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 = (no - ne), 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: T1↓ = 279.2 K (T1↑ = 279.4 K), T2↑ = 270 K (T2↓ = 268.9 K), T3↓ = 218 K (T3↑ = 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) ↔ Pa3[combining macron] (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-HF hydrogen bonds can be a driving force of the observed phase transitions.

Sequence of phase transitions in (NH4)3SiF7

“…Calorimetric measurements revealed the succession of four reversible phase transitions below room temperature: T 1 = 263 ± 1 K; T 2 = 198 ± 1 K; T 3 = 194 ± 1 K; T 4 = 170 ± 1 K, and an absence of transformation into the Pm3m phase up to decomposition. It was found that the application of uniaxial pressure does not change the orientation of the four-fold axis of (NH 4 ) 3 SiF 7 , similar to (NH 4 ) 3 GeF 7 , 11 which is in agreement with the fact that they are isostructural at room temperature. Therefore, both (NH 4 ) 3 SiF 7 and (NH 4 ) 3 GeF 7 differ from (NH 4 ) 3 TiF 7 in which the high-pressure parent phase Pm3m is attainable.…”

“…The uniaxial pressure increase (up to destruction of the sample) did not change the orientation of the fourth-order axis in (NH 4 ) 3 SiF 7 , similar to that found earlier for (NH 4 ) 3 GeF 7 . 11 Thus, silicon and germanium compounds differ from (NH 4 ) 3 TiF 7 , where small uniaxial pressure reoriented this axis at 90 degrees. 15 Fig.…”

“…Thus, the calorimetric measurements have shown that the cationic substitution Si → Ge in compounds (NH 4 ) 3 M 4+ F 7 is accompanied by a small broadening of the temperature range of the P4/mbm phase stability (T 1 = 280 K in (NH 4 ) 3 GeF 7 ) and an increase in the number of the phase transformations. 11…”

“…The presence of the optical anisotropy Δn c and the position of extinction along the [110] 11 Further temperature lowering leads to an additional cooling decrease in the symmetry of (NH 4 ) 3 SiF 7 at T 3 . It can be seen (Fig.…”

“…Recently, we found that the isostructural compound (NH 4 ) 3 GeF 7 (space group P4/mbm) underwent several reversible phase transitions below room temperature with an unusual sequence P4/mbm ↔ Pbam ↔ P2 1 /c ↔ Pa3. 11 Moreover, some other fluoride double salts (NH 4 ) 3 M 4+ F 7 (M 4+ = Ti 4+ , Sn 4+ ) undergo phase transitions but of different sequences, and a correlation with the M 4+ ionic radius was found. [11][12][13][14][15][16] Therefore, one might expect the temperature variation would also destabilise the room temperature structure of the silicon double fluoride salt, leading to phase transitions.…”