133Cs NMR Spin-Lattice Relaxation Time and Chemical Shift Studies on Cs2MX4 Crystals with Sr2GeS4 and β-K2SO4 Structures Performing no Low Temperature Phase Transition

Abstract

133Cs NMR spectra and spin-lattice relaxation times T1 in crystalline Cs2ZnCl4 and Cs2ZnBr4 with -K2SO4 structure, and Cs2CdI4 and Cs2HgI4 with Sr2GeS4 structure, which have no phase transition in the lower temperature regions, weremeasured to clarify the relation between the interionic covalency and crystal structures. Two central lines corresponding to two crystallographically inequivalent Cs sites were observed in all compounds. One of the two peaks in β-K2SO4-type compounds appears below 40 ppm, but another peak in those compounts and the both lines in Sr2GeS4-type compounds show larger shifts, 130 - 200 ppm. The temperature dependences of T1 observed in Sr2GeS4-type compounds were close to the theoretical behaviour, calculated by considering contributions from normal lattice vibrations. Deviations from the calculation obtained for β-K2SO4-type systems are attributable to the difference in interionic interactions, i. e., partial covalency, in the crystals