Band 55, Heft 1-2

The application of hyperfine interaction techniques to problems in solid state physics has been stead-ily growing over the last decade with the use of radioactive ion beam facilities such as ISOLDE at CERN. New applications of the e - -γ Perturbed Angular Correlation (PAC) technique are underway at ISOLDE using probe nuclei with highly converted cascades. In this paper we present the motivation for extend-ing PAC experiments to elements/isotopes not usable with the conventional γ-γ PAC technique. Examples will be shown where the long-lived 73 As/ 73 Ge, the short lived 127 Ba/ 127 Cs and the 197m Hg/ 197 Hg decay cascades are applied to measurements of nuclear quadrupole interactions in materials.

Nuclear magnetic resonance has been used to study the diffusion of lithium on a Ru(001) single crystal surface. The quadrupolar interaction of the radioactive probe nucleus 8 Li was utilized for this. In an online experiment the 8 Li nuclei are produced, thermalized and highly polarized before they land on the surface studied, the parity violating β-decay revealing the sought after NMR/NQI nuclear information through the spatial asymmetry of the decay electrons. As a function of substrate temperature, alkali metal coverage and magnetic field the nuclear spin-lattice relaxation measurements show the existence of two distinctly different diffusion barriers on the surface, valued at 0.45 eV and 0.15 eV. The former is attributed to jumps from step to terrace sites, whereas the latter is the barrier between adjacent terrace sites. On the substrate Si(l 11)7x7 the relaxation measurements suggest a much higher diffusion barrier around 0.8 eV.

We report solid-state 17 O NMR determinations of the oxygen chemical shift (CS) and electric field gradient (EFG) tensors for a series of 17 O-enriched organic compounds containing various functional groups. In several cases, analysis of the n O magic-angle-spinning (MAS) and static NMR spectra yields both the magnitude and relative orientations of the 17 O CS and EFG tensors. We also demonstrate the feasibility of solid-state 17 O NMR as a potentially useful technique for studying molecular structure and hydrogen bonding.

A variety of two dimensional two-and three-dimensional NQR experiments are reviewed, showing their application to the determination of field gradients for important sites in peptides and proteins, for assigning connected transitions of nuclei with spin ≥ 5/2, and for determining hexadecapolar coupling constants. The quadrupole coupling tensor for 63 Cu in the active site of a protein, bovine erythrocyte (Cu, Zn) superoxide dismutase, has been measured and is compared with the results of ab initio calculations.

A cross-relaxation technique is described which involves two spin contacts per double reso-nance cycle. The result is an improvement in signal to noise ratio particularly at low frequencies. Experimental spectra and analyses are presented: 14 N in ammonium sulphate showing that the tech-nique gives essentially the same information as previous studies; 14 N in ammonium dichromate determining e 2 Qq/h as (76±3) kHz and η = 0.84±.04; 7 Li in lithium acetylacetonate for which the spectrum (corrected for Zeeman distortion) yields e 2 Qq/h = (152 ±5) kHz and η=.5 ±.2. Calculated spectra are presented to demonstrate the η dependence of the line shapes for 7 Li.

The electric field gradient (efg) at the Nb site in the intermetallic compounds Nb 3 X (X = Al, In, Si, Ge, Sn) was measured by the Perturbed Angular Correlation (PAC) method using the well-known gamma-gamma cascade of 133 -482 keV in 181 Ta from the β - decay of 181 Hf, substituting approximately 0.1 atom percent of Nb. The PAC results show that V zz drops by nearly 40% when X changes from Al to In, and by about 25% when X changes from Si to Ge and Sn. This behavior is most probably related to the change in the degree of sp hybridization in these compounds. The V zz values of the studied compounds do not follow the well known universal correlation for the efg's in non-cubic metals but the observed trend is well reproduced by results of ab-initio electronic structure calculations. In the case of Nb 3 Al a linear temperature dependence of the quadrupole frequencies was observed in the temperature range of 6.5 to 1210 K.

We have measured the hyperfine interactions in YbVO 4 and YbPO 4 using 172 Yb perturbed angular correlation spectroscopy for 90 -1094 keV and 1094 -79 keV γ-γ cascades. The quadrupole interaction frequency varied with temperature from 0 to 2 GHz. Precise information was obtained from both cascades independently even using the cascade with the intermediate state of the half life as short as 1.6 ns.

We report on a time differential perturbed angular correlations (TDPAC) study of the hyperfine interactions at 181 Ta in Hf 2 Ni 7 and Zr 2 Ni 7 . The samples were prepared by arc melting proper amounts of constituents together with a small amount of Hf containing radioactive 181 Hf atoms. A further heat treatment was necessary to obtain samples of the required crystal structure (monoclinic symmetry in the C2/m space group). As known, two non-equivalent lattice sites of Hf/Zr metal exist in this structure. The PAC measurements were performed at 20 - 1123 K. Two quadrupole interactions were observed, corresponding to two possible crystallographic positions of the probe atom in the studied structure. The electric quadrupole interaction parameters at 181 Ta in Hf 2 Ni 7 are υ Q1 (300K) = 586.7(65) MHz with η 1 = 0.79(4) and υ Q2 (300K) = 466.8(41) MHz with η 2 = 0.20(3), and in Zr 2 Ni 7 they are υ Q1 (300K) = 498.4(22) MHz with η 1 = 1.0 and υ Q2 (300K) = 418.0(50) MHz with η 2 = 0.35(3). The presence of a magnetic interaction, reported in the literature, was not confirmed.

We present a detailed theoretical and experimental NQR multiple-pulse spin-locking study of spin-lattice relaxation and spin diffusion processes in the presence of paramagnetic impurities in solids. The obtained diffusion equation was obtained allows to find the time dependence of the magnetization in the effective field. The spin lattice relaxation times were calculated, both for direct and diffusion regimes, as functions of the correlation time and multiple-pulse parameters. Measurements of relaxation times in rotating frame allow to determine the diffusion coefficient and the radius of the diffusion barrier in γ-irradiated polycrystalline samples of NaCIO 3 .

A pulsed NQR spectrometer specifically designed to facilitate real-time tuning of pulse sequence parameters is described. A modular approach based on the interconnection of several rack-mounted blocks provides easy access to all spectrometer signals and simplifies the task of modifying the spectrometer design. We also present experimental data that demonstrates the ability of the spectrometer to increase the signal to noise ratio of NQR measurements by automatically adjusting the pulse width in the strong off-resonant comb pulse sequence.

A new method for increasing the probability of detecting nuclear resonance signals is demon-strated experimentally. It is well known that the detection of signals with a low signal to noise ratio (SNR) results in missed detections of false alarms. In situations where the noise is correlated or where limited data is averaging, it may not be possible to achieve a desired SNR through averaging alone. We present an alternative approach in which a feedback algorithm automatically adjusts pulse parameters so that the SNR and probability of correct detection are increased. Experimental results are presented for the detection of 14 N NQR signals.

A new TDPAC spectrometer which is based on the principle of event by event data acquisition (list mode) in order to reduce a background, avoid systematic errors and increase efficiency of the setup is described. The principal demand on the new spectrometer is the simultaneous measurement of all events containing the time and energy signals of the four detectors appearing in a preset common time window. For this purpose an 8-parameter list mode data acquisition system has been developed.

A two-dimensional spectroscopic technique is presented. Application of the technique to de-termine all the relaxation rates for a multi-level quadrupole system is discussed along with ex-perimental requirements. Theoretical and experimental data are analyzed for the 14 N, three level, spin 1 system. The analysis shows that all three relaxation rates can be obtained from a single two-dimensional spectrum, and that only 3 peaks in the 3 x 3 two-dimensional intensity matrix are needed to completely specify the problem. The 4-level spin 3/2 system is also examined. In theory, all six relaxation rates can be independently evaluated (for a suitable system) without necessitating measurement at the low frequency Zeeman separated levels.

35 Cl NQR measurements revealed two phase transitions at 14.5 K and 70 K in NH 4 ICI 4 . By deuteration of the ammonium ions, the respective transition temperature increased to 27 K and 74 K. In the lowest-temperature phase of NH 4 ICI 4 , by the deuteration a remarkable change of the temperature dependence of the 35 Cl NQR frequencies was observed. The NQR spin-lattice relaxation-time minimum, ascribable to the modulation effect of the electric field gradient (EFG) due to a motion of the ammonium ions, was observed at 7 K and 11 K in NH 4 ICl 4 and ND 4 ICI 4 , respectively. The activation energy of the ammonium motion and the fluctuation fraction q' /q of the EFG were determined to be 0.32 kJ mol -1 and 1.3x10 -3 for NH 4 ICI 4 , and 0.30 kJ mol -1 and 0.88x10 -3 for ND 4 ICI 4 . The experimental result suggests that the inter-ionic interaction between the ammonium ion and the ICl 4 - complex anion is weakened by the deuteration. However, the frequency shift observed in the lowest-temperature phase is much larger in ND 4 ICI 4 than in NH 4 ICI 4 . This suggests that the lattice distortion in NH 4 ICI 4 is suppressed to a great deal by the tunneling rotation of NH 4 + ions. The effective inter-ionic interaction responsible for the lattice distortion, the transition temperature and the enhancement of tunneling of NH 4 + as compared with ND 4 + is discussed for NH 4 ICI 4 as well as for previously reported NH 4 AuCI 4 and α-NH 4 HgCl 3 .

Radioactive atoms located on cation sites in oxide matrices can be used to monitor phase transitions by measuring the electric or magnetic hyperfine interactions by means of Perturbed Angular Correlations spectroscopy. The article illustrates three types of phase transitions studied with 111 In tracers and their daughter nuclei 111 Cd, namely magnetic, structural and REDOX phase transitions in binary and ternary polycrystalline or single-crystalline oxides. In this context, we also discuss the question of identifying the probes' lattice site(s), the scaling of the Electric Field Gradients in oxides, the influence of the (impurity) probes themselves on the phase transitions, and the occurrence and mechanisms of dynamic interactions. Recent results on 111 In in pure and Li-doped In 2 S 3 will also be presented.

NMR spectra of 73 Ge (nuclear spin I = 9/2) in perfect single crystals of germanium with different isotopic content were measured at 80, 300, and 450 K. The observed specific line shapes gave evidence of the isotopic disorder, in particular, abnormal broadening of the spectrum was found for the magnetic field directed along the [111] axis. Local lattice deformations in the germanium crystal lattice due to "isotopic disorder" were calculated in the framework of the adiabatic bond charge model. The results were applied to study random non-cubic crystal field interactions with the nuclear quadrupole moments and corresponding effects on NMR spectra. The simulated second moment of the resonance frequency distributions caused by the magnetic dipole-dipole and electric quadrupole interactions are used to analyze the lineshapes, theoretical predictions being in a qualitative agreement with the experimental data.

The 35 CI NQR frequency (υ Q ) and spin lattice relaxation time ( T 1 ) in 2,6-dichloropyridine, 2 amino 3,5-dichloropyridine and 6 chloro 2-pyridinol have been measured as a function of pressure up to 5.1 kbar at 300 K, and the data have been analysed to estimate the temperature coefficients of the NQR frequency at constant volume. All the three compounds show a non linear variation of the NQR frequency with pressure which can be described by a 2 nd order polynomial in pressure. The rate of change of the NQR frequency with pressure is positive and decreases with increasing pressure. The spin lattice relaxation time T 1 , in all the three compounds shows a small increase with pressure, indicating that the relaxation is mainly due to the torsional motions.

NMR, NQR, powder X-ray diffraction, DTA and AC conductivity were measured in RMBr 4 (R = Ag, Cu; M = Al, Ga) and RM 2 Br 7 (R = Li, Ag; M = Al, Ga). In RMBr 4 , the activation energy of Cu + diffusion was evaluated from 63 Cu NMR and was in good agreement with that from 81 Br NQR. In CuAlBr 4 , the e 2 Qq/h value of 63 Cu NMR and the η value of 27 AI NMR changed linearly with decreasing temperature, although the e 2 Qq/h value of 27 AI NMR did not change so much. These temperature dependences are supposed to be due to Cu + diffusion and not to a variation of the lattice constants. In RM 2 Br 7 , the activation energy was obtained from the spin-lattice relaxation time T 1 of 81 Br NQR and is ascribed to a modulation of the cation diffusion. The line width of 7 Li NMR in LiAl 2 Br 7 was about 5.9 kHz in the low-temperature phase and 0.4 kHz for the high-temperature phase. The 27 AlNMR spectrum was broadened by the quadrupole interaction and unchanged up to 400 K, suggesting that diffusion of Li + ions occurs in the high-temperature phase.

External influences on the 14 N NQR of ferroelectric NaNO 2 , such as changes of the temperature, electric field, stress, gamma-ray irradiation, and impurity doping are reviewed. They normally cause either a frequency shift or a broadening of the 14 N NQR lines and are strongly related to the change of the spontaneous polarization.

A search was made to detect the possibility that individual impurity atoms in two-phase, binary alloys nucleate embryonic crystals. The alloy system studied was Ni-Al and the impurities were 111 ln probe atoms. Local surroundings of the probes characteristic of the crystal phases were detected through quadrupole interactions with 111 Cd daughter nuclei using the method of perturbed angular correlation of gamma rays. Analysis of site fractions of the probes led to two alternative interpretations that could not be distinguished on the basis of the present measurements: 1.) 111 ln probes in the two-phase domain between Ni 2 Al 3 and NiAl segregate to Ni 2 Al 3 with a segregation energy of about 0.10 eV; or 2.) 111 ln probes nucleate embryonic crystals of Ni 2 Al 3 , with the driving force for nucleation being an attraction between In atoms and Ni vacancies that is known to be strong in NiAl.

The 123,123 Sb, 35 Cl, 79,81 Br and 127 I NQR Spectra of the complexes Cs 3 Sb 2 X 9 (X = Cl, Br, I), prepared from CsX and SbX 3 aqueous solutions, were studied at 77-400 K. Analysis of the^ temperature coeffi-cients of the quadrupole transition frequency (v), quadrupole coupling constant (e 2 Qq zz ) and asymme-try parameter of the electric field gradient (η) was carried out.

127 I and 14 N NQR have been investigated in two cadmium idodide -pyridine compounds, the inter-calate CdI 2 +5.9 pyridine and the CdI 2 Py 2 complex. In the intercalate, iodine is mainly ionic and nitro-gen like in pure pyridine; in the complex, iodine exhibits 30% covalency while nitrogen, coordinated to the central cadmium atom of the complex, and presents coupling constants e 2 Qq and η smaller than in pure pyridine. The difference between the two compounds is also revealed by the thermal coefficient of the quadrupole coupling constants and the relaxation. Results on the other title compounds are also given.

NQR (nuclear quadrupole resonance) spectroscopy is potentially one of the best ways of characterizing the bonding of metal cations such as zirconocene cations and Ag + to halogen donor atoms in weakly coordinating anions and neutral ligands such as halocarbons. Known coordination of organoiodine atoms to silver ions was studied by 127 I NQR spectroscopy in [Ag(p-C 6 H 4 I 2 )](PO 2 F 2 ), [Ag(o-C 6 H 4 I 2 )]N0 3 , [Ag(CH 2 I 2 ) 2 ]PF 6 , and the explosive [Ag(CH 2 I 2 )]NO 3 ; a possible case of iodocarbon coordination to a mercury atom in C 5 Cl 5 HgCl ·CH2I2 was also investigated. Both the low-frequency (ca. 300 MHz) and the high-frequency (ca. 500 MHz) NQR signals were detected for each compound, and, for the first time, the quadrupole coupling constants e 2 Qq zz /h and asymmetry parameters ηof coordinated organohalogen atoms were obtained. As compared to the free iodocarbons, substantial (4 - 12%) reductions of e 2 Qq zz /h and substantial increases in η (to about 20 - 40%) were found in the iodocarbon complexes of silver, but not of mercury. From approximate theoretical analysis of the data, it appears that the electronic interaction of silver and iodine is quite substantial.

Time-differential PAC measurements have been made using the high specific activity of 111 In, both in aqueous solutions of the ClO - 4 , NO - 3 and Cl - at pH values between 1.0 and 9 and temperatures between 186 and 293 K.

We present Perturbed Angular Correlation (PAC) results of the electric-nuclear quadrupole interaction at lxl Ta located at the nonequivalent cation sites of cubic C-Sm 2 O 3 and monoclinic B-Sm 2 O 3 free of defects. The application of the empirical Cd/Ta correlation found in binary oxides allowed us to correlate the interactions with each site and to identify the single Sm site populated in 111 In-implanted B-Sm 2 O 3 experiments. The preferential site occupation of Cd and Ta in B-Samaria is discussed in terms of the “chemistry” of the probe-atoms.

The 35 Cl and 79 Br NQR spectra of chalcogen halide complexes of aluminium, gallium, titanium, zirconium, hafnium, niobium, tantalum, molybdenum, tungsten, rhenium, iron, ruthenium, osmium, iridium, rhodium, platinum, palladium and gold are discussed. Three structure types of these complexes have been distinguished by X-ray structure analysis: type I with AX 2 ligand and [MX n „A m ] coordination polyhedron; type II with AX 3 ligand and [MX n+m ] coordination polyhedron; type III, dimeric complexes with M-X-M bridge (where X = Cl, Br and A = S, Se, Te). The formation of secondary M-X-A or M-X-M bonds is characteristic of most structures. The spectra were interpreted by a Townes. Dailey approximation with allowance for the electronic configuration of the metal, mutual influence of ligands and structure features of complexes. Systematic investigation of a big series of chalcogen halide complexes-analogues allowed the following changes in 35 CI and 79 Br NQR frequencies on secondary bonding to be established for intraligand halogen atoms: A decrease in frequency for type I complexes and an increase in frequency for type II complexes; for halogen atoms in the coordination polyhedron: a decrease in frequency for p metals and transition metals with d>6, and an increase in frequency for metals with d< 6.

The crystal structure of the room temperature phase (RTP) of the title compound was determined at 297 K (monoclinic, space group C2/c, a = 1384.2(2), b = 1377.8(3), c = 755.5(2) pm, β = 121.58(1)°). A complicated disorder was found for the cation. A phase transition from the low-temperature phase (LTP) to the RTP was found at (224 ±1) K (T c ). The 1 H NMR spectra showed a sharp motional narrowing at ca. T=T c , indicating the occurrence of a reorientational motion of the cation in the RTP in accord with the disorder. It was found that another reorientational motion is excited in the LTP. Four 81 Br NQR lines (132.71, 115.38, 61.54 and 59.31 MHz at 77 K) and two Sb NQR lines (53.78 and 33.76 MHz at 77 K) were found in the LTP, while a single 81 Br NQR line was observed at T> 276 K (ca. 121.80 MHz at 300 K). Crystal dynamics are discussed on the basis of the temperature dependence of the NQR, 1 H NMR line width, and 1 H NMR T 1 .

The temperature dependences of 2 H NMR spectra and the spin-lattice relaxation time T\ were measured for [Co(H 2 O)f,][SiF 6 ]. The variation of the spectrum above room temperature can be explained by the reorientation of [Co(H 2 O) 6 ] 2+ about the C 3 axis. The activation energy E a and the jumping rate at infinite temperature K 0 for the three site jump of [Co(H 2 O) 6 ] 2+ were obtained as 82 kJmol -1 and 2x 10 17 s -1 from the spectral simulation. Below room temperature, the spectral line shape was dominated by the 180° flip of the water molecule. The minimum of T 1 caused by the 180° flip of the water molecule was observed at ca. 260 K. The jumping rate of the 180° flip of the water molecule was estimated from the 2 H NMR T 1 and the spectral simulation. E a = 38 kJmol -1 and K 0 = 6x 10 15 s -1 for the 180° flip of the water molecule were obtained from T 1 .

The temperature and angular dependences of the 2 H NMR spectrum were measured for single crystal [Cu(H 2 O) 6 ][PtCl 6 ]. (e 2 Qq/h, η) = (132 kHz, 0.72) were obtained for the D nucleus averaged by the fast 180° flip of the water molecule at 297 K. D 1 (236 kHz, 0.10) and D 2 (246 kHz, 0.09) were obtained for two unequal D nuclei of the water molecule at 133 K. Below T c , at least three nonequivalent waters were found to exist. The jumping rate for the 180° flip of the water molecule (k) was obtained from the simulation of 2 H NMR spectra at H 0 ||[111] direction. The activation energy E a and the jumping rate at infinite temperature were estimated from the temperature dependence of k to be 24kJmol -1 and 1 x 10 13 s -1

Stable crystalline 2-chlorothiophene has two glass transitions at 164 and 186 K. 35 CI NQR measurements were carried out between 77 and 200 K. Two NQR signals with full widths of about 100 kHz at half maximum were observed in this temperature range. The spin-lattice relaxation times T 1 were measured at the two peak frequencies. The activation energy obtained from the results of the T 1 measurements showed a fairly good agreement with those estimated from calorimetric measurements.

The pharmaceutical^ important iron-dextran complexes (ferritin models) were studied by Mössbauer spectroscopy in frozen solution at 87 K and in lyophilized form at 297 and 87 K. The quadrupole splittings of the lyophilized forms of iron-dextran complexes measured at 297 and 87 K were slightly different. The quadrupole splittings of several complexes in lyophilized form and frozen solutions measured at 87 K were also different. Slight differences in the quadrupole split-ting were found for various iron-dextran complexes. Mössbauer spectra of iron-dextran complexes were better fitted using two quadrupole split doublets. Slight differences in the corresponding quadrupole splittings and doublet ratios were observed for various complexes.

Human adult hemoglobin modified by both pyridoxal-5'-phosphate and glutaraldehyde in the oxy-form was studied by Mössbauer spectroscopy. Mössbauer spectra were measured at 87 and 295 K (hemoglobin in lyophilized form) and at 87 K (hemoglobin in frozen solution). The values of the quadrupole splitting for modified oxyhemoglobin were found to be lower then those of oxyhemoglobin without modifications in lyophilized form and frozen solution, respectively. The Mössbauer spectra of modified oxyhemoglobin were also analyzed in terms of the heme iron inequivalence in α-and β-subunits of the tetramer. Differences of the tendencies of temperature dependencies of quadrupole splitting for modified and non-modified oxyhemoglobin in lyophilized form were shown.

The structure o f iodates of α- and β-alanine ( Ala) (2(β-Ala • HIO 3 ) • H 2 O , β-Ala-2HIO 3 , D L-Ala• HIO 3 • 2H 2 O, L-Ala • HIO 3 ) and L-lysine (L-Lys) (L-Lys • HIO 3 , L-Lys • 2HIO 3 ,L-Lys • 3HIO 3 , L-Lys • 6HIO 3 ) have been investigated by means of iodine-127 NQR, IR spectroscopy and X-ray diffraction

Chlorine NQR was studied for the isostructural hexachlorometallate mixed system. The study shows that Isomorphic hexachlorometallate solid solutions exhibit often impurity induced local structural order because of their relatively clear local site symmetry. This is manifested in the formation of a few satellite lines near the original resonance line and results from the random distribution of impurities on the lattice sites of the corresponding counterpart ions. Using the point charge model and a simple binomial function for the occupation probability of the guest ions on the host lattice sites, the position and the line intensity could be determined, the results of which are in good agreement with the NQR-observation. The temperature region of lattice dynamics in the crystal seems to shift in proportion to the impurity content. This fact explains the gradual change of the transition temperature in the mixed crystal between two starting materials.

Arsenic NQR in Co(AsF 6 ) 2 .2AsF 3 reveals three lines: one near 122 MHz and two near 50 MHz. Relations of the spectral lines to the structure and to thermal vibrations are discussed. Possible influence of paramagnetic Co is examined.

The temperature dependence of the 35 Cl NQR line and its relaxation has been measured from 77 K to room temperature. The known displacive phase transition near 162 K is reflected in the change of the slope of the temperature dependence of the quadrupole line, but not so in the chlorine or proton relaxation. The results are discussed in view of the proposed structural changes and thermal vibrations.

The influence of impurities on the 14 N NQR lineshape of Na 1-x Ag x NO 2 and [NaNO 2 ] 1-x -[BNO 3 ] x (B = Na, K) at room temperature has been investigated. Carrying out spectral analysis in conjunction with classification of the local field inhomogeneities according to the structurally isomorphic, Na 1-x Ag x NO 2 , and anisomorphic [NaNO 2 ] 1-x [BNO 3 ] x systems, enabled an under-standing of the microscopic nature of impurity-induced local disorder. The iso-and anisomorphic systems reveal their own unique features of the impurity induced local disorder. They are charac-terized by a static, random distribution of impurities in the isomorphic system and a fast motion of the impurity-induced mobile point defects in the anisomorphic system. However, for both systems, neither a change of the 14 N NQR frequency nor a multisplitting of the lines is observed because of the relatively low symmetry.

The temperature dependence of 127 I NQR and DSC as well as the crystal structure at room temperature of the title compound were determined. This compound shows a first-order phase transition of an order-disorder type at 245 K. Eight 127 I(v 1 :m = ±1/2 ↔ ±3/2) NQR lines of 79.57, 81.86, 82.56, 83.36, 84.68, 87.72, 88.34, and 88.86 MHz, and corresponding eight 127 I(v 2 : m = ±3/2 ↔±5/2) NQR lines were observed at liquid nitrogen temperature. Three 127 I(υ i ) NQR lines wfth an intensity ratio of 1:1:2 in the order of decreasing frequency were observed just above the transition point and two NQR lines except for the middle-frequency line disappeared around room temperature. This temperature behavior of NQR lines is very similar to that observed in [N(CH 3 ) 4 ] 2 Hgl 4 . Another first-order phase transition takes place at 527 K. The structure of the room-temperature phase was redetermined: orthorhombic, Pnma, Z = 4, a = 1342.8(3), b = 975.7(2), c = 1696.5(3) pm. The NQR result of three lines with an intensity ratio of 1:1:2 is in agreement with this structure. The thermal displacement parameters of atoms in both cations and anions are large.

(T The crystal structure of [C(NH 2 ) 3 ]HgBr 3 was determined at room temperature: monoclinic, space group C2/c, Z = 4, a = 775.0(2), b = 1564.6(2), c = 772.7(2) pm, β = 109.12(2)°. In the crystal, almost planar HgBr3 - ions are connected via Hg ··· Br bonds, resulting in single chains of trigonal bipyramidal HgBr 5 units which run along the c direction. [C(NH 2 ) 3 ]HgI 3 was found to be isomorphous with the bromide at room temperature. The temperature dependence of the halogen NQR frequencies (77 < 77K < ca. 380) and the DTA measurements evidenced no phase transition for the bromide, but a second-order phase transition at (251 ± 1) K (T c1 ) and a first-order one at (210 ± 1) K for the iodide. The transitions at T c2 are accompanied with strong supercooling and significant superheating. The room temperature phase (RTP) and the intermediate temperature phase (ITP) of the iodide are characterized by two 127 I (m=1/2↔3/2) NQR lines which are assigned to the terminal and the bridging I atoms, respectively. There exist three lines in the lowest temperature phase (LTP), indicating that the resonance line of the bridging atom splits into two. The signal intensities of the 127 I (m =1/2↔3/2) NQR lines in the LTP decrease with decreasing temperature resulting in no detection below ca. 100 K. The 127 I (m=1/2↔3/2) NQR frequency vs. temperature curves are continuous at T cl , but they are unusual in the LTP. The T 1 vs. T curves of 1 H NMR for the bromide and iodide are explainable by the reorientational motions of the cations about their pseudo three-fold axes. The estimated activation energies of the motions are 35.0 kJ/mol for the bromide, and 24.1, 30.1, and 23.0 kJ/mol for the RTP, FTP, and LTP of the iodide, respectively

The quadrupole hyperfine interactions of 111 Cd/ 111 In probe nuclei in single-crystalline SrTiO 3 perovskite samples were investigated using Perturbed Angular Correlation spectroscopy. Three electric field gradients were detected and their fractions and hyperfine parameters were measured in the temperature range from 26 to 700 K. The fraction f 0 having a vanishing quadrupole frequency, as expected for 111 Cd on substitutional sites in a cubic lattice, starts to develop around 300 K and reaches 100% at 700 K. Two well-defined EFG's having closely lying quadrupole frequencies and asymmetry parameters of ω Qi = 49.1(3) Mrad/s, 77, η 0 = 0.10(2) and ω Q2 = 51.8(3) Mrad/s, η 2 = 0.12(2), at room temperature, were identified in the temperature range from 250 to 530 K, and their (100) orientation in the lattice was determined. They are associated with electronic defects at the probe atoms.

Phase transitions in the ferroelectric perovskite BaTiO 3 were studied for 111 In-implanted poly-crystalline samples by measuring the electric field gradients by means of Perturbed Angular Correlation spectroscopy. The phase transitions between the orthorhombic ⇔ rhombohedral ⇔ tetragonal ⇔ cubic lattices were investigated in 2 -10 K steps, for increasing and decreasing temperatures, in order to determine their hysteresis. The transition parameters are compared with results from measurements of the spontaneous polarization, electric susceptibility and neutron scattering.

This paper discusses first-principles cluster investigations on the electronic structure of YBa 2 Cu 3 Ü 7 and La 2 CuO 4 performed at the University of Zürich. In particular, electric field gradients have been calculated at the copper sites and their dependence on applying uniaxial strain is studied. Magnetic hyperfine interactions have been evaluated from spin-polarized calculations, and the transfer of spin density from neighboring copper ions is explained. An interpretation is given of the nuclear quadrupole resonance spectra in doped La 2 CuO 4 .

Electric field gradients have been measured at substitutional lattice sites in ternary semiconductors using Perturbed γ -γ Angular Correlation spectroscopy (PAC). The experimental results for A I B III C 2 chalcopyrite structure compounds and • A II B 2 III C 4 IV defect chalcopyrites are compared with ab nitio calculations. The latter were carried out with the WIEN code that uses the Full Potential Linearized Augmented Plane Wave method within a density functional theory. The agreement between experiment and theory is in most cases very good. Furthermore, the anion displacements in AgGaX 2 -compounds (X: S, Se, Te) have been determined theoretically by determining the minimum of the total energy of the electrons in an elementary cell.

A non-zero electric field gradient tensor, detected by probes that occupy sites with cubic point group symmetry, was observed in many AB0 3 perovskite-type compounds. This breakdown of local cubic symmetry is commonly associated with the presence of oxygen vacancies around the probe. This effect in BaTi x Hf 1-x O 3 , with x = 0.7, 0.5, 0.3, 0.1, 0.05 and 0.01 is studied in this work. The cell parameters were obtained at laboratory temperature using XRD spectroscopy. The hyperfine parameters were measured at a 181 Ta probe in the B site using Perturbed Angular Correla-tions (PAC) spectroscopy as a function of both temperature and composition. As a common trend, a static asymmetric and distributed quadrupolar interaction, strongly dependent on composition has been observed. The results, together with those corresponding to 1 > x ≥ 0.75, are analyzed using the point-charge model in terms of polarized oxygen vacancies, different covalence of the Ti-0 and Hf-0 bonds with computational simulation for the lattice positions of cations and oxygen vacancies.

We report here first-principles determination of the electric-field gradient (EFG) tensor at the Cd im-purity located at cation sites in rutile TiO 2 . As far as we know, these represent the first ab initio calcu-lations at impurity sites with the FP-LAPW method in oxide systems. We used super-cells to simulate the diluted impurity in the crystal. The free-relaxation process performed in our study shows that the changes in distances of the oxygen nearest-neighbours to the impurity are not isotropic as was suppo-sed in a previous study within the muffin-tin approximation. Our prediction for the EFG component of major absolute value agrees well with the experiment and is opposite in sign and direction if isotropic relaxations are assumed. Our value of the asymmetry parameter η also compares very well with the ex-perimental value.

A comparison of the experimental and calculated nuclear quadrupole coupling constants of diatomic halogen, interhalogen, trihalide ions and complexes of pyridine is a good test of reliability of semiempirical and non-empirical theories as well as their usefulness in the interpretation of quadrupole coupling data from the qualitative and quantitative point of view. Therefore, a practical way of gaining insight into the bonding properties and electronic structure of such systems consists of combining semiempirical MO calculations with experimental data to derive reliable information. In the present work we report the results of ab initio studies of several iodine containing molecules and ions using the calculated NQCC as a test of the quality of the wavefunctions. To demonstrate the quality of our calculations, we compare the calculated halogen-halogen bond length and QCC with the corresponding experimental values for the compounds studied. With a few exceptions, the overall agreement with experiment is most satisfactory. The results on dipole moments and energy levels are also in a good agreement with the experimental data, however these quantities were measured only in the few cases.

The electron density and quadrupole coupling constants of molecules containing Sb are analysed. The NQCC for antimony, calculated using the extended basis 6-311G** are much lower than the experimental data, while the use of the small 3-21G* basis led to NQCC closer to the experimental ones.

Dynamic processes in solids exhibiting structurally incommensurate phases are briefly reviewed, and the application of NMR and NQR is discussed. The unique utility of these methods, - arising due to, on one hand, the microscopic resonant nature of the probe used and, on the other, the presence of periodic, though incommensurable, structure - , is brought out by presenting recent results in a prototype system (Rb 2 ZnCl 4 ) in the presence of randomly quenched disorder. In particular, the interesting new methodology of measuring, by analysing NQR spin echo modulation, ultra-slow diffusion like collective motions of ensembles of atoms in the presence of pinning effects due to disorder is illustrated with new results.

Some recent progress in solid state 47,49 Ti NMR is described and reviewed. The metallic-state work described covers metals such as hep titanium, TiB 2 , a number of intermetallics such as TiAl 2 and TiAl 3 · The inorganic work covers the various titanium oxide based materials including the TiO 2 polymorphs, anatase, rutile and brookite. The gel work covers the evolution of crystalline titania from gels formed by hydrolysis of titanium isopropoxide. Some complementary data from 17 O and 13 C NMR and powder X-ray diffraction is also included.

The deuteron NMR lineshapes in the tilted antiferroeelectric smectic liquid crystalline phases are evaluated for: i) the case of a discrete short pitch modulation P 1 = N • d where N = 2,3,4, ii) for a superposition of two modulations p 1 and p 0 where p 0 ≫p 1 as well as iii) for the case where in addition rapid molecular exchange between adjacent layers takes place due to translational diffusion. The results are compared with the experimental deuteron NMR spectra of α-CD 2 deuterated MHPOBC in the Sm C * A phase measured at different angles between the magnetic field direction and the normals to the smectic layers. The alternating tilt structure of the Sm C * A phase is confirmed

The negative thermal expansion in ZrW 2 O 8 was investigated on a microscopic scale by temperature dependent measurements of the electric field gradients at the nuclear probe 187 W(β - ) 187 Re using time differential perturbed angular correlation spectroscopy. Two distinct nuclear quadrupole interactions I V zz Re1 l= 18.92(4) • 10 21 V/m 2 , η Re1 = 0.0 and I V zz Re1 l = 4.55(2) • 10 21 V/m 2 , η Re1 = 0.053(3) were observed at 295 K, which are assigned to the two crystallographically distinct W0 4 tetrahedra of the room temperature structure. Ab initio calculations of electron densities and electric field gradients with 1:7 Re-impurities using the full potential linearized augmented plane wave package WIEN97 yield the electric field gradients V zz Re1 = 12.63 • 10 21 V/m 2 , η Re1 = 0.0 and V zz Re2 =4.90 • 10 21 V/m 2 , η Re2 =0.0. The observed temperature dependence of the nuclear quadrupole interactions agrees well with the structural phase transition at 428 K observed by neutron and x-ray diffraction. Our experiments corroborate the suggested mechanism of coupled librations of rigid ZrO 6 octahedra and WO 4 tetrahedra, which is an alternative description of transverse vibrations of oxygen atoms in Zr-O-W bonds, for the negative thermal expansion in ZrW 2 O 8

Long-range antiferromagnetic (AF) order in the doped spin ladder compound of Sr 2.5 Ca 11.5 Cu 24 O 41 is investigated by heat capacity, magnetization and Cu-NMR/NQR measurements. We suggest that this AF order is primarily responsible for the chain Cu moments

Ab initio Hartree-Fock molecular orbital calculations were applied to the crystalline imidazole and its derivatives in order to examine systematically the effect of possible N-H---N type hydrogen bond-ing on the nuclear quadrupole interaction parameters in these materials. The nitrogen quadrupole coupling constant (QCC) and the asymmetry parameter (η) of the electric field gradient (EFG) were found to depend strongly on the size of the molecular clusters, from single molecule, to dimer, trimer and to the infinite molecular chain, i.e., crystalline state, implying that the intermolecular N-H -N hydrogen bond affects significantly the electronic structure of imidazole molecule. A certain correla-tion between the QCC of 14 N and the N-H bond distance R was also found and interpreted on the basis of the molecular orbital theory. However, we found that the value of the calculated EFG at the hy-drogen position of the N-H group, or the corresponding QCC value of 2 H, increases drastically as R -3 when R is shorter than about 0.1 nm, due probably to the inapplicability of the Gaussian basis sets to the very short chemical bond as revealed in the actual imidazole derivatives. We suggested that the ob-served N-H distances in imidazole derivatives should be re-examined.

This paper emphasizes the fact that valuable information on the dynamics of high-T c super-conductors is concealed in that part of the spin-lattice relaxation which arises from quadrupolar interactions. We briefly discuss the problem how to disentangle magnetic and quadrupolar time dependent interactions if both are present, thus leading to multiexponential magnetization recovery laws. We then discuss two examples from our studies of the high-T c superconductor YBa 2 Cu 4 O 8 where the identification of a quadrupolar contribution to the spin-lattice relaxation has been es-sential to draw the relevant conclusions. One example is concerned with the charge fluctuations associated with an electronic crossover showing up in the oxygen relaxation. The second example is dealing with the separation of charge and spin excitations and the validity of the single-spin fluid model.

Time Differential Perturbed Angular Correlation (TDPAC) experiments using 111 In as probe were performed in order to determine the nature of In-sites in In-ferrierite (In/FER), before and after the incorporation of Pt through the hyperfine interaction between the nuclear quadrupole moment of the probe's nucleus and the extranuclear electric field gradient (EFG). Extended X-ray Absorption Fine Structure (EXAFS) measurements were performed to measure the absorbance above the Pt's L III absorption edge in the transmission mode in order to determine the local environment of Pt atoms in Pt/FER and In Pt/FER catalysts. TDPAC spectra of 111 In in In/FER and InPt/FER taken at 500°C in air indicated that there are no changes in the indium surroundings after the incorporation of Pt. Three sites were found for indium in both samples. Two of them correspond to the two sites of indium in ln 2 O 3 and the third one is attributed to In in exchange sites of the zeolites. Their population, quadrupole frequencies and asymmetry parameters didn't change with the incorporation of Pt. EXAFS experiments showed similar near neighborhood Pt-0 distances and Pt coordination number in Pt/FER and In Pt/FER samples. There is no evidence of the presence of any In-Pt bimetallic entity in In Pt/FER catalyst.

A reinvestigation of the nitrogen-14 NQR in thiourea has permitted to record, apparently for the first time, the resonances above 169 K. The first order and second order of the transitions at 169 K and 202 K, respectively, appear clearly from the temperature dependence of the resonance frequencies; the modulated structure, incommensurate or commensurate, is also seen on the 14 N NQR spectrum. This study is the first step of a new investigation of thiourea and its inclusion compounds.

Specific heat and 35 Cl nuclear quadrupole resonance (NQR) measurements of the channel thio-urea-hexachloroethane inclusion compound are presented. Experimental NQR data confirm the results of atom-atom potential calculations that the guest sublattice comprises two types of nearly commensurate finite molecular chains, having different structure and separated by domain walls. Specific heat measurements show phase transition near 96 K.

133 Cs NMR spin-lattice relaxation times(T 1 ) in crystalline Cs 2 CdI 4 were measured at 225 - 373 K. The critical exponent ( of T\ observed near the normal-incommensurate transition in the normal phase was determined to be 0.62 ± 0.03, in good agreement with the predicted value for three-dimensional XK-model. The frequency dependent T 1 in the incommensurate phase could be explained by the fluctuation of amplitudon and small gap phason.

Quantum chemistry methods -ab initio -have been used for the assignment of the 35 Cl-NQR resonance frequencies to particular chlorine atoms substituted in positions 4 and 6 of pyrimidine. The so-called solid state effect, that is the influence of intermolecular interactions, in particular hydrogen bond formation, on the NQR parameters has also been studied.

The structure of NASICON-type compounds, Na 1+x Sc x Ti 2-x (PO 4 ) 3 (O ≤ x ≤ 2), and the dynamics of Na + have been investigated by 23 Na NMR spectroscopy. It was found that the 23 Na 1D and 2D MQMAS spectra depend on the Na concentration, suggesting strongly that the Na + ions are distributed between two crystallographically nonequivalent sites, one is a special position with axial symmetry, and the other a position of low symmetry. The chemical exchange between these different sites in the crystal takes place at room temperature, which may cause the high Na ion conduction of this material

For the first time the excitation of a quadrupole spin echo by a sequence of radio frequency pulses with the filling frequencies ω Q and ω Q ± Δω Q is theoretically and experimentally considered, where ω Q is the resonance frequency of the raised transition and Δω Q the offset within the half-width of the NQR line. It is shown that in this case the amplitude of the observable signals does not depend on the offset size, and the echoes appear at times which depend on the intervals between pulses, on the ratio Δω Q /ω Q , and on the offset sign.

35 Cl NQR frequencies and spin-lattice relaxation times T 1Q were measured in p-ClC 6 H 4 CO 2 H(PCB A) and p-ClC 6 H 4 CO 2 D(PCBA-d 1 ) at 77-333 K. T 1Q in PCB A gave a shallow minimum of 8.0 ms at ca. 110 K, which could be explained by a double proton transfer mechanism in the carboxylic acid dimer referring to 1 H NMR data giving a T 1H minimum at almost the same temperature. PCBA-d 1 , showed temperature dependent NQR frequencies quite analogous to those in PCBA, whereas their T 1Q behaviour was quite different in its minimum value and its temperature as well as temperature gradient. These results were explained by suppressed deuteron tunnelling and the Ubbelohde effect.

Field-swept NMR spectra of 93 Nb are examined for a sample of BaNbS 2.96 which has a semicon-ductive temperature dependence of the resistivity at low temperature, and a sample of BaNbS 3.03 which has a metallic temperature dependence at low temperature. Both spectra are so-called NMR powder patterns perturbed by a quadrupole interaction with a quadrupole frequency υ Q = 3e 2 qQ/2I(2I - l)h of 2.5 [MHz]. No appreciable difference in the spectra has been observed between the two samples, suggesting that the amplitude of the conduction electron density is too small to perturb the electronic environments around the niobium atoms. The general behavior of the nuclear spin-lattice relaxation of 93 Nb in both compounds is consistent with an interpretation that the relaxation at low temperature is dominated by a Korringa mechanism and at higher temperature by a quadrupole interaction.

The static and dynamics properties of hydration water molecules in (±)-[Co(en) 3 ]Cl 3 were studied by means of 1 H, 2 H, and , 17 O solid state NMR. By 'H pulsed field gradient (PFG) NMR the apparent diffusion coefficient of mobile water through a micropore along the crystalline unique c-axis was found to be 1.0 x 10 -9 m 2 s -1 . The 2 H NMR spectrum at 141 K consists of two components, one being a Pake doublet corresponding to a quadrupole coupling constant (QCC) of (226 ± 2) kHz and an asymmetry parameter of the electric field gradient η of 0.08 ± 0.01, and another being a Gaussian line with a linewidth of 3.5 kHz. The 17 O NMR spectrum at 300 K also consists of a narrow Gaussian peak and a broad powder pattern with a second order quadrupole effect corresponding to QCC = (6.3 ± 0.5) MHz and η = 0.55±0.02. The broad and narrow components are assigned to water molecules accommodated at general 12g positions and special 2a and 2b positions in the trigonal lattice with space group P1c1. From the ratio of the populations at these positions their potential energy difference was estimated to be between (2.7 ±0.1) and (3.5 ± 0.1) kJmol -1 . The 2 H NMR spectrum at room temperature indicates a finite quadrupole interaction which is attributable to the rapid rotation of water molecule about the molecular C 2 -axis. When the water content exceeds 2.7, the QCC is reduced sharply to (5.0 ±0.1) kHz at 285 K, suggesting that there occurs rapid rotation of water and rapid exchange of 2 H between nonequivalent positions.