A non-typical sequence of phase transitions in (NH4)3GeF7: optical and structural characterization

Abstract

Single crystals of germanium double salt (NH₄)₃GeF₇ = (NH₄)₂GeF₆·NH₄F = (NH₄)₃[GeF₆]F were grown and studied by the methods of polarization optics and X-ray diffraction. The birefringence Δn = (n_o − n_e), the rotation angle of the optical indicatrix ϕ(T) and unit cell parameters were measured in the temperature range 100–400 K. Three structural phase transitions were found at the temperatures: T_1↓ = 279.2 K (T_1↑ = 279.4 K), T_2↑ = 270 K (T_2↓ = 268.9 K), T_3↓ = 218 K (T_3↑ = 227 K). An unusual sequence of symmetry transformations with temperature change was established: P4/mbm (Z = 2) (G₁) ↔ Pbam (Z = 4) (G₂) ↔ P2₁/c (Z = 4) (G₃) ↔ Pa (Z = 8) (G₄). The crystal structures of different phases were determined. The experimental data were additionally interpreted by a group-theoretical analysis of the complete condensate of order parameters taking into account the critical and noncritical atomic displacements. Strengthening of the N–H⋯F hydrogen bonds can be a driving force of the observed phase transitions.