Anomalous Temperature Dependence of the 99Tc Quadrupole Coupling and Disordering of Cesium Positions in CsTcO4

Abstract

A “Tc and 133Cs solid-state NMR study of polycrystalline CsTcO4at a field of 7.04 T and temperatures from 90 to 430 K revealed first order quadrupole effects. 99Tc quadrupole coupling constant, c0, shows anomalous positive temperature coefficients and decreases from 2.0 MHz at 430 K to 0.45 MHz at 90 K. The asymmetry parameter, η (99Tc), is zero at 430 K and at 210 K. The high-temperature minimum of η is associated with a phase transition from orthorhombic to tetragonal and the second minimum reflects an orientational change of the principal axes qxxand qyyof the FFG-tensor. The 133Cs NMR spectra are characterised by a superposition of the two powder multiplets from the first order couplings which arise from magnetically and crystallographically nonequivalent cesium sites, Cs(1) and Cs(2); the population ratio of Cs (2) and Cs(1), p2/P1, varies from 2.0 at 120 K to 6.5 at 373 K. The nonequivalence and population changes are attributed to the temperature change of the crystal field potential in the vicinity of the cations. © 1992, Verlag der Zeitschrift für Naturforschung. All rights reserved.