Unusual sequence of phase transitions in (NH4)3TiF7 detected by optic and calorimetric studies

• First phase transitions are found in double salt fluorides A2MeF6·AF.
• (NH4)3TiF7 shows unusual symmetry changes on heating: cubic → P4/mnc → 4/m.
• Twin structures were described assuming the existence of cubic parent phase Pm3¯m.
• Excess entropies correspond to significant structural changes at phase transitions.

For the first time structural transformations were observed for one of the compounds belonging to the family of double salt fluorides A3MeF7 = A2MeF6·AF. Single crystals of (NH4)3TiF7 = (NH4)2TiF6·NH4F were grown by evaporation of the aqueous solution. Polarizing optical observations, heat capacity and birefringence Δn = (no − ne) measurements were performed in a wide temperature range 100–400 K. Two reversible, first order structural phase transitions were found to be followed by an unusual sequence of symmetry changes: cubic (G3) (T2↓ = 287.5 K; T2↑ = 291–310 K) ↔ P4/mnc (G2) (T1↓ = 358.5 K; T1↑ = 360 K) ↔ 4/m (G1). The twin structures observed were described assuming the existence of cubic parent phase G0 with Pm3¯m symmetry.

(NH4)2TiF6·NH4F undergoes two reversible ferroelastic phase transitions with unusual sequence of the symmetry change G3-cubic → G2-P4/mnc → G1-4/m  . Complex twinning of the tetragonal phases is explained assuming the existence of high temperature cubic parent phase with m3¯m symmetry.Figure optionsDownload as PowerPoint slide