Dynamics of structural phase transitions in (CH3NH3)CdCl42-type compounds

Abstract

The dynamical properties of a model describing the structural phase transitions in (C${\mathrm{H}}_3$N${\mathrm{H}}_3$)${}_2{}^{}{}_{}{}^{}\mathrm{CdCl}_4^{}{}_{}{}^{}$ perovskite-type layer compounds as orientional order-disorder transitions of the C${\mathrm{H}}_3$N${\mathrm{H}}_3$ groups, are investigated. It is shown that the soft order-parameter-fluctuation modes observed in the low-frequency Raman spectra of (C${\mathrm{H}}_3$N${\mathrm{H}}_3$)${}_2{}^{}{}_{}{}^{}\mathrm{CdCl}_4^{}{}_{}{}^{}$ and (C${\mathrm{H}}_3$N${\mathrm{H}}_3$)${}_2{}^{}{}_{}{}^{}\mathrm{MnCl}_4^{}{}_{}{}^{}$, can be understood on the basis of the same model used to explain the static properties of these systems. The nonlinear coupling of the motion of the C${\mathrm{H}}_3$N${\mathrm{H}}_3$ groups among four equilibrium sites with the Cd${\mathrm{Cl}}_4$ octahedral matrix is not essential in the high-temperature phases, but is necessary to describe the monoclinic ground state. The planewise antiphase motion of the C${\mathrm{H}}_3$N${\mathrm{H}}_3$ groups is included in the model to explain the observed ${{\Gamma }_2}^{+}$ mode of the orthorhombic room-temperature phase and the ${{\Gamma }_4}^{+}$ mode of the tetragonal low-temperature phase. If the interlayer coupling is assumed to be much weaker than the intralayer one, similar fluctuation frequencies are obtained for the planewise antiphase modes as for the "planewise in phase" modes corresponding to the condensing order-parameter modes of the four-site model.