An EPR study of structural phase transitions in vanadium oxyfiuoro anionic compounds

Abstract

Ammonium oxyfluoro vanadate, (NH₄)₃VO₂F₄, exhibits some interesting polymorphic transitions involving changes in fluxional behaviour of anionic units. Further, the ammonium ions in this compound are found to be in two chemically inequivalent forms, the distinction arising due to possible differences in hydrogen bonding. The present work deals with an examination of the role of hydrogen bonding on the fluxional behaviour of the anionic groups, and thereby on phase transitions, by the EPR technique, using paramagnetic V⁴⁺ species created by gamma irradiation as the probe. This is done by studying the consequences of (a) successive substitution of ammonium ions by sodium, and (b) successive removal of NH₄F units from the parent compound. It is found that, of the two kinds of inequivalent ammonium ions, only one has a profound influence on the observed phase transitions in [NH₄]₃VO₂F₄, Na[NH₄]₂VO₂F₄ and [NH₄]₂VO₂F₃. This indicates the existence of a possible hydrogen bonding between this ammonium ion and the anionic complex. In these compounds the low-temperature phase appears to be electrically ordered. In compounds Na[NH₄]₂VO₂F₄ and Na₃VO₂F₄, the VO₂F^3- ₄ units are found to be freely reorienting at room temperature and these compounds do not exhibit any phase transitions. On the other hand the absence of phase transitions in compounds [NH₄]₃V₂O₄F₅ and NH₄VO₂F₂ appears to be due to structural changes in the anionic units following removal of NH₄F units from the parent compound.

Key Words:

• structural phase transitions
• electron paramagnetic resonance
• tetravalent vanadium
• oxyfluoro complexes of vanadium
• ammonium oxyfluoro compounds
• radiation damage.