Crystallographic features of ammonium fluoroelpasolites: dynamic orientational disorder in crystals of (NH₄)₃HfF₇ and (NH₄)₃Ti(O₂)F₅

Abstract

A classical elpasolite-type structure is considered with respect to dynamically disordered ammonium fluoro-(oxofluoro-)metallates. Single-crystal X-ray diffraction data from high quality (NH 4 ) 3 HfF 7 and (NH 4 ) 3 Ti(O 2 )F 5 samples enabled the refinement of the ligand and cationic positions in the cubic Fm " 3 3m (Z = 4) structure. Electron-density atomic profiles show that the ligand atoms are distributed in a mixed (split) position instead of 24e. One of the ammonium groups is disordered near 8c so that its central atom (N1) forms a tetrahedron with vertexes in 32f. However, a center of another group (N2) remains in the 4b site, whereas its H atoms (H2) occupy the 96k positions instead of 24e and, together with the H3 atom in the 32f position, they form eight spatial orientations of the ammonium group. It is a common feature of all ammonium fluoroelpasolites with orientational disorder of structural units of a dynamic nature. research papers Acta Cryst. (2017). B73, 1-9 Anatoly A. Udovenko et al. Crystallographic features of ammonium fluoroelpasolites 9

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

“…Two independent tetrahedral ammonium groups were also assumed to be disordered. These results are consistent with NMR data suggesting the dynamical nature of structural disorder in (NH 4 ) 3 ZrF 7 and K 3 ZrF 7 [13][14][15].…”

“…According to [13,14], ammonium compounds have a cryolite-like structure with a cubic face-centered lattice (sp. gr.…”

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

“…In favour of this option there are experiments [3] that have shown that as the symmetry decreases as a result of two phase transitions, the fourth-order axis of the crystal remains. The model of the oxofluor complex (NH4)3Ti(O2)F5 proposed in [2] was confirmed by recent structural studies [4]. At the same time, there is the opinion that the structure of this substance is constructed from complexes in the form of a pentagonal bipyramid [5].…”

“…A structural complex in the form of a pentagonal bipyramid is realized in the isostructural fluorine compounds (NH4)3HfF7 and (NH4)3ZrF7 [4,12]. The crystals have the structure of an ordered perovskite (elpasolite) at ambient conditions with a cubic face-centred lattice (the space group, sp.…”

“…References [13,14] contain information about the implementation of a complex sequence of five non-ferroelectric phase transitions including a transition between two cubic phases m Fm3  F23 upon the process of cooling the crystal (NH4)3ZrF7. This paper presents the results of a study on the number of physical properties of the crystal (NH4)3HfF7, whose structure m Fm3 was established recently at room temperature [4].…”