Showing papers on "Nuclear quadrupole resonance published in 1994"

Cu NMR and NQR Studies of High-Tc Superconductor La2-xSrxCuO4.

Abstract: Results of extensive Cu NMR and NQR studies in the superconducting La 2- x Sr x CuO 4 with x =0.075∼0.24 have been presented. The rapid decrease of the nuclear spin-lattice relaxation rate, 1/ T 1 , and the spin Knight shift below T c confirm microscopically the existence of the superconducting phase with lower T c than 38 K for 0.10 ≤ x ≤0.24. For x =0.10, magnetic anomalies emerge at low- T in some small parts of the sample, e.g . T -independent T 1 and broadening of spectrum, having some relevance to the magnetic order in La 2- x Ba x CuO 4 with x =0.125. For x >0.15, the T 1 T =const. behavior and the existence of the residual spin Knight shift far below T c have revealed that the superconductivity is of a gapless type with a finite density of states at the Fermi level caused by the mixture of different structures, i.e. tetragonal and orthorhombic.

Nuclear quadrupole resonance spectroscopy

Abstract: A method of detecting the presence of a given quadrupolar nucleus in a sample, the resonance frequency and spin-lattice relaxation time of the nucleus varying with a given environmental parameter, comprises determining a range of the resonance frequency which corresponds to a selected range of the parameter, applying excitation to the sample to excite nuclear quadrupole resonance, and detecting the resonance response signal, the excitation being such that a detectable resonance response signal can be excited over the resonance frequency range, and being such as to produce a constant flip angle of less than 45 degrees actual.

Narcotics and explosives detection by 14 N pure nuclear quadrupole resonance

Abstract: Pure nuclear quadrupole resonance (NQR) of 14N nuclei is quite promising as a method for detecting explosives such as RDX and contraband narcotics such as cocaine and heroin in quantities of interest. Pure NQR is conducted without an external applied magnetic field, so potential concerns about damage to magnetically encoded data or exposure of personnel to large magnetic fields are not relevant. Because NQR frequencies of different compounds are quite distinct, we do not encounter false alarms from the NQR signals of other benign materials. We have constructed a proof-of-concept NQR explosives detector which interrogates a volume of 300 liters (10 ft3). With minimal modification to the existing explosives detector, we can detect operationally relevant quantities of (free base) cocaine within the 300-liter inspection volume in 6 seconds. We are presently extending this approach to the detection of heroin base and also examining 14N and 35,37Cl pure NQR for detection of the hydrochloride forms of both materials. An adaptation of this NQR approach may be suitable for scanning personnel for externally carried contraband and explosives. We first outline the basics of the NQR approach, highlighting strengths and weaknesses, and then present representative results for RDX and cocaine detection. We also present a partial compendium of relevant NQR parameters measured for some materials of interest.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Apparatus for and method of nuclear resonance testing

Abstract: Apparatus for Nuclear Quadrupole Resonance testing a sample comprises: means (102) for applying excitation to the sample to excite NQR resonance, the excitation comprising at least one excitation pulse, the or each pulse covering a selected excitation frequency range, for the or each pulse the phase of the excitation varying generally non-linearly with the excitation frequency over the selected range; and means (106) for detecting the NQR response signal.

Successive Phase Transitions and High Ionic Conductivity of Trichlorogermanate (II) Salts as Studied by 35C1 NQR and Powder X-Ray Diffraction

Abstract: A series of trichlorogermanate (II) salts (AGeCl 3 , A = Rb, Cs, CH 3 NH 3 , and (CH 3 ) 4 N) have been synthesized and characterized by 35 Cl NQR, 35 Cl NMR, AC conductivity, DTA, and X-ray diffraction techniques. In the temperature range studied two, two, five, and four phases were confirmed for the Rb, Cs, CH 3 NH 3 , and (CH 3 ) 4 N salts, respectively. From the 35 Cl NQR and structural data, isolated pyramidal GeCl 3 − anions were recognized in the low temperature phases. With increasing temperature the relaxation times of the 35 Cl NQR decreased exponentially and the signals disappeared far below the melting point. This suggests that the reorientation of the anion about the pseudo three-fold axis is excited

63/65Cu/139La-NMR Study on Antiferromagnetic Ordering in High-Tc Oxides La2-xSrxCuO4(x〓0.115) and La2-xBaxCuO4(x〓0.125)

Abstract: La-based high- T C oxides La 2- x Ba x CuO 4 (LBCO) and La 2- x Sr x CuO 4 (LSCO) show a local minimum in the curve of T C versus x around 0.125 for LBCO and around 0.115 for LSCO. In order to investigate the origin of this local minimum, temperature dependence of 63/65 Cu/ 139 La-NMR/NQR spectra has been studied on spray-dry-made samples with various Ba/Sr-concentrations. Existence of the antiferromagnetic ordering was confirmed by a significant broadening in the resonance lines at low temperatures below 35 K for LBCO (\(x\cong 0.125\)) and 32 K for LSCO (\(x\cong 0.115\)). By the analysis on the anisotropy in the broadening, the direction of the ordered field was found to be within the CuO plane. The hyperfine field and the Neel temperature showed a maximum at the Ba/Sr-concentration around which T C is most suppressed, suggesting the crucial interplay between the antiferromagnetic ordering and the suppression in T C .

209Bi NMR and NQR investigation of the small-gap semiconductor Ce3Bi4Pt3.

Abstract: PHYSICAL REVIEW B 15 JUNE 1994-I VOLUME 49, NUMBER 23 Bi NMR and NQR investigation of the small-gap semiconductor Ce3Bi4pt3 A P Reyes, * R H Heffner, P C Canfield, J D Thompson, and Z Fisk Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 14 February 1994) Bi nuclear quadrupole resonance We report measurements of the temperature dependence of the frequency v&, the Knight shift K, and the spin-lattice relaxation rate 1/Tl in the small-gap semiconduc- tor Ce3Bi4Pt3 between 1 8 and 300 K Corresponding measurements also are reported for the nonmag- netic metallic isomorph La3Bi4Pt3 The v& data in the Ce compound show a characteristic departure behavior when the sample is cooled below TM =80 K, the temperature of the from metallic-to-insulating susceptibility maximum, attributable to a loss of low-frequency vibrational modes in the insulating state The Knight shift has both isotropic and axial components; this anisotropy originates from the presence of Ce via a transferred hyperfine coupling between Ce 4f and conduction electrons An s fexchan-ge 0 4 eV is found, consistent with hybridization in other rare-earth intermetallic compounds constant A change in the scaling between the susceptibility and both the isotropic and axial Knight shifts at tem- perature T~ provides evidence that hybridization between the Ce 4f orbitals and the conduction elec- trons is responsible for the gap structure The temperature dependence of the 1/Tl data is consistent with a model electronic density of states possessing a temperature-independent gap 5 of 180 K and a bandwidth of the order of 1600 K The temperature dependence of 1/Tl can also be fit well with a temperature-dependent gap with 5(0) also = 180 K I imum leads to an estimation INTRODUCTION for the Kondo temperature gap in Ce343, as mea- sured by resistivity, is temperature dependent, reaching a maximum of about 120 K below about 50 K By con- trast, recent far-infrared re6ectivity measurements report a charge gap of about 450 K, with a significant density of states below this energy Inelastic neutron scattering ex- periments yield a spin gap of about 150 K, and are some- what ambiguous regarding the existence of discernible states in the gap A small linear coefficient of specific 3 mJimol Ce K ) has also been measured heat y The nonmagnetic analog to Ce343 is LasBi~Pt3 (re- ferred to here as La343), which is metallic down to 4 K The substitution of La for Ce in Ce343 decreases the low-temperature resistivity and eventually destroys the energy gap The properties of both Ce343 and La343 are reviewed in Refs 1 and 3 Nuclear magnetic resonance probes the local spin den- sity and electronic and magnetic structure of materials and can therefore yield information on the microscopic nature of these hybridization-gap systems For example, measurements of the Knight shift have an inherent ad- vantage over susceptibility measurements because impur- ities and other phases generally do not affect the Knight shift The NMR spin-lattice relaxation rate is sensitive to the excitation of quasiparticles across the gap structure, yielding information about the spin gap Finally, nuclear quadrupole resonance can be used to probe the charge distribution surrounding the nucleus through a measure of the local electric field gradient (EFG) In Curie-Weiss paramagnets one can in principle ob- serve NMR at either the magnetic- or nonmagnetic-ion site However, the NMR at the site of the magnetic ion can be rendered unobservable by the rapid relaxation of the paramagnetic ion itself When NMR is carried out at Tz of about 320 The compound Ce3Bi4Pt3 (hereafter referred to as Ce343) is one of about a dozen small-gap semiconducting compounds involving either rare-earth or actinide met- als ' Most of these semiconductors possess a cubic struc- ture, and the behavior of their lattice parameters suggests a mixed-valence state for the electrons The dc suscep- f tibility typically increases with temperature, passes through a maximum, and then falls again, exhibiting local-moment behavior at high temperatures with nearly the full local-moment value This behavior is characteris- tic of dense Kondo systems, where the reduction in the susceptibility at low temperatures is produced by the hy- bridization of the local moments with the conduction electrons In some materials, this hybridization is as- sumed to produce an energy gap, which is usually less than 1000 K ' The conditions for gapping the entire Fer- mi surface, just two electrons per site and the intersec- tion of the level with only one conduction band at the Fermi surface, are most easily met in cubic materials Several issues are of current interest regarding these small-gap semiconductors First, although the gap has been assumed to arise from hybridization, it is important to find good experimental evidence for this hypothesis Second, the magnitudes of both the charge (5, ) and spin (5, ) gaps need to be established For example, strong- coupling models predict that the ratio of 6, to 5, is at least 15 Finally, the temperature dependence of the gap should be investigated, as well as whether or not there are intrinsic states within the gap In this paper we present nuclear magnetic and nuclear quadrupole resonance (NMR and NQR) experiments on Ce343, a cubic small-gap semiconductor with a suscepti- bility maximum near 80 K This susceptibility max- f f K The transport 1994 The American Physical Society

Abrupt but continuous antiferromagnetic transition in nearly stoichiometric La2CuO4+ delta.

Abstract: Zero-field La139 nuclear quadrupole resonance (NQR) has been used to characterize the antiferromagnetic transition in nearly stoichiometric single crystals of La2CuO4+ (Neel temperatures TN between 275 and 318 K). The onset of the NQR Zeeman splitting at TN is abrupt but continuous, indicative of a second-order phase transition with either a crossover in critical behavior just below TN or a very small critical exponent 0.1. The absence of any anomaly at TN in the La139 electric field gradient indicates no accompanying structural distortion. © 1994 The American Physical Society.

Effect of Deuteration on the Phase Transitions and on the Critical Dynamics in Ammonium Hexachlorometallates

Abstract: Recent and novel data obtained from chlorine NQR measurements on natural and deuterated (NH 4 ) 2 MCl 6 compounds are discussed with special regard to the influence of the ammonium-ion dynamics on the structural stability of these crystals. The temperature dependence (4.2 K to 350 K) of the chlorine NQR frequency ν Q and relaxation rates T 1 −1 , T 2 −1 obtained from the natural ammonium salts of Sn, Pd, Os, Pb, Te, Se and from the deuterated salts of Sn, Te and Se are analysed. Slight deviations from the normal temperature behaviour of ν Q and T 1 −1 are found in Sn, Pd and Os compounds which stay cubic in the whole temperature range. The ammonium compounds of Pb and Te undergo a structural transformation between 80 K and 90 K from the cubic to a trigonal phase which is distinguished by the preservation of the single line spectrum of the chlorine NQR below T c1

Explosives detection by nuclear quadrupole resonance (NQR)

Abstract: Pure nuclear quadrupole resonance (NQR) of 14N nuclei is quite promising as a method for detecting explosives such as RDX and contraband narcotics such as cocaine and heroin in quantities of interest. Pure NQR is conducted without an external applied magnetic field, so potential concerns about damage to magnetically encoded data or exposure of personnel to large magnetic fields are not relevant. Because NQR frequencies of different compounds are quite distinct, we do not encounter false alarms from the NQR signals of other benign materials. We have constructed a laboratory prototype NQR explosives detector which interrogates a volume of 300 liters (10 ft3). This paper presents abbreviated results from a demonstration of the laboratory prototype NQR explosives detector conducted at the Federal Aviation Administration Technical Center in May 1994 on RDX-based explosives.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Cu Nuclear Quadrupole Resonance Study of CuGeO3.

Abstract: We have observed the nuclear quadrupole resonance (NQR) of Cu in the spin-Peierls system CuGeO 3 . The NQR signals have been observed at 34.23±0.02 MHz for 63 Cu and 31.66±0.02 MHz for 65 Cu at 4.2 K under zero external field. These signals persist down to 1.3 K with neither line splitting nor broadening due to the internal magnetic field. This fact indicates there is no long-range order of Cu spins. The 63 Cu nuclear spin-lattice relaxation rate 63 (1/ T 1 ) rapidly drops below 15 K which clearly demonstrates an opening of the energy gap in the excitation spectrum. However, no anomaly was detected in the Cu NQR frequency at the transition temperature. It was also found that 63 (1/ T 1 ) above 15 K smoothly extrapolates to a finite value at 0 K. This may suggest the spin dynamics in the paramagnetic state is described in terms of the one-dimensional antiferromagnetic Heisenberg model with S =1/2.

Materials characterisation by nuclear quadrupole interaction

Abstract: The power of solid state NMR to characterise solids by determining the nuclear quadrupole coupling of the constituent nuclei is demonstrated for a number of compounds of current interest in a materials science laboratory. Recent results are presented for the nuclei 17 O, 23 Na, 27 Al, 39 K, 71 Ga, 91 Zr, 93 Nb, and 139 La. These were derived using a variety of FT NMR techniques including static, magic angle spinning and frequency stepped spin echo NMR spectroscopy

NMR and NQR study of the electronic and structural properties of Al-Cu-Fe and Al-Cu-Ru quasicrystals

Abstract: [sup 27]Al and [sup 63,65]Cu NMR is reported for powdered stable Al-Cu-Fe and Al-Cu-Ru icosahedral quasicrystals and crystalline approximants, and for an Al-Pd-Mn single-grain quasicrystal. [sup 27]Al NQR spectra at 4.2 K were observed in Al-Cu-Fe and Al-Cu-Ru samples. From quadrupole-perturbed NMR spectra at different magnetic fields, and from zero-field NQR spectra, a wide distribution of local electric-field gradient (EFG) tensor components and principal-axis-system orientations was found at the Al site. A model EFG calculation based on a 1/1 Al-Cu-Fe approximant successfully explained the observed NQR spectra. The average local gradient is largely determined by the [ital p]-electron wave function at the Al site, while the width of the distribution is due to EFG lattice contribution. Comparison of [sup 63]Cu and [sup 27]Al NMR shows the EFG distribution at the two sites is similar, but the electronic contribution to the EFG is considerably smaller at the Cu site, in agreement with a more [ital s]-type wave function of the conduction electrons. Overall spread of EFG values is well reproduced by calculation based on the approximant. However, the experimental spectra indicate a much larger number of nonequivalent sites when compared with the simulated NQR spectra based on the 1/1 approximant. Themore » short-range, local chemical order is well represented by the approximant, but differences in coordination must be included at intermediate range in the quasicrystal. Measured [sup 27]Al Knight shift, magnetic susceptibility, and nuclear spin-lattice relaxation time as a function of temperature indicate reduced density of states at the Fermi level by a factor of 7 or 8 from the value in Al metal, consistent with the notion of a pseudogap for these quasicrystals. No differences in measured parameters were detected as a function of composition of the quasicrystalline alloys.« less

139La NQR Study of the Magnetic Properties of La2-xMxCuO4 (M=Ba, Sr) for 0 ≤x ≤0.08

Abstract: The magnetic properties of La 2- x M x CuO 4 (M=Ba, Sr) for 0 ≤ x ≤0.08 have been investigated from 1.5 K to 300 K by the nuclear quadrupole resonance (NQR) of 139 La ( I =7/2). The electrical quadrupole relaxation process of 139 La in La 2 CuO 4 is dominant above 50 K in spite of the existence of the internal field at the La site due to the antiferromagnetic ordering of Cu-moments. At low temperature, the magnetic relaxation process becomes dominant. From the divergence of the nuclear relaxation rate, T 1 -1 , of 139 La, we obtain the magnetic phase diagram which consists of two distinct magnetic phases for 0 < x < 0.05. The internal field at the La-site changes discontinuously at T C * at which the temperature dependence of T 1 -1 shows a peak. This change can be interpreted by decrease of the transferred hyperfine fields from Cu spins through the apical oxygen of the CuO 6 octahedron below T C * .

Observation of Magnetic Ordering in La 2- xSr xCuO 4 around x=0.115–La-NQR Study–

Abstract: The nuclear quadrupole resonance (NQR) study of 139 La in La 2- x Sr x CuO 4 was systematically carried out in the Sr-content range from 0.085 to 0.15 at 1.4 and 4.2 K in zero field. The linewidth of the 139 La NQR spectrum for the (±1/2→±3/2) transition was found to increase especially at x =0.115, associated with the occurrence of magnetic ordering. This is hence considered to be responsible for the decrease of the superconducting transition temperature around x =0.115. Since the spectrum does not broaden with for the (±5/2→±7/2) transition down to 1.4 K, the ordered Cu-moments are considered to exist in the Cu-O 2 sheets.

Application of multipulse sequences in remote NQR

Abstract: We describe NQR devices for detecting plastic explosives, mines and drugs.

Composite pulses in nuclear quadrupole resonance

Abstract: It is shown in this paper that composite pulses optimized for NMR situations can be used also for NQR experiments. Specifically identical composite pulses can be used in both cases and are most effective for single crystal NQR experiments. A number of composite pulses used for NMR are calculated exactly for the NQR case and shown to be as good as or better than existing pulses designed exclusively for NQR.

Nuclear magnetic resonance in the antiferromagnetic heavy-fermion system UCu5: spin reorientation at 1 K

Abstract: The magnetic properties of UCu5 with the C15b-type cubic crystal structure, which shows an antiferromagnetic transition at 15 K and, further, an unidentified transition at 1 K, were investigated by using the nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) techniques. The measurements were performed for both crystallographically inequivalent Cu sites with cubic and trigonal symmetry in all the paramagnetic, antiferromagnetic and unidentified states. The results clearly indicate that the low-temperature state below I K is another antiferromagnetic state with a different spin structure from the high-temperature state. From the temperature variation of the field-swept spectrum, we propose a non-collinear quadruple-q (4-q) structure for the state between 15 and 1 K, which is different from the structure previously proposed by Murasik et al based on their neutron diffraction result. Furthermore, we propose a collinear single-q (1-q) structure for the state below 1 K, which cannot be distinguished from the 4-q structure by neutron diffraction. Assuming a simple model for hyperfine interactions and the magnetic structures, we consistently explain the complicated zero-field spectra obtained both above and below 1 K. The analysis shows clearly the spin reorientation at 1 K and that the variation of the local field at Cu sites is reasonably interpreted in terms of only the spin reorientation. The analysis also indicates that the electric field gradient changes significantly at 1 K, which is in contrast to no change at 15 K. The relation between the spin reorientation and the gap formation at 1 K is discussed.

Bromine NQR and Crystal Structures of Tetraanilinium Decabromotricadmate and 4-Methylpyridinium Tribromocadmate

Abstract: The 79,81 Br NQR spectra of tetraanilinium decabromotricadmate (1) and 4-methylpyridinium tribromocadmate (2) were studied as function of temperature and their crystal structures were determined. (C 6 H 5 NH 3 ) 4 Cd 3 Br 10 (1): Space group D 2h 15 − Pbca, Z = 4, a = 2507.8(7) pm, b = 1995.4(5) pm, c = 763.0(2) pm. Characteristic for the structure are trioctahedral units [Cd 3 Br 10 ] condensed to planes. Within the units the octahedra are face connected and further condensed to planes via common corners. Two types of hydrogen bonds are observed. The 81 Br NQR lines with frequencies (MHz, 298 K) of 62.98, 52.59, 43.39, 41.82, and 40.71 are little temperature dependent with positive and negative coefficients. The wide frequency range of the NQR lines is reflected by the wide range of the intraionic distances, 263 ≤ d(Br − Cd)/pm ≤ 300

Structural properties of CaBaLaCu3Oy studied by NMR/NQR method

Abstract: The first NMR/NQR study of the high- T c superconductor CaBaLaCu 3 O y has been carried out on a sample enriched with 43 Ca and 135 Ba which allowed one to observe signals from all cations. Direct evidence was obtained that the superconducting compound contains less Ca than the starting composition and a large part of Ca and Cu forms a spurious dielectric phase. The composition of the superconducting phase is close to (Ca 0.5 La 0.5 )(Ba 1.25 La 0.75 )Cu 3 O y as was proposed for the first time by Keller-Belest et al. A highly asymmetric electric-field gradient observed at the Ba and La sites in the Ba/La plane indicates that the local environment of these ions is not tetragonal in spite of the overall tetragonal crystal symmetry. A symmetric field gradient, however, is found for the cations in the central Ca/La plane. No evidence for cation ordering or a special superstructure of the oxygen could be detected.

Distribution of nonequivalent aluminum sites revealed in Al-Cu-Ru and Al-Cu-Fe quasicrystals by 27Al NQR.

Abstract: The distribution of nonequivalent aluminum sites was studied in Al-Cu-Fe and Al-Cu-Ru stable icosahedral quasicrystalline phases using [sup 27]Al NQR spectra taken at 4.2 K. The observed spectra---which give directly the distribution of nonequivalent aluminum sites---were broad, asymmetric, and structureless. A simple electric-field-gradient model calculation accounted for the spectral width in terms of a wide distribution of local atomic environments, and an estimate for the lower limit on the number of nonequivalent aluminum sites was found.

27Al-NMR and NQR Studies in the antiferromagnetic heavy fermion CePd2Al3

Abstract: Systematic measurements of the NQR spectrum and the nuclear-spin-lattice relaxation rate, 1/ T 1 , of 27 Al in the heavy-fermion antiferromagnetic (AF) compound CePd 2 Al 3 have revealed that the magnetic nature dramatically changes from showing long-range and inhomogeneous AF ordering for annealed and as-cast polycrystals, respectively, to a disappearance of any type of magnetic ordering for single crystals. A close relationship between the magnetic ordering and the structural disorder in the Al layers isolated from the Ce–Pd layers is demonstrated. The exchange coupling via the Al layers along the c -axis, which leads to the AF ground state, critically depends on the occupation and distribution of Al atoms as proposed by Mentink et al . It is pointed out that the randomness remaining inevitably in the Al layers masks the intrinsic magnetic excitation in the AF ordered state.

35Cl nuclear quadrupole resonance study of anion dynamics in the paraelectric phases of Cs2ZnCl4 and Rb2ZnCl4

Abstract: The dynamics of the ZnCl4 anion in two compounds, one exhibiting a structural transition to an incommensurate phase (Rb2ZnCl4) and another not having any structural transition at all (Cs2ZnCl4), are studied through 35Cl NQR frequency ( nu ) and relaxation time (T1Q) measurements as a function of temperature T in their high-temperature (paraelectric) phases. Estimates of the potential barriers hindering the reorientational motions of ZnCl4 anions along axes parallel and perpendicular to the a axis of the crystal structure are obtained from the T1Q data. These results are analysed in conjunction with a qualitative comparison of relative mean square amplitudes of such motions obtainable from frequency data, in order to investigate the differences in the dynamics and structure of the ZnCl4 anion. There seems to be correlation between the dynamics of the anion in this phase and the possibility that the compound may exhibit a structural instability at low temperatures in this family.

Influence of the synthesis conditions and carbonate content on the properties of Y2Ba4Cu7O14+x

Abstract: A study of the thermodynamic, magnetic, structural and physical properties of the Y 2 Ba 4 Cu 7 O 14+ x (247) phase is presented in this paper. The stability range of the 247 phase in the P – T phase diagram forms a narrow band between that of the 124 and 123 phases. The critical temperature T c of fully oxygenated ceramic samples varies from 50 to 93 K as the content of the carbonate changes from 6000 to 500 ppm. Susceptibility measurements performed on pure sintered samples with low CO 2 content show a T c onset at 93 K with a sharp transition. At a low magnetic field a shoulder caused by weak links has been observed. NQR measurements of ceramic samples and single-crystal X-ray investigations proved a high structural perfection of the samples with T c #62;90 K can clearly showed the existence of many defects in the materials with lower T c . The increase of T c from 4 to 90 K in the 247 crystals is coupled with a decrease of the apical bond length.

NMR-NQR Studies of High-Temperature Superconductors

Abstract: Nuclear Magnetic Resonance (NMR) played an important role in understanding “classical” superconductors such as aluminum [1] when the Bardeen-Cooper-Schrieffer (BCS) theory provided the first microscopic explanation of this extraordinary state of matter. Thus it was logical that soon after the discovery of the new high-temperature superconductors (HTSC) by G. Bednorz ad K. A. Muller [2], the NMR and nuclear quadrupole resonance (NQR) community devoted much activity to these new materials. The first results which were reported by mid-1987 [3–6] dealt with many of the important parameters which can be studied by NMR/NQR such as electric field gradient, Knight shift and relaxation. In the last few years the activity has steadily expanded.

Structures and librational motions of p -dichlorobenzene in Werner clathrates

Abstract: Two structures of the Ni(SCN)2(4-EtPy)4 Werner clathrate (4-EtPy=4-ethylpyridine) and one structure of Ni(SCN)2(4-AcPy)4 clathrate (4-AcPy=4-acetylpyridine) withp-dichlorobenzene (PDCB) as a guest molecule have been elucidated. The host molecules have the usual octahedral “propeller” conformations and form channel-type cavities. Effective librational frequencies of the PDCB molecule in these clathrates, and in 14 other Werner clathrates, have been determined from the temperature dependence of the35Cl nuclear quadrupole resonance (NQR) frequencies.