THE INFLUENCE OF IMPURITY CATION SIZE ON THE STRUCTURAL STABILITY OF (RB(1-X)M(X))(2)ZNCL4 (X=0.03 M=NA, K, NH4 RB AND CS) - A CL-35 NQR STUDY

The influence of the size of the cation impurity (M) on the dynamics o f the ZnCl4 anion and on the structural stability of the (Rb(1-x)M(x)) (2)ZnCl4 system (M = Na, K, NH4 and Cs) is investigated through Cl-35 NQR frequency and relaxation studies as a function of temperature. The mean square amplitude of torsional motions of ZnCl4 tetrahedra about an axis parallel to the a-axis of the crystal structure and about axes perpendicular to this direction, as well as the corresponding activat ion energies, are estimated from the experimental data. The results sh ow that, while parallel motions are insensitive to the presence of imp urity, the perpendicular motions develop an anharmonic potential when the impurity cation is larger than the host cation, which also suppres ses the structural phase transition temperature. Smaller impurity cati ons, on the other hand, have minimal effect on the anion dynamics and on the phase transition temperature.