Dynamic orientational disorder in crystals of fluoroelpasolites, structural refinement of (NH4)3AlF6, (NH4)3TiOF5 and Rb2KTiOF5

Abstract

Room-temperature crystal structures of triammonium hexafluoroaluminate, (NH4)3AlF6 (I), and triammonium oxopentafluorotitanate, (NH4)3TiOF5 (II), were refined, and the crystal structures of dirubidium potassium oxopentaflourotitanate, Rb2KTiOF5, at 297 K (III) and 218 K (IV) were determined using single-crystal X-ray diffraction techniques. In ammonium fluoroelpasolites [(I) and (II)], the ligand (O, F) atoms are located in the mixed 24e + 96j position of the Fm3m unit cell. The 24e position is occupied by the ligand atoms predominantly in (III) and fully in (IV). `Ordered' N1 and Rb atoms are tetrahedrally displaced from the 8c position into the 32f site, and the H atoms of the disordered ammonium group N2 are statistically distributed in the 96k and 32f positions. The Ti atoms in (II) and (IV) are shifted from the 4a position to 24e thus allowing identification of the O and F atoms in the octahedron on a local scale. The disorder in the crystals is of a dynamic nature. Unique Raman spectra of Rb2KTiOF5 under the laser beam of 1064 nm indicate fast octahedral reorientations resulting in physical equalizations of the Ti—O and Ti—F distances as well as in the appearance of totally synchronous Ti—O and Ti—F stretching vibrations at 750 cm−1. This phenomenon is assumed to also take place under X-rays.