Soft mode behavior and anharmonicity of some two-dimensional layer compounds

Abstract

The far infrared reflection spectra of the layer structure compounds of the type (C_nH_2n+1NH₃)₂MCL₄, n=1,2,..., (M=Cu,Cd) have been measured in the region 10–200 cm⁻¹ as function of temperature between 20 and 300 K. The change of the number of modes in the spectra at ∼120 K is in agreement with a structural phase transition where the layer structure changes from not entirely square to square with increasing temperature. The presence of two‐phonon transitions in the spectra is an indication for anharmonicity of the potential energy. The observed strong increase of the damping of several modes with increasing temperature is explained by interaction with a (infrared inactive) soft mode of the two‐dimensional layer. The critical temperature of this soft mode has been determined as 180 K for (CH₃NH₃)₂CuCl₄ and 160 K for (CH₃NH₃)₂CdCl₄.