Crystallographic, elastic, and Raman scattering investigations of structural phase transitions in RbCdF3and TlCdF3

Abstract

A simple model of force constants allowed us to forecast that structural phase transitions due to $M{\mathrm{F}}_6$ octahedra rotations may occur in the fluoperovskites $\mathrm{AM}{\mathrm{F}}_3$ containing $M$ ions large enough to determine the dimensions of the cell and leave the $A$ ions free to move from the octahedra. Thus we focused our attention on $A\mathrm{Cd}{\mathrm{F}}_3$ compounds. We successfully prepared large single crystals of RbCd${\mathrm{F}}_3$, TlCd${\mathrm{F}}_3$, and CsCd${\mathrm{F}}_3$ by the Bridgman-Stockbarger method. These crystals have the cubic perovskite structure at room temperature. The x-ray experiments show that a tetragonal distortion with $\frac{c}{a}>1\mathrm{}$ occurs at 124 K in RbCd${\mathrm{F}}_3$ and 191 K in TlCd${\mathrm{F}}_3$. For CsCd${\mathrm{F}}_3$, no distortion was observed between 4 and 300 K. The occurence of a phase transition in RbCd${\mathrm{F}}_3$ and TlCd${\mathrm{F}}_3$ has been confirmed by the study of elastic wave velocities as a function of temperature. The anomalous decreasing of the elastic constant $C_{44}$ with decreasing temperature is consistent with slight rotations of Cd${\mathrm{F}}_6$ octahedra. The study of both superstructure reflexions and Raman-scattering spectrum allows us to retain the space group $D_{4h}^{ 18}$ for the low-temperature phase. Two low-frequency modes show a behavior of soft modes very similar to that already observed in SrTi${\mathrm{O}}_3$ and KMn${\mathrm{F}}_3$.