Аннотация:Reinvestigations of the room temperature structure of ammonium heptafluorostannate (NH4)(3)SnF7=(NH4)(2)SnF6 center dot NH4F = (NH4)(3)[SnF6]F by both powder and single crystal X-ray diffractions have revealed that its real symmetry is Pa-3 (Z = 8) instead of Pin-3m (Z = 1) suggested earlier. Polarizing optical observations, heat capacity, X-ray powder, and single crystal measurements were performed in a wide temperature range (100-420 K). A reversible structural phase transition of the first order between two cubic modifications Pa-3 <-> Pm-3m was found at about T-0 = 360 K. The structural models associated with partially disordered and totally ordered high and low temperature phases, respectively, comply with a large value of phase transition entropy.
