Influence of the substitution of the central Me atom in ammonium oxyfluorides on phase transitions: IR spectroscopy study

Comparative investigations of infrared absorption spectra of ion-molecular crystals of the ammonium-containing oxyfluorides (NH 4 ) 3 WO 3 F 3 and (NH 4 ) 3 MoO 3 F 3 have been performed. It has been found that the parameters of the vibrational spectra exhibit anomalies in the ranges of internal vibrations of the Me O 3 F 3 octahedral groups and ammonium ions near the phase transition temperatures. A comparative analysis of the IR spectra allows us to affirm that the studied phase transition in (NH 4 ) 3 WO 3 F 3 is primarily associated with the ordering of octahedral groups, whereas in (NH 4 ) 3 MoO 3 F 3 , it is mainly due to the ordering of ammonium groups and the distortion of ammonium ions by the crystal environment.