E.P.R. study of molecular dynamics and phase transition in γ-irradiated (NH4)3VO2F4

Electron paramagnetic resonance (E.P.R.) of γirradiated polycrystalline (NH4)3VO2F4 has given clear indication of the existence of trans [VO2F4]4- species in the solid consistent with the predictions of VSEPR theory. Clear evidence was obtained for the occurrence of a first order phase transition at 418 +- 3 K with a large coexistence region (418 to 273 K) below the transition temperature. This transition is associated with the motion of [VO2F4]3- units which are present in fast reorienting form in the high temperature phase. This transition of fast reorienting to rigid forms has been found to be reversible. Differential scanning calorimetry lends support to this E.P.R. observation, clearly showing the existence of a first order phase transition at 418 K and the coexistence of the phases over a wide range of temperatures. This is the first report of a phase transition is this compound. The hyperfine constants were essentially temperature independent between 440-130 K. However, an abnormal increase in Asub(parallel to) was found at 77 K. This observation, in conjunction with the anomalous change (decrease) of the Q-value of the microwave cavity around 130 K, suggests the possible occurrence of yet another phase change probably associated with the onset of ordering of electric dipoles (ferro or anti-ferroelectric transitions). (author)