Sequence of phase transitions in (NH4)3SiF7

Abstract

Single crystals of silicon double salt (NH₄)₃SiF₇ = (NH₄)₂SiF₆·NH₄F = (NH₄)₃[SiF₆]F were grown and studied by the methods of polarization optics, X-ray diffraction and calorimetry. A sequence of symmetry transformations with the temperature change was established: P4/mbm (Z = 2) (G₁) ↔ Pbam (Z = 4) (G₂) ↔ P2₁/c (Z = 4) (G₃) ↔ P (Z = 4) (G₄) ↔ P2₁/c (Z = 8) (G₅). Crystal structures of different phases were determined. The experimental data were also interpreted by a group-theoretical analysis of the complete condensate of order parameters taking into account critical and noncritical atomic displacements. Strengthening of the N–H⋯F hydrogen bonds can be a driving force of the observed phase transitions.