Raman studies of a lambda -type phase transition in solid Cs2CdCl4.

Abstract

An unusual second-order-like phase transition in the perovskite-type layered compound ${\mathrm{Cs}}_{2}$${\mathrm{CdCl}}_{4}$ has been observed by Raman studies of the solid. The phase transition took place over a range of temperatures from 435 \ifmmode^\circ\else\textdegree\fi{}C to about 460 \ifmmode^\circ\else\textdegree\fi{}C. At any temperature within this temperature range Raman bands characteristic of the different cadmium coordination in each of the separate phases were detected simultaneously with relative intensities that were dependent on the temperature. Analysis of the Raman spectra indicated that the structure of ${\mathrm{Cs}}_{2}$${\mathrm{CdCl}}_{4}$ transformed from the ${\mathrm{K}}_{2}$${\mathrm{NiF}}_{4}$-type layered-perovskite-like low-temperature phase II (octahedral cadmium coordination) to the ${\mathrm{K}}_{2}$${\mathrm{SO}}_{4}$-type-structure high-temperature phase I (tetrahedral cadmium coordination). Relative-intensity measurements of bands characteristic of each cadmium coordination as a function of temperature indicated an equilibrium distribution. The enthalpy and entropy associated with the change of coordination were of the order of those usually observed on fusion. These large values are consistent with the major structural rearrangement from a network octahedral structure to discrete tetrahedral coordination. Although local order around the cadmium ion increased in the high-temperature phase the overall order of the lattice was considerably reduced because of the orientational disorder of the spherical, discrete ${\mathrm{CdCl}}_{4}^{2\mathrm{\ensuremath{-}}}$. Similar studies have been performed for solids of composition ${\mathrm{Cs}}_{2}$${\mathrm{CdCl}}_{4}$\ensuremath{\cdot}0.5CsCl, ${\mathrm{Cs}}_{2}$${\mathrm{CdCl}}_{4}$\ensuremath{\cdot}CsCl, and ${\mathrm{Cs}}_{2}$${\mathrm{CdCl}}_{4}$\ensuremath{\cdot}2CsCl to follow the formation of ${\mathrm{Cs}}_{3}$${\mathrm{CdCl}}_{5}$ which is stable above 395 \ifmmode^\circ\else\textdegree\fi{}C. The presence of the peak due to ${\mathrm{CdCl}}_{4}^{2\mathrm{\ensuremath{-}}}$ at temperatures as low as 365 \ifmmode^\circ\else\textdegree\fi{}C signaled the onset of the reaction to ${\mathrm{Cs}}_{3}$${\mathrm{CdCl}}_{5}$.