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

Abstract

New fluoride compound (NH 4) 2 KZrF 7 with the elpasolite structure (sp. gr. Fm-3m) was synthesized by partial cationic substitution in (NH 4) 3 ZrF 7 cryolite. Due to small difference in ionic radii of K + and NH 4 + , non-equivalent crystallographic positions 8c and 4b are occupied by both cations. Investigations of thermal, optical and structural properties in a wide range of temperature revealed two phase transitions Fm-3m ↔ P4 2 /ncm ↔ P4 2 /nmc of the first order. The results obtained are discussed in comparison with the data for the previously studied related cryolite (NH 4) 3 ZrF 7 .

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

“…Summarizing the above, we can conclude that chemical pressure is one of the most effective tools in the search and design of new fluorine materials. This statement is confirmed by the results of recent pioneering research of the first synthesized complex fluorine elpasolite (NH 4 ) 2 KZrF 7 [40]. X-ray and optical studies have shown that its structure in the initial phase is characterized by the 2 same cubic symmetry F m3m that was determined for the initial (NH 4 ) 3 ZrF 7 cryolite [32].…”

“…X-ray and optical studies have shown that its structure in the initial phase is characterized by the 2 same cubic symmetry F m3m that was determined for the initial (NH 4 ) 3 ZrF 7 cryolite [32]. On the other hand, the temperature behavior of structure and optical properties has demonstrated that elpasolite undergoes only two structural transformations F m3m (T 1 =333 K) ↔ P 4 2 /ncm (T =329 K) ↔ P 4 2 /nmc [40] instead of six phase transitions accompanied by stronger structural distortions observed in cryolite. It cannot be ruled out that such a strong difference in the structural parameters of two related crystals is due to a small difference in the ionic radii of potassium and ammonium, which led to the occupation of nonequivalent crystallographic positions 4c and 8c by both cations that was found by the refinement of structure.…”

“…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].…”

“…DTA setup was used to study the effect of hydrostatic pressure on the temperatures and enthalpy/entropy of the F m3m ↔ P 4 2 /ncm ↔ P 4 2 /nmc phase transitions [40]. A copper container with a powder sample of 0.05 g and the reference quartz sample were glued onto opposite junctions of a highly sensitive (∼400 µV/K) germanium-copper thermocouple.…”

Fluxional seven-coordinated fluorido- and oxofluoridotantalates

“…In the case of transition metal fluorides and oxide fluorides, molecules adopt all three stereochemistries, for example, ReF 7 has a PB structure (Vogt et al, 1994). Isolated ZrF 7 3À (Zachariasen, 1954;Hurst & Taylor, 1970;Udovenko & Laptash, 2008a;Pogoreltsev et al, 2019) and HfF 7 3À (Granzin & Saalfeld, 1988;Udovenko et al, 2017) are pentagonal bipyramidal in A 3 MF 7 (A = NH 4 , K; M = Zr, Hf). In orthorhombic crystal NaBaZrF 7 , the anion is a CTP (Gao et al, 1992), whereas in orthorhombic compounds of similar composition: ABZr(Hf)F 7 (A = Rb, Tl; B = Ca, Cd) (Meddar et al, 2008) and KPdZr(Hf)F 7 (Mü ller & Mü ller, 1995) as well as MMnZrF 7 (M = Tl, Rb, NH 4 , K) (El-Ghozzi et al, 1994), they are PBs.…”