Temperature behaviour of optical phonons near Tc in triglycine sulphate and triglycine selenate

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Abstract
Infrared reflection spectra and Raman spectra at several temperatures between 2 K and 350 K have been observed for triglycine sulphate and the isomorphous compound triglycine selenate. A complete group theoretical enumeration of the phonon modes is given and the observed frequencies have been tabulated for different temperatures. The tables also include an attribution of most of the observed lines to motions of the molecular groups in the crystal. The observed lines are classified into several groups according to the temperature dependence of their frequencies and intensities. The spectra of the TGS-family are discussed in comparison with the spectra of other systems performing an order-disorder transition. Our results support the order-disorder model of the phase transition as reported recently by Mitsui, Itoh, Keve, Kay, Kleinberg et al.