(NH 4 ) 3 HfF 7 : Crystallooptical and calorimetric studies of a number of successive phase transitions

Abstract

Single crystals of (NH4)3HfF7 were grown. Polarising optical observations as well as measurement of the heat capacity and birefringence n(Т) were performed in the temperature range of 200-310 K. Reversible phase transitions at temperatures T0 = 290 K, Т1 = 280.5 K, Т2≈ 273 K, Т3 = 266 K, Т4 = 259 K, Т5 = 231 K, Т6 = 229 K were found. Observations in polarised light make it possible to suggest a sequence of changes in the symmetry groups for these transitions: m Fm3 Т0→ cub. Т1→ mmm (1) Т2→ mmm (2) Т3→ mmm (3) Т5→ 2/c. The T-p phase diagram was studied and the temperature boundaries of the stability of the distorted crystalline phases were determined. A significant change of the entropy at successive phase transitions ΣΔS = 10.6 J/mol•K indicates a disordering of the initial cubic phase.

Structural, thermal and optical properties of elpasolite-like (NH4)2KZrF7

“…Rln6, (Hf) Rln4 [19,20]. However, the change in entropy at each of the transformations was turned out to be significantly less than Rln2.…”

“…More recent comprehensive studies revealed that ammonium zirconium and hafnium cryolites undergo a more complex sequence of structural distortions [18][19][20][21]. Careful polarization-optical, X-ray, calorimetric and Raman investigations as well as the group-theoretical analysis have shown that (NH 4 ) 3 ZrF 7 and (NH 4 ) 3 HfF 7…”

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

“…On the other hand, the latter fluorides show rather different sensitivity to cationic and anionic replacement. Thermodynamic parameters of the ammonium hafnium compound, including the temperatures and entropies of transformations, are very close to those of the zirconium crystal [32,33]. However, the complete or even partial replacement of the tetrahedral ammonium cation with spherical potassium in A 3 ZrF 7 leads to a significant decrease in the number of phase transitions and a change in their sequence [37,38,39,40].…”

“…Other situation was observed in fluorides with seven-coordinated anionic polyhedra which can be formed as a monocapped trigonal prism (A 2 MF 7 ; A: Rb, NH 4 , K; M: Ta, Nb) [24,25,26,27,28,29,30] or a pentagonal bipyramid (A 3 MF 7 ; A: K, NH 4 ; M: Hf, Zr) [31,32,33,34,35,36]. The variation of cations and central atom in the former compounds is accompanied by strong change in the symmetry and structural disorder in the initial and distorted phases.…”