NQR Investigation of Anion Dynamics in Rb2Zn(Cl1 - xBrx)4

Abstract

Abstract The structural stability of Rb2Zn(Cl1 - xBrx)4 (x = 1% and 3%) is investigated using the 35Cl quadrupole resonance frequency and spin lattice relaxation time (T1Q) in the paraelectric (PE) phase. The PE to incommensurate (IC) phase transition temperature T1, observed at 302 K in the pure compound, is lowered to 300 K for x = 1 % and to 293 K for x = 3%. These effects are smaller than in the earlier reported case of 3% Cs substitution in Rb2ZnCl4 , which reduces T1 by 25 K. While bigger cation substitution affects only the barrier for anion dynamics about directions perpendicular to the a-axis (direction of low temperature IC modulation wave), bigger anion substitution is found to affect the barrier for anion dynamics about the a-axis as well. The present study also indicates that the cusp like dip in T1Q observed while approaching T1 from above (characterizing the soft mode condensation associated with the structural phase transition) is essentially unaffected by anion impurity substitution, in contrast to the case where bigger cation substitution tends to smear the transitional effects. These results seem to suggest the dominant role played by cations in stabilizing the PE phase of these A2BX4 systems, in comparison to anions.