Anomalous temperature dependence of the 99Tc quadrupole coupling and disordering of cesium positions in CsTcO4

A 99Tc and 133Cs solid-state NMR study of polycrystalline CsTcO4 at a field of 7.04 T and temperatures from 90 to 430 K revealed first-order quadrupole effects. 99Tc quadrupole coupling constant, CQ, 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 qxx and qyy of the EFG-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. (orig.)