined at room temperature (a = 1089 pm, 6 = 845 pm, c = 832 pm, space group Pnma, Z = 4). For comparison, guanidinium dichloroacetate was studied by 35 Cl NQR and by X-ray diffraction, too: P2 1 /c, Z = 4 , a = 804pm, b = 1202 pm, c = 1080 pm, ß = 131.58°. For guanidinium chlorodifluoroacetate and chlorodifluoroacetic acid, the 35 Cl spin lattice relaxation time T 1 and the line width have been followed up as a function of temperature. Therefrom, the activation energies of the reorientation motion of the group -CF2C1 have been determined to be 14 kJ • mol -1 (from T 1 ) and 12.5 kJ • mol - 1 (from Δv) for the pure acid and 9.2 kJ • mol -1 and 8.8 kJ • mol -1 , respectively, for the guanidinium salt.

Tetrachloro-o-benzoquione (TOB) crystallizes in two different forms. The two forms, Sample I and Sample II, show different phases and NQR spectra. When subjected to pressures higher than about 40 MPa the a phase is converted to the y phase. X-ray powder patterns studies as well as DTA measurements have confirmed this result. Heating the y phase above 310 K reverses this process.

The chlorine-35 NQR has been investigated in the mixed-stack charge-transfer complex tetrathiafluvalene- chloranil. The neutral-to-ionic (N-I) phase transition is clearly seen on the NQR spectrum. Among several features of the spectra recorded and their counterpart in the sample behaviour, the evidence for the change in the crystal symmetry and the quite likely existence of neutral domains in the ionic phase below the N - I transition temperature are the most striking ones.

The temperature and isotope dependence of the 35,37 Cl-NQR spin-lattice relaxation rate near the cubic to trigonal phase transition at T c1 ≌ 87 K has been investigated in both (NH 4 ) 2 TeCl 6 and (ND 4 ) 2 TeCl 6 salts. In the temperature range of ~ 20 K above and ~ 10 K below TCl, the relaxation obeys the relation (T 1 • T) 2 ∞(T-TCl). This behaviour corresponds to a direct relaxation mechanism in presence of a strongly damped rotational soft mode of the TeCl 2 6 - tetrahedra and is discussed on the basis of cation-anion coupling near the critical region. Below TCl we observe an extra contribution to the 35,37 Cl relaxation rate which is tentatively ascribed to the dynamics of the NH 4+ tetrahedra.

Trihalogenomethyl derivatives of 4-chloroacetanilide, 4-ClC 6 H 4 NHCOCCl 3 ,4-ClC 6 H 4 NHCOCF 2 Cl, and 4 - ClC 6 H 4 NHCOCF 3 were prepared. The 35 Cl NQR and 79 FNMR spin-lattice relaxation times T 1Q and T 1F , respectively, of these compounds were measured at various temperatures. On heating, T 1Q of the CCl 3 and CF 2 Cl groups decreased rapidly above ca. 110 and 150 K, respectively, showing the onset of reorientation of the trihalogenomethyl group about the pseudo-triad axis. The activation energies were determined to be 27 and 13 k J mol -1 for the CCl 3 and CF 2 Cl reorientation, respectively. The temperature dependences of the 19 F NMR T 1F of the CF 2 Cl and CF 3 groups lead to activation energies of 13 and 4.3 kJ mol -1 , respectively, for the same reorientations. T 1F of the CF 3 group observed above ca. 200 K was attributed to the spin-rotational interaction. The T 1Q -1 of ring-chlorine in 4 - ClC 6 H 4 NHCOCCl 3 and 4 - ClC 6 H 4 NHCOCF 3 can be interpreted by the T n law with n = 2.2 and 1.1, respectively, whereas that of 4-ClC 6 H 4 NHCOCF 2 Cl by the sum of two contributions: T n law (n = 2.0) and the modulation mechanism due to the CF 2 Cl reorientation

The 35 Cl NQR spin-lattice and spin-spin relaxation times, T 1Q and T 2Q , respectively, and the 1 HNMR spin-lattice relaxation time T 1H at 32 and 60 MHz were determined for [(CH 3 ) 3 S] 2 SeCl 6 as functions of temperature. The rapid decrease of observed above ca. 250 K with increasing temperature was attributed to the onset of reorientation of the [SeCl 6 ] 2- complex anion with the activation energy E a = 42 + 5 kJ mol -1 . When cooled from ca. 250 K, T 1Q showed an anomalous decrease. This T 1Q decrease was explained by electric field gradient modulation related to some cationic motion. Possible origins of the cationic motion are discussed

The 35 Cl NQR spin-lattice relaxation time T 1Q , spin-spin-relaxation time T 2Q , and 1 H NMR spin-lattice relaxation time in the rotating frame T 1Q in Mg(H 2 O) 6 SnCl 6 were measured as functions of temperature. Above room temperature T 2Q increased rapidly with increasing temperature, which can be explained by fluctuations of the local magnetic field at the chlorine nuclei due to cationic motions. From the T 1Q experiments, these motions are found to be attributable to uniaxial and overall reorientations of [Mg(H 2 O) 6 ] 2 + ions with activation energies of 95 and 116 kJ mol - 1 , respectively. Above ca. 350 K, T 1Q decreased rapidly with increasing temperature, which indicates a reorientational motion of [SnCl 6 ] 2 - ions with an activation energy of 115 kJ mol -1 .

The temperature dependence of the 35 Cl NQR spin-latticew relaxation time T 1Q has been determined for the three resonance lines observed in [ ( C 2 H 5 ) 3 NH] 2 SnCl 6 . The higher frequency lines fade out around 150 K upon heating, whereas the lowest line shows up to room temperature no anomaly, although these three lines are assigned to chlorines belonging to the same complex anion. The T 1Q values of the higher two lines decrease exponentially around the fade-out temperature, where T 1q of the lowest line shows no such behavior. These results are explained by the onset of uniaxial reorientations of the octahedral complex ions by 90° about the Cl -Sn -Cl axis containing the lowest frequency chlorines. The activation energy ( E a )of this reorientation (22-24 kJ mol -1 ) is the lowest so far reportd for [SnCl 6 ] 2 - ions. E a about the other axes is 67 kJ mol -1 , indicating a remarkable anisotropic reorientation.

The temperature dependences of NQR frequencies and spin-lattice relaxation times, T 1Q , of 35 Cl in (N 2 H 5 ) 2 SnCl 6 were measured. Four NQR signals distributed in a fairly wide frequency range were observed, the lowest-frequency line exhibiting an anomalous positive temperature coefficient. The highest-frequency line showed a steep temperature dependence. These results could be interpreted by considering the intermolecular interaction between CI and - NH 2 in N 2 H 5 + . Below ca. 250 K, T 1Q of the upper three signals exhibited a gradual decrease upon heating, explainable by lattice vibrations, while a shallow T 1Q minimum, ascribable to the modulation of electric quadrupole interaction, was observed at ca. 110K for the lowest-frequency signal. The reorientation of [SnCl 6 ] 2- was elucidated to be highly anisotropic, in that it reorients about one of the CI-Sn-Cl axes much more frequently than about the other two axes. The activation energies were determined to be 62 and 94 kj mol -1 for the respective reorientations.

The crystal structure of the title compound was determined at 291 K : monoclinic, space group C 5 2h -P2 1 /n, Z = 4, a = 1693.2(1), b = 579.3(1), c = 1232.5(1) pm, and ß = 107.21 (1)°, R = 0.0415. A sharp decrease of of Cl NQR found at T ≳ 270 K for the CCl 3 group is attributed to the reorientation of the group over a potential barrier of ca. 50 kJ/mol. Intermolecular interactions between the CCl 3 and the neighboring atoms seem to dominate the magnitude of the potential barrier. T 1 -1 of the 35 Cl NQR of the Cl atom on the benzene ring obeyed the T 2 law well in the range 80 ≲ T/K ≲ 270, while at T ≳ 280 K it deviated from the T 2 law.

The crystal structure of cyclohexylhemiacetal (cycHx-CH) was determined at 296 K : monoclinic, space group P2 1 /c, a =1028.7 (9), b = 609.5 (1), c = 1811.9 (4) pm, and ß = 99.79 (3)°, Z = 4, R = 0.0552. The three 35 Cl NQR lines in cycHx-CH, T 1 , T 2 , and T 2 *, were measured by a pulsed method at 80-300 K. Below 200 K T 1 -1 obeyed the T law well, indicating that the spin lattice relaxation is governed by lattice vibrations. The reorientation of CCl 3 seems to be responsible for the sharp T: drop observed above 250 K. Shoulders in the T 1 vs. 1/T curves indicate the presence of T 1 minima at about 240 K. A fluctuation of the EFG due to a dynamic disorder of hydrogen atoms in the OH groups is assumed to explain the T 1 minima.

The 35 Cl NQR temperature dependence in (PNCl 2 ) 3 and (PNCl 2 ) 4 was measured between 77 K and 293 K. The dependence in both compounds was smoothly varying. A sudden decrease in T 2 * near 112 K indicated the presence of a strong broadening mechanism in (PNCl 2 ) 3 . The nature of this mechanism is interpreted in terms of the Cl spin-lattice relaxation

Measurements of the spin-lattice relaxation time for rhenium in KReO 4 and NH 4 ReO 4 are reported; relaxation rates are extremely fast due to the large quadrupole moment of Re. Calculations of the electric field gradient in KReO 4 show that the EFG is a very sensitive function of the orientation of the anion in the unit cell as well as of the unit cell dimensions, and that geometrical effects make a major contribution to the temperature dependence. Neutron diffraction in these two salts has been employed to determine the temperature dependence of structural parameters over a wide range of temperature, and preliminary results are reported.

The spin-lattice relaxation time, T 1 , of the 23 Na-NMR line in NaNO 2 is measured between 25 K and 160 K at two magnetic field strengths, 1.1 T and 6.9 T. The temperature dependence of T 1 for the center line, observed on a polycrystalline sample prepared by precipitation from aqueous solution, is given by a monotonous curve. T 1 increases gradually as the temperature decreases. On the other hand for a single crystal, which is made by a modified Bridgman method, the temperature dependence of T 1 shows two deep dips below 150 K and a frequency dependence which cannot be explained by ordinary BPP theory. The dominant relaxation mechanism above and below 150 K is also investigated.

23 Na two-dimensional nutation NMR was applied to Na 1+x * Zr 2 S x P 3-x O 12 with x = 0, 1, and 2. The nutation spectrum in each compound consists of two clearly separated peaks corresponding to the two crystallographically inequivalent sodium sites. In the compound with x = 2 the nuclear quadrupole coupling constants (e 2 Qq/h) and the asymmetry parameters (η) were by spectral simulation estimated to be 1.0 MHz and 0 for one site, and 1.5 MHz and 0.8 for the other site. The spectrum varies considerably on heating; the two peaks merge into a single peak at high temperatures due to fast ion exchange between the two different sites. For x = 0 and x = 1 the nutation spectra did not show any significant temperature dependence, suggesting that the rates of the sodium ion exchange are negligibly low.

A 99 T c and 133 Cs solid-state NMR study of polycrystalline CsTcO 4 at a field of 7.04 T and temperatures from 90 to 430 K revealed first-order quadrupole effects. 99 T c quadrupole coupling constant, C q , shows anomalous positive temperature coefficients and decreases from 2.0 MHz at 430 K to 0.45 MHz at 90 K. The asymmetry parameter, η ( 99 Tc) , is zero at 430 K and at 210 K. The high-temperature minimum of η is associated with a phase transition from orthorhombic to tetragonal and the second minimum reflects an orientational change of the principal axes q xx and q yy of the EFG-tensor. The 133 Cs NMR spectra are characterised by a superposition of the two powder multiplets from the first-order couplings which arise from magnetically and crystallographically nonequivalent cesium sites, Cs (1) and Cs (2); the population ratio of Cs (2) and Cs (1), p 2 /p 1 , varies from 2.0 at 120 K to 6.5 at 373 K. The nonequivalence and population changes are attributed to the temperature change of the crystal field potential in the vicinity of the cations.

The 35 Cl NQR frequency and spin-lattice relaxation time of solid chlorobenzene and chloropentafluorobenzene at temperatures from 77 K to the melting points have been measured and explained by thermoactivated librations and reorientations of the molecules around the normal to their plane. The activation energies of these motions have been estimated

The application of two-dimensional spectroscopy to nuclear quadrupole resonance (NQR) is reviewed with special emphasys on spin 3/2 nuclei. A new two-dimensional level crossing double resonance NQR nutation technique based on magnetic field cycling is described. This technique allows for a determination of both the electric quadrupole coupling constant and the asymmetry parameter for spin 3/2 nuclei in powdered samples even in cases where the quadrupolar signals are too weak to be observed directly. It works if the usual double resonance conditions are met, i.e. if the spin-lattice relaxation times are not too short if the quadrupolar nuclei are dipolarly coupled to "strong" nuclei. Variations of this techique can be also used for 2 D "exchange" NQR spectroscopy and NQR imaging.

By simple examples it is shown how fine structures in rotational spectra of free molecules can be used to analyse nuclear quadrupole coupling. It is demonstrated that microwave Fourier transform spectroscopy is an adequate tool for these studies. Examples for different nuclei illustrate the present possibilities

The nutation frequency of a quadrupolar spin I depends on the quadrupole interaction. From nutation spectra, therefore, one obtains information about the direct surrounding of the atom. The theory of the straightforward and of more complicated nutation NMR experiments is outlined. Two applications of nutation NMR, to 23 Na in zeolites and 27 Al in alumina, are shown. In this way in CaO • 6 Al 2 O 3 penta-coordinated aluminum has been detected

A rotating-frame NQR imaging (ᵨNQRI) procedure based on pure NQR is reported. The technique is suitable for powdery or crystalline solid materials containing quadrupole nuclei. The spatial information is encoded by gradients of the radio frequency amplitude of the excitation pulse. A special deconvolution procedure has been developed for the analysis of the spatial information encoded in the pseudo-FID of the NQR signal in the case of powder geometry. The technique is applicable to rotationally symmetric as well as to asymmetric electric field gradient tensors. The RF gradients are suitably produced by surface coils. The prominent advantage of surface-coil NQR is that objects larger than the coil diameter can be investigated. The imaging procedure then provides depth resolution in a range of the order of the coil radius. The technique is particularly suitable for the detection of gradients of physical parameters influencing the NQR line shift and of spatial distributions of the chemical composition. Examples are stress or temperature gradients. Twodimensional images have been produced by rotating the sample step by step. For each orientation a profile across the sample is evaluated as the projection on the direction of the radio-frequency gradient. A projection-reconstruction formalism then permits the rendering of two-dimensional NQR images.

The 14 N-nuclear quadrupole coupling constants χ aa (14N <2>) and χ aa ( 14 N (1) ) for the ground-states of the dimers 14 N (2) 14 N (1) • • • HCCH and 14 N (2) 14 N (1) • • • HC 15 N have been corrected for zero-point effects and for the electrical effects of the subunit HX to give two estimatesχ( 14 N) = -5.01 (13) and - 5.07 (8) MHz, respectively, for the coupling constant of the isolated 14 N 2 molecule

The positive muon proves to be a very versatile and sensitive magnetic resonance probe: implanted in virtually any material its polarisation may be monitored via the asymmetry in its radioactive decay, giving information on the sites occupied by the muon in lattices or molecules, and the local fields experienced at these sites. The scope of these experiments has been greatly extended by the development of a technique of cross relaxation or level crossing resonance which allows quadrupole splittings on nuclei adjacent to the muon to be measured. The principles of the technique and the conditions necessary for detection of the spectra are described, together with a number of applications. Of especial interest is the manner in which muons mimic the behaviour of protons in matter. In metal lattices, for instance, muons invariably adopt the same interstitial sites as do protons in the dilute hydride phases, so that they can be used to study problems of localisation and diffusion common to those of hydrogen in metals. Studies of the muon level crossing resonance in copper have given valuable information on the crystallographic site, electronic structure and low temperature mobility of the interstitial defect. In semiconductors, muons are expected to trap at other impurities - notably acceptors - in processes analogous to the passivation of dopants by hydrogen. Muon resonance offers the exciting prospect of spectroscopic study of these passivation complexes. In molecular materials, substitution of protons by muons can be thought of rather like deuteration. Muons implanted in ice produce a significant change in the quadrupole coupling constant of adjacent 17 O nuclei which may be traced to the effects of the large muon zero point energy; the resonance spectrum also exhibits temperature dependent features which may be informative on the nature and lifetime of defects in the ice structure. Muon level crossing resonance has already been studied in an oxide superconductor and this relatively young field is now wide open for quadrupole interaction studies in other materials, using a variety of nuclei.

Exchange between positions with different resonance frequencies can be studied by two-dimensional spectroscopy originally developed for the NMR case. Cross intensity reflects changes of the resonance frequency due to different line shifts or anisotropic interactions. We now present analogous experiments carried out with pure NQR of polycrystalline samples. The specific requirements are discussed in comparison to 2D exchange NMR spectroscopy. The method has been demonstrated with chloroform and p-chlorobenzotrichloride, where the resonances of the 35 Cl nuclei were investigated. Isomeric changes taking place in these compounds reveal themselves as cross-peaks provided the right ranges of the temperature and the mixing time are chosen. The corresponding dependences were recorded and are discussed.

Since many years the 19 F-TDPAD method (TDPAD = time differential perturbed angular distribution) has been used to investigate electric field gradients in fluorine compounds. In these experiments always a strong reduction of the observed interaction amplitudes is observed. To explain these characteristics, a simple kinematic model is suggested.

A completely automatic computer-controlled NMR/NQR double resonance spectrometer is described. It features automatic tuning of the low, variable frequency power amplifier, thus permitting untended use over long periods, with high sensitivity and signal reproducibility. The sample is transferred between the low-frequency, zero-field region and the high-field region using compressed air and the possibility of switching on a field of several tens of gauss during the transfer of the sample is also included

Powder Zeeman NQR (PZNQR) spectra of 63 Cu, 75 As, and 35 Cl in various compounds have been observed at 77 K. On the basis of the types of simulated integral curves of PZNQR absorption, the estimated asymmetry parameters of the electric field gradients have been divided into three classes; (1) measurable, (2) small (smaller than about 0.05), and (3) very small (probably zero). The asymmetry parameters have been found to be very small ( Cu, NQR frequency = 26.704 MHz) in Cu,0;0.40 ( 63 Cu, 32.617 MHz) in KCu(CN) 2 ; very small ( 75 As, 116.820 MHz) in As 2 O 3 ; very small ( 75 As, 78.950 ; 35 Cl, 25.0567 and 25.4036 MHz) and small ( 35 Cl, 24.9581 MHz) in ASCl 3 ; very small ( 75 As, 98.501 and 99.046 MHz) in As(C 6 H 5 ) 3 ; small (38.389 MHz), 0.15 (38.464 MHz), and 0.14 ( 38.503 MHz) for 35 Cl in hexachlorobenzene; small ( 35.847 MHz) and very small ( 39.154,39.603 , and 39.615 MHz) for 35 Cl in l-chloro-2-(trichloromethyl)-benzene; small ( 32.619 MHz) and very small ( 32.878 MHz) for 35 Cl in 1,4-dichlorobutane; very small ( 37.828 and 38.051 MHz) for 35 Cl in 1,1,1 -trichloroethane.

Results of a study, both theoretical and experimental, of a new kind of NQR spin echo technique are reported and discussed

The Zeeman effect in a rotating frame is considered for the case when the quadrupolar nuclear spin system is acted on by two radio-frequency fields.

The behavior of induction signals during steady-state pulse irradiations in 14 N NQR was investigated experimentally. It has long been known that the signal response to a long sequence of η pulses will dramatically increase for pulse spacings less than T 2 . This increase is exponential, and can result in signals comparable to optimally prepared Free Induction Decays. Because these "SORC" signals recur as long as the pulsing continues, very efficient signal-averaging can result. The dependence of these quasi steady-state signals on pulse parameters and on frequency offset are presented, together with a discussion of the applicability of the method.

A Matec pulsed instrument, already equipped with a laboratory-built data system, has been considerably improved by adding new capabilities, such as versatile pulse sequence programming. Among other things, this allows automated T 1 relaxation measurements. The instrument has been employed to record the 14 N NQR FT spectrum of cyanuric chloride and measure its T 1 relaxation time from 77 K to the melting point at 418 K. The four-line 14 N spectrum indicates the presence of two chemically inequivalent sites. The intensity ratio of the lines identifies the site pertaining to a couple of symmetry-related 14 N nuclei. The lines smoothly decrease in frequency up to the melting point, where they remain narrow and almost equally spaced. Considering previous results at 4.2 K, this indicates that a single phase is stable over the entire accessible temperature range. The relaxation time T 1 decreases smoothly with increasing temperature, with a T -2 law, from nearly 180 s at 77 K to about 1 s near the melting point, where a precipitous decrease starts. The high T 1 values found are consistent with the lack of nuclei having high magnetic moment.

Two-dimensional NQR spectroscopy is experimentally accomplished by direct detection; forbidden transitions are also recorded. Two-dimensional spectra with cross-peaks are used in assigning lines in complex spectra. A method is described of increasing the 14 N NQR sensitivity by saturating the line v 0 whilst observing the NQR at a frequency v_. The presence of a low magnetic field is shown to improve the conditions for localised NQR observations.

The Nuclear Quadrupole Resonance Spectra (NQRS) Database contains all the numerical data of resonance frequencies that have been published since 1950, the year NQR was discovered, through the end of 1989. The total number of records compiled is 10,155 as of May, 1991. One record usually covers data of a particular polymorphic modification of a crystalline substance. About 3/4 of all the records deal with organic or organometallic substances. Recent changes in the data content and format, development of online and stand-alone search software, statistics of the database, and updating problems are described. A personal computer version of the database with load/search software was prepared and is described in the Appendix