Sequence of phase transitions in (NH 4 ) 3 SiF 7 .

Single crystals of silicon double salt (NH4 )3 SiF7 = (NH4 )2 SiF6 ·NH4 F = (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 ) ↔ P1[combining macron] (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-HF hydrogen bonds can be a driving force of the observed phase transitions.