Sequence of phase transitions in (NH₄)₃SiF₇

Abstract

Single crystals of silicon double salt (NH)SiF = (NH)SiF·NHF = (NH)[SiF]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) ↔ Pbam (Z = 4) (G) ↔ P2/c (Z = 4) (G) ↔ P1[combining macron] (Z = 4) (G) ↔ P2/c (Z = 8) (G). 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.

Chemical pressure as an effective tool for tuning the structural disordering and barocaloric efficiency of complex fluorides (NH4)3MF7 (M: Sn, Ti, Ge, Si)

“…Detailed procedures and results of the individual experimental studies of the thermal and structural properties of fluorides (NH 4 ) 3 MF 7 , were published by us earlier [26][27][28][29][30][31][32][33][34]. Here we provide a brief description of samples preparation and the experimental methods used to determine the thermodynamic parameters needed to evaluate barocaloric efficiency.…”

“…The title compounds (ammonium heptafluorometallates, (NH 4 ) 3 MF 7 = (NH 4 ) 2 MF 6 •NH 4 F were synthesized from fluoride aqueous solutions [26][27][28][29]. (NH 4 ) 2 MF 6 of a reagent grade were used as starting materials, together with the reagent NH 4 F: (NH 4 ) 2 MF 6 + NH 4 F = (NH 4 ) 3 MF 7 .…”

“…One of the features of the compounds (NH 4 ) 3 MF 7 ≡ (NH 4 ) 2 MF 6 •NH 4 F ≡ (NH 4 ) 3 [MF 6 ]F (M: Sn, Ti, Ge, Si) is related to their structure, which can be considered in terms of two three-dimensional interpenetrating sublattices, one of which consists of octahedra MF 6 and tetrahedra NH 4 , and the other is composed by 'independent' F atoms linked to the ammonium groups by the N-H• • • F bonds. At room temperature, these complex fluorides exhibit a strong difference in symmetry, mainly due to the very wide spectrum of the ionic radius of the central atom M (Å): Sn (0.690) -Pa 3 Ti (0.605) -P4/mnc, Ge (0.530) -P4/mbm, Si (0.400) -P4/mbm (figures 1(a)-(c)) [26][27][28][29]. These crystalline phases are characterized by varying degrees of disordering of octahedral and tetrahedral structural units and are differently stable with respect to changes in temperature and pressure.…”

“…As a result, linear extrapolation of the Pm 3m phase stability boundary showed that the transformation between tetragonal and high temperature cubic phases in germanium and silicon compounds could be realized at atmospheric pressure at an even higher temperature (table 1). Therefore, it is not surprising that the P4/mbm phase turned out to be stable up to the decomposition temperature: (NH 4 ) 3 GeF 7 (~330 K) and (NH 4 ) 3 SiF 7 (~450 K), which was accompanied by the appearance of phases (NH 4 ) 2 GeF 6 and (NH 4 ) 2 SiF 6 [28,29,32,33].…”

Calorimetric, dilatometric and DTA under pressure studies of the phase transitions in elpasolite (NH4)2KZrF7

“…Numerous studies of complex fluorides and their solid solutions have shown that anionic-cationic substitution can significantly affect the formation of the type of structure, its change during phase transitions and the behavior of physical properties [1,2,3,4,5,6,7]. For example, the substitution of the central atom or monovalent cations in six-coordinated compounds A 2 A'MF 6 (A, A': Cs, Rb, K, NH 4 ; M: Lu, Sc, Fe, Ga, Al) [4,6,8,9,10,11,12,13] and (NH 4 ) 2 MF 6 NH 4 F (M: Si, Ge, Ti, Sn, Pb) [14,15,16,17,18,19,20,21,22,23] does not change the cubic F m3m symmetry of the initial phase but leads to strong change in the succession of structural distortions associated with the nonferroelectric transformations and their parameters (degree of the structural disordering, temperature, entropy, sensitivity to external pressure, etc. ).…”