Lattice dynamics in perovskite-type layer structures. FIR studies of (CH3NH3)2MnCl4and (CH3NH3)2FeCl4

Abstract

The far-infrared reflectivity of (CH₃NH₃)₃MnCl₄(=MAMC) and of (CH₃NH₃)₂FeCl₄(=MAFC) single crystals was studied at several temperatures. By means of a Kramers-Kronig analysis, the frequencies of the long-wavelength infrared-active phonons were determined. Because of the considerable gap (300-900 cm^-1) between the external and the internal modes of the organic ions these can be treated as rigid bodies at far-infrared frequencies. In addition, the authors present the results of a structure analysis of MAMC above and below the continuous phase transition at 394K which turns out to be of the order-disorder type. In the high-temperature tetragonal modification, three strong and two weak IR-active modes are observed for E perpendicular to c.