The structure and phase transitions in oxyfluoride (ND4)2MoO2F4

Abstract Single crystals of (ND 4 ) 2 MoO 2 F 4 containing 95% deuterium were produced by repeated recrystallization of (NH 4 ) 2 MoO 2 F 4 in heavy water. The effect of deuteration on the parameters of successive phase transitions was investigated by differential scanning microcalorimetry and polarization optics. X-ray analysis of the original and distorted phases was performed. It was found that deuteration does not change the sequence of crystal phases typical for (NH 4 ) 2 MoO 2 F 4 , but leads to a shift in the phase transition Cmcm → Pnma towards the tricritical point. The mechanism of phase transitions in (ND 4 ) 2 MoO 2 F 4 was associated with both the ordering and the displacement of atoms.