The far-infrared reflectivity of (CH 3 NH 3 ) 3 MnCl 4 (=MAMC) and of (CH 3 NH 3 ) 2 FeCl 4 (=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.
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Lehner et al. (1975) studied this question.
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