Structural transformation between two cubic phases of (NH4)3SnF7

• Published in: Journal of fluorine chemistry 2015-10, Vol.178, p.86-92
• Main Authors: Flerov, I.N., Molokeev, M.S., Laptash, N.M., Udovenko, A.A., Pogoreltsev, E.I., Mel’nikova, S.V., Misyul, S.V.
• Format: Article
• Language: English
• Subjects: Calorimetry; Crystal optics; Cubic fluorides; Phase transitions; X-ray
• ISSN: 0022-1139; 1873-3328
• DOI: 10.1016/j.jfluchem.2015.06.024

(NH4)3SnF7 undergoes one reversible structural phase transition of the first order between two cubic modifications Pa−3↔Pm−3m. The structural analysis has shown that octahedral SnF6 and tetrahedral NH4 groups are ordered in Pa−3 phase, while they are disordered in Pm−3m phase. •A reversible structural phase transition of the first order at T0=360K is found in (NH4)3SnF7.•The structural analysis shows transition between two cubic modifications Pa−3↔Pm−3m.•Octahedral SnF6 and tetrahedral NH4 groups are ordered in Pa−3 phase, while they are disordered in Pm−3m phase.•The large entropy change at phase transition point agrees well with an order-disorder transformation. Reinvestigations of the room temperature structure of ammonium heptafluorostannate (NH4)3SnF7=(NH4)2SnF6·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 Pm−3m (Z=1) suggested earlier. Polarizing optical observations, heat capacity, X-ray powder, and single crystal measurements were performed in a wide temperature range (100–420K). A reversible structural phase transition of the first order between two cubic modifications Pa−3↔Pm−3m was found at about T0=360K. 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.