Nuclear Quadrupole Relaxation Research Articles

Nuclear quadrupolar relaxation due to hydrogen diffusion in metal-hydrogen systems

Nuclear electric quadrupolar relaxation of deuterium or quadrupolar metal nuclei, due to diffusing hydrogen or deuterium, is estimated to dominate magnetic dipolar relaxation in metal-hydrogen systems. The longitudinal magnetization recoveries for quadrupolar relaxation can be linear combinations of several exponentials and expressions for the exponents depend on the spin I, whether a spin-temperature exists in the system and whether a static quadrupole interaction exists. Some limiting cases and examples are discussed.

Temperature dependence of the nuclear quadrupole coupling and relaxation time of 25Mg in Mg metal

The temperature dependences of the quadrupole coupling (e2qQ/h) and relaxation time (T1) for 25Mg in metallic magnesium have been studied in the range 150 K to 400 K. The Korringa product T1T=74(2) s K.

Nuclear quadrupolar relaxation due to hydrogen diffusion in metal-hydrogen systems

Nuclear electric quadrupolar relaxation of deuterium or quadrupolar metal nuclei, due to diffusing hydrogen or deuterium, is estimated to dominate magnetic dipolar relaxation in metal-hydrogen systems. The longitudinal magnetization recoveries for quadrupolar relaxation can be linear combinations of several exponentials and expressions for the exponents depend on the spin I, whether a spin-temperature exists in the system and whether a static quadrupole interaction exists. Some limiting cases and examples are discussed.

Superconductivity-induced softening of phonons in multilayered CuO 2 superconductors

The softening of phonons can be driven by the appearance of another superconducting state in a superconducting state. Such a transition of the second order is possible in multilayered CuO 2 superconductors. The soft mode must be an optical mode which vibrates CuO 2 planes out of phase. The softening can explain a tiny but sharp anomaly of the transverse nuclear quadrupole relaxation rate around T=35 K in double-layered YBa 2 Cu 3 O 7− δ with critical temperature T c≈92K. However, the present theory predicts the absence of anomalies in single-layered compounds such as La 2− x Sr x CuO 4.

Temperature Dependences of NQR Frequencies and Nuclear Quadrupole Relaxation Times of Chlorine in 2,6-Lutidinium Hexachlorotellurate (IV) as Studied by Pulsed NQR Techniques

Abstract The temperature dependences of the 35Cl NQR frequencies vQ and the nuclear quadrupole spin-lattice relaxation times T1Q in 2,6-lutidinium hexachlorotellurate (IV) was observed at various temperatures between 80 and 343 K. This crystal undergoes a phase transition at Tc = 229 K. A single and three pairs of 35Cl NQR frequencies were observed above and below Tc , respectively. The hysteresis of the phase transition and a discontinuity in the temperature dependence of the resonance frequencies at Tc indicate that this phase transition is of first order. Although the resonance frequencies of the pairs in the low temperature phase are very close to one another, T1Q and below Tc could be accurately determined by measuring the Fourier transform spectra of each line. Above ca. 250 K, T1Q showed an exponential decrease which is attributable to the overall reorientational motion of [TeCl6]2- with an activation energy of 82 kJ mol-1

A Study of Anonic Motions in Solid Rubidiumand Cesium Tetrachloroaurates(III)by Measuring the Temperature Dependence of Chlorine Nuclear Quadrupolar Relaxation Times

Abstract The temperature dependences of 35Cl NQR frequencies and 35Cl nuclear quadrupolar relaxation times T1Q and T2Q were determined above 82K for Rb[AuCl4] and CsfAuClJ up to ca. 340 and 320 K, respectively. Below these temperatures two 35Cl NQR frequencies were observed for each complex whereas no resonance could be detected above the respective temperatures. For both complexes, T1Q observed below ca. 270 K was interpreted as mainly determined by the rotary lattice vibration around the pseudo-C4(C4) axis and each Cl-Au-Cl diagonal axis of the square planar complex anion, whereas T1q observed above ca. 270 K was definitely shown to be determined through the reorientation of the anions about their C′4 axis. The activation energies for the C′4 reorientation of the anions was evaluated to be 71 and 53 kJ mol-1 for the rubidium and cesium salts, respectively. The anionic dynamics are discussed by assuming the motion of the anions within and out of the C4 potential wells at lower and higher temperatures, respectively.

Chlorine nuclear quadrupole resonances and motion of pyridinium ions in pyridinium tetrachloroaurate(III)

The temperature dependences of 35Cl NQR frequencies and of chlorine nuclear quadrupole relaxation times, T 1Q have been determined for pyridinium tetrachloroaurate(III). The 35Cl NQR frequency of a single line appearing is 27.769 MHz at 293 K. With decreasing temperature, the NQR line is observed to become gradually broad and cannot be detected below ca. 230 K. 35Cl T 1Q is observed to decrease with decreasing temperature from 800 μs at 350 K to 170 μs at 273 K. This fast relaxation rate may be responsible for the disappearance of the NQR signal described above. T 1Q data are discussed by referring to the 1H NMR T 1 data already determined. The dominant relaxation mechanism for T 1Q below ca. 350 K can be attributed to the fluctuation of the field gradient due to the motion of the pyridinium ions in the crystal. Above ca. 360 K, T 1Q decreases rapidly with increasing temperature suggesting the onset of a motion of [AuCl 4] − ions. The activation energy for the motion is estimated to be 67 kJ mol −1. When the Larmor frequency of 1H T 1 measurements is near to the NQR frequencies, anomalous temperature dependences of 1H T 1 are observed, suggesting the occurrence of dipolar—quadrupolar cross relaxation with chlorine nuclei.

Phase transitions and anionic dynamics of dimethylammonium hexachlorotellurate(IV) as studied by pulsed nuclear quadrupole resonance of chlorine

The temperature dependence of 35Cl NQR frequencies has been reinvestigated for dimethylammonium hexachlorotellurate(IV) using an FT NQR spectrometer. A new line is observed just above the lowest-frequency line already reported for the intermediate-temperature phase. In the lowest-temperature phase, the temperature dependence is definitely determined by finding new lines. The NQR frequencies can be precisely measured even in the vicinity of the phase transitions. The temperature dependence of the nuclear quadrupole relaxation time, T 1Q of 35Cl and 37Cl nuclei has also been observed. In a temperature range 64–160 K, the isotope ratio, T 1Q( 37Cl)/ T 1Q( 35Cl) is 1.5 for each line, suggesting that the quadrupolar relaxation arises mainly from the libration of the complex anion. Above 160 K, T 1Q decreases very rapidly with increasing temperature for each line. This can be interpreted in terms of the onset of the reorientation of the anion as a whole, which is responsible for the fade-out phenomenon of the two lines occurring near 220 K. The nature of the phase transitions and the anionic dynamics are discussed in detail. Especially, the phase transition at 96 K which is rather unusual and is explained through softening of the librations and the rotational displacement of the anions successively operated.

Nuclear quadrupole resonance and relaxation in incommensurate and commensurate ferroelectrics

Abstract The 35Cl, 14N nuclear quadrupole resonance and nuclear quadrupole double resonance spectra have been measured in a number of systems exhibiting structural, incommensurate and commensurate ferroelectric phase transitions. In this contribution we illustrate the power of NQR spectroscopy for the study of the incommensurate and commensurate ferroelectric phases as well as for the study of structural phase transitions in models of biological membranes and related systems. We specifically studied the sequence of structural phase transitions in the Octylammonium N-Octylammoniumchloride layer structure compound and the Paraelectric to Ferroelectric transition in (CH3NH3)HgCl3. The dynamical properties of the above transitions are reflected in the temperature dependence of the NQR frequencies as well as in the time anomalous behaviour of the nuclear quadrupole spin-lattice relaxation times. From the above experimental data one can extract detailed information about the microscopic nature of the above transition.

A Thermodynamic and Dynamic Model for Nuclear Quadrupolar Relaxation in Liquid Alloys*

Article A Thermodynamic and Dynamic Model for Nuclear Quadrupolar Relaxation in Liquid Alloys* was published on January 1, 1988 in the journal Zeitschrift für Physikalische Chemie (volume 156, issue 2).

Nuclear quadrupole relaxation of [formula omitted

Nonexponentiality of the nuclear quadrupole relaxation of spin- 3 2 was carefully examined for the nonextreme narrowing case. Two decay constants for 7Li longitudinal relaxation in 11 M aqueous solution (D 2O) of LiCI were determined. The ratio of two decay constants is interpreted on the basis of anisotropic reorientational motions of a hydration complex of lithium cation.

Temperature dependence of the structure of the antifluorite K2OsCl6: evidence for precursor tetragonal clusters near TC

Neutron-diffraction profiles were obtained from a powder sample of K2OsCl6. Measurements were taken at six temperatures above TC, the temperature of the cubic-to-tetragonal structural phase transition, and at one temperature below TC. Profile-refinement analysis yielded temperature-dependent Debye–Waller factors that behaved as expected from the elastic properties of a harmonic cubic crystal. Of intrinsic interest is the conclusion that the data contain the first neutron-diffraction evidence for the occurrence of precursor clusters. These long-lived fluctuations, which possess the symmetry of the tetragonal phase but which occur in the cubic phase, were first identified from chlorine nuclear quadrupole relaxation data in K2OsCl6.

81Br Nuclear Quadrupole Resonance of Cs2ZnBr4 and Crystal Stability of Compounds of A2MX4 Type

Abstract The temperature dependences of 81Br nuclear quadrupole resonance frequencies and quadrupolar relaxation times of Cs2ZnBr4 were measured. The librational motion of [ZnBr4]2− about the Zn–Br axis parallel to the a axis in the crystal is more active than those about the other axes. The reason why this compound shows no phase transition can be accounted for by the molecular packing, especially the packing of [ZnBr4]2− along the a axis. General examination of the relative size of cation/anion in the class of compounds of A2MX4 type led to a criterion which may be used to rationalize existence and absence of a phase transition.

Molecular Motion in GeCl4 as Studied by 35Cl Nuclear Quadrupole Resonance

Abstract Temperature dependence of 35Cl nuclear quadrupole resonance frequencies and quadrupole relaxation time was measured of GeCl4 in the metastable phase. Above 160 K, molecular reorientation is seen active; its activation energy is 63.5 kJ mol−1. This energy probably prevents GeCl4 from undergoing a rotational phase transition.

Nuclear Quadrupolar Relaxation m Liquid Alloys and Nearest Neighbour Dynamics

A theory of the quadrupolar relaxation rate, RQ , for ideal liquid binary metallic mixtures is presented. The theory predicts cancellation after a few tenths of a picosecond of the two and three particle correlation terms in the fluctuation of the electric field gradient (EFG) for pure metals, and prevailing of the three particle term for binary mixtures. This is due to the high symmetry in the arrangement of the atoms around a probe nucleus. The prevailing of the three particle term in binary systems leads to a longer correlation time (a few picoseconds) of the fluctuating EFG and explains the experimentally observed higher values of RQ and their quadratic concentration dependence. The validity of the theory has been confirmed by experimental data. Deviations for some real systems are discussed.

Structural Phase Transition and Molecular Motion of p-Chlorobenzamide as Studied by the Temperature Variations of 35Cl NQR Frequencies, Chlorine Nuclear Quadrupolar Relaxation Times, and 1H Magnetic Dipolar Relaxation Times

The temperature dependences of 35Cl NQR frequencies, 35Cl (37Cl) nuclear quadrupolar relaxation times (T1ClQ), 1H NMR absorption, and of 1H NMR relaxation times were observed for p-chlorobenzamide, p-ClC6H4CONH2 and its N-deuterated analog. The existence of three 35Cl NQR lines already reported was confirmed for the known low temperature phase below 316 K. A single sharp 35Cl NQR line was detected in the high temperature phase up to 447 K just below the melting point 451 K. At lower temperatures, three different 35Cl T1ClQ values attributable to the fluctuation of the electric field gradient due to rotational oscillation were observed. With increasing temperature, these T1ClQ decreased rapidly and yielded almost the same value immediately below Ttr suggesting the onset of a slow motion. The 1H T1, of the high temperature phase gave a T1, minim um and could be explained through the 180° flip motion of the phenyl ring with an activation energy o f 44 kJ mol-1.

Orientation Dependence of the Nuclear Quadrupole Spin-lattice Relaxation of 23Na in NaNO2

The nuclear quadrupole relaxation probability W1 of 23Na in a single crystal of NaNO2 has been studied by applying a selective 180° pulse to the centre line and monitoring the exponential approach to equilibrium of the satellites. The orientation dependence of W1 at 170 K is similar in form to that at 298 K, indicating that the lattice motions responsible for the relaxation are similar at both temperatures. Ratios of the M-tensor components obtained by fitting the W1 data have been compared with the results of various point charge calculations. It indicates that the relaxation is primarily caused by interaction with the four nearest oxygen atoms and confirms that the NO2 groups oscillate and reorient primarily about the c axis.

Anomaly of Nuclear Quadrupole Relaxation in Cu2O Prepared at Low Temperatures

The 63Cu and 65Cu NQR transitions in powder samples of cuprous oxide have been investigated from 77 to 500 K and at room temperature after annealing up to 1100 K Significant differences in T1 , NQR linewidth Δv, and their temperature dependences were found among the samples prepared in different ways. For C u20 samples obtained in low temperatures (below 380 K), the temperature dependence of T1 below 380 K is o f activation character with Ea = 0.07 eV. These results are interpreted in terms of an electron hopping mechanism. Thermal processing of these samples permits to obtain irreversible electronic state and then the spectroscopic parameters are the same as for the samples obtained in high temperatures (above 1320 K).

Nuclear quadrupole relaxation of atoms and ions in polar solvents

Abstract A simple continuum model for quadrupole relaxation rates of atoms and large ions in polar solvents is obtained by generalizing the recent Hynes-Wolynes model. Whereas the original model considers fluctuations in the electric field gradient at the quadrupolar nucleus to arise solely from rotational motions of solvent dipoles, the present model also allows for fluctuations due to translational solvent motions. It shows that when both translational and rotational motions of polar solvent molecules occur simultaneously the rate of spin-lattice relaxation is reduced relative to the case where relaxation is induced by rotational solvent motions alone. Predicted rates from an amended version of the original Hynes-Wolynes model and from the generalized model are compared with experimental rates for 131 Xe, 127 I − and 81 Br − in several solvents.

Electric field fluctuations and bond character in liquid alloys

Electric field fluctuations within a liquid alloy can be measured by nuclear quadrupolar spin relaxation rate R Q of noble gas atoms imbedded in the liquid. The method is demonstrated using Xe as a prove in liquid Te alloys, viz. TeAg, Ga, Ge, In, Sn, Tl. It is proposed to use R Q in order to derive a measure of ionic contribution to bonding. Combining the R Q data with conductivity data, and applying a simple model approach, we estimate the fraction of metallic-covalent-ionic bonds for each of the alloys.