Sequence of phase transitions in (NH4)3SiF7

Abstract

Single crystals of silicon double salt (NH4)(3)SiF7 = (NH4)(2)SiF6 center dot 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) (G(1)) <-> Pbam (Z = 4) (G(2)) <-> P2(1)/c (Z = 4) (G(3)) <-> P (1) over bar (Z = 4) (G(4)) <-> P2(1)/c (Z = 8) (G(5)). 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 center dot center dot center dot F hydrogen bonds can be a driving force of the observed phase transitions.