Abstract
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 100–400K. Two reversible, first order structural phase transitions were found to be followed by an unusual sequence of symmetry changes: cubic (G3) (T2↓=287.5K; T2↑=291–310K)↔P4/mnc (G2) (T1↓=358.5K; T1↑=360K)↔4/m (G1). The twin structures observed were described assuming the existence of cubic parent phase G0 with Pm3¯m symmetry.
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