STRUCTURAL INSTABILITY AND ANION DYNAMICS IN THE SYSTEM (RB1-XCSX)2ZNCL4 - A CL-35 NUCLEAR-QUADRUPOLE RESONANCE STUDY

The dynamics of the ZnCl4 anion in the paraelectric phases of (Rb]1-xC sx)2ZnCl4 with x = 0.005, 0.01, 0.03 and 0.05 are investigated through Cl-35 nuclear quadrupole frequency nu and spin-lattice relaxation tim e (T1Q) studies. The data, collected as a function of temperature for different lines in these compounds, are analysed to obtain the hinderi ng potential barrier heights and the relative mean square amplitudes f or librational motions parallel and perpendicular to the a-axis of the crystal structure. A comparison of these results shows that the prese nce of impurity affects only perpendicular motions and the correspondi ng potential becomes anharmonic, probably developing a double-well str ucture, as the concentration of impurity increases. As the paraelectri c-incommensurate transition temperature is suppressed by the presence of impurity, the above results suggest that the perpendicular motions have an important role to play in stabilizing the paraelectric phase i n this system.