Vibrational spectroscopic, optical and thermal properties of a deuterated ferroic crystal, [(CH2OD)3CND3]2SiF6—An experimental and theoretical study

[(CH2OD)3CND3]2SiF6 ferroic crystal, abbreviated as D-(TRIS)2SiF6 crystal was investigated. Optical (linear birefringence) and thermal (differential scanning calorimetry, DSC) measurements indicated a solid–solid phase transition (PT) of the first order at 185 K. The vibrational infrared spectra of powdered D-(TRIS)2SiF6 crystal in Nujol and Fluorolube mulls were studied in the wide range of temperature, from 308 K to 133 K. The temperature changes of wavenumber, width, centre of gravity position and intensity of the bands were analyzed to clarify the molecular mechanism of the phase transition and the contribution of SiF62− and CH2OD groups to the PT. A wide range (4000–400 cm−1) of internal vibrations of the DTRIS+ and SiF62− ions was investigated and the assignment of the observed infrared absorption bands was discussed. Raman spectra of the crystal at room temperature (RT) were recorded and discussed in relation to the structure of isostructural (TRIS)2SiF6 crystal in the range (4000–400 cm−1). Theoretical calculations were made based on density functional theory (DFT). The calculated normal vibrational modes of the molecules, their intensities and frequencies were compared with those obtained from experimental data.