Disorder and phase transitions in oxyfluoride (NH4)3Ta(O2)2F4

Calorimetric, X-ray, dielectric and DTA under pressure measurements have been performed on oxyfluoride (NH4)3Ta(O2)2F4. The succession of nonferroelectric phase transitions was found associated with the order-disorder processes. The comparative analysis tantalate with related niobate has revealed the important role of the central atom in the physical properties behavior, mechanism of structural distortions and barocaloric effect in oxyfluorides with the eight-coordinated anionic polyhedra.

The successive phase transitions in (NH4)3Ta(O2)2F4 are associated with the ordering disordered cubic structure Fm3¯m. The mechanism of structural distortions in oxyfluorides with the eightcoordinated anionic polyhedra is considerably affected by the substitutions of central atom.Figure optionsDownload as PowerPoint slideHighlights
► (NH4)3Ta(O2)2F4 undergoes the succession of nonferroelectric phase transitions.
► Excess entropy is associated with the order-disorder processes.
► Physical properties and barocaloric effect depend on the type of central atom.