ESR OF AS0 2 1- IN CALCITE 439 The hyperfine structure separations in the ESR spectrum of the As0 2 2-molecule-ion was examined as a function of temperature. Upon warming from 4.2° to 4000K the separation!? are observed to a decrease by approximately 10%, see Fig. 3. Measurements above 4OO 0 K were not attempted due to the onset of thermal decay of the defect center. Unlike the behavior of the CO 2 -molecule-ion in ca1cite,23 no evidence was un-covered in this ESR spectrum to suggest reorientation of the As0 2 2-molecule-ion in this temperature range. No attempt has been made to interpret this hyperfine structure temperature dependence. 23 J. A. McMillan and S. A. Marshall, J. Chem. Phys. 48, 467 (1968). THE JOURNAL OF CHEMICAL PHYSICS ACKNOWLEDGMENTSThe authors would like to :express their appreciation to E. Olsen of the Field Museum of Natural History for having made available calcite single crystals and to G. A. Johnson for his valuable assistance.A deuteron magnetic resonance study is made of single crystals of CuSO,·5D20 between 130° and 360 o K. The electric quadrupole coupling constant, the asymmetry parameter, and the directions of the principal axes of the field-gradient tensor are determined for each deuteron at 133.2° and 294.8°K. At low temperatures ten pairs of lines are observed, corresponding to five nonequivalent stationary water molecules. At temperatures above room temperature, five pairs of lines are observed; these are shown to result from fast molecular reorientation about the bisecting axis of each water molecule. An activation energy for the 180° reorientational motion is derived for each water molecule and is discussed in relation to the lone. pair coordination of the oxygen atom of the water molecule and the hydrogen-bond strength. The fieldgradient tensor for each stationary water molecule is discussed in relation to the structure of the hydrogenbond system. For relatively weak asymmetric hydrogen bonds of O-H·· ·0, an empirical equation, eQq/h= 31O.0-3.0XI90.6/R(0·· ·H)3 (kHz), is found to represent the observed values well, where R(O·· ·H) is the O···H distance in angstrom units. For strong hydrogen bonds, there is a significant deviation from the above relation, indicating that the covalent effect is of importance in these cases. Coordination to a paramagnetic ion has no marked effect on the quadrupole coupling constant, but it appears to cause a rotation of the directions of principal x and y axes about the O-H bond. Two possible cases of the principal y-axes directions for the field-gradient tensor of the reorienting water molecule are analyzed in detail, and the available experimental data are classified into two groups. Theoretical formulas for the DMR spectra in the paramagnetic crystals are derived and compared with that in diamagnetic crystals. A local field due to the cupric ions is derived from the observed paramagnetic shift at 133.2°K for each deuteron and is found to be due to the electron nuclear spin dipole coupling. The spectra are observed at temperatures up to ...
This paper presents the frequency dependance of the proton spin-lattice relaxation time T1 at several temperatures and pressures in TTF-TCNQ(D4) and TTF(D4)-TCNQ. It is shown that only backward (q = 2 kF) and forward (q = 0) scatterings contribute to the nuclear relaxation induced by the modulation of the hyperfine field in these one-dimensional conductors. At medium fields, H 0 ~ 30 kOe, the frequency dependence of T1 originates from the diffuse character of the spin density wave excitations around q = 0, leading to T1-1 αH0- 1/2 . The enhancement of T1 -1, is at low fields, limited by the existence of a finite interchain coupling (tunnelling type). We find, within a RPA analysis, close correlations between the pressure and temperature dependences of the spin excitations diffusion constant and the collision time derived from the longitudinal conductivity. The interpretation of the NMR data in terms of a Hubbard model excludes both big U and small U pictures. However, we point out the importance of the electron-electron interactions on the relaxation rate of TTF-TCNQ. We derive a ratio U/4 t II ~ 0.9 for the TCNQ chain. We also assume that besides charge density waves fluctuations existing between 300 K and the phase transition at 53 K, electron-electron interactions make an important contribution to the temperature dependence of the spin susceptibility. Finally, we give a unified description of quasi one dimensional conductors in which the various systems are classified according to the transverse tunnelling coupling and the electron lifetime. It follows from this description that for TTF-TCNQ and its derivatives, transverse couplings (tunnelling and Coulomb) are large enough to justify the use of a mean-field theory
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