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

• Published in: Solid state sciences 2016-11, Vol.61, p.155-160
• Main Authors: Bogdanov, Evgeniy V., Mel'nikova, Svetlana V., Pogoreltsev, Evgeniy I., Molokeev, Maxim S., Flerov, Igor N.
• Format: Article
• Language: English
• Subjects: Crystals; Deuteration; Deuterium; Distortion; Heavy water; Phase transformations; Scanning; X-rays
• ISSN: 1293-2558; 1873-3085
• DOI: 10.1016/j.solidstatesciences.2016.08.012

Single crystals of (ND4)2MoO2F4 containing 95% deuterium were produced by repeated recrystallization of (NH4)2MoO2F4 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 (NH4)2MoO2F4, but leads to a shift in the phase transition Cmcm → Pnma towards the tricritical point. The mechanism of phase transitions in (ND4)2MoO2F4 was associated with both the ordering and the displacement of atoms. [Display omitted] •A molybdenum oxyfluoride appeared to be more susceptible to the replacement of protons by deuterons in comparison with a related tungsten oxyfluoride, as evidenced by a higher degree of deuteration (95% versus 85%) for the same number of recrystallizations in heavy water.•The D → H substitution did not lead to significant changes in the stability ranges of the initial and distorted phases, but caused a marked approximation of the phase transition at T1 to the tricritical point.•The mechanism of phase transitions in (ND4)2MoO2F4 were associated with both ordering processes and displacement of the central atom as well as partial ordering of some F/O atoms.