Abstract
Phase transitions of the (NH4)2WO2F4 crystal were studied by Raman spectroscopy in the range from 10 to 350 K. The mechanism of two phase transitions at T1 = 201 and T2 = 160 K was proposed. The significant spectra changes occur in the range corresponding to the W–O vibrations. The first temperature phase transition is due to the ordering of the quasi-octahedral groups [WO2F4]2– and partial ordering of ammonium groups. Experimental data allow for attributing the first (T1 = 201 K) phase transition to the first order close to the tricritical point. The noticeable changes of the Raman spectrum have been found in the range corresponding to the ammonium vibrations below the temperature T2. The second phase transition is associated with the further ordering of ammonium groups. Room temperature (296 K) experiments have been carried out under high hydrostatic pressure up to 10 GPa. Above 2 GPa, new spectral features appear, allowing for the assumption of the existence of a new high-pressure phase of (NH4)2WO2F4, which is mainly connected with ordering of the [WO2F4]2– quasi-octahedral groups.