N. M. Laptash

Structure and Internal Mobility of Complex Ions in Ammonium Pentafluorotitanate According to XRD and NMR Data

The crystal structure of NH4TiF5 (I) was determined (monoclinic crystals, a = 14.683(1), b = 6.392(1), c = 20.821(2) Å, β = 110.538(2); space group P21/n, Z = 4). Structure I is built from infinite zigzag chains of TiF6 octahedra linked by their cis-vertices in the [101] direction; the chains are connected by the ammonium ions forming hydrogen bonds. The chains of octahedra with the surrounding ammonium ions are staggered in the crystal lattice of I. 19F and 1H NMR spectroscopy was used to study the dynamics of the complex ions in NH4TiF5 in the temperature range 270-530 K. Types of ionic motion in the fluoride and proton subsystems were determined, and activation energies evaluated.

Polymorphism of KNaNbOF5 crystals

X-ray crystallography is used to analyze the concomitant polymorphism of KNaNbOF5 crystals. The second β-modification of the compound, crystallizing in the tetragonal crystal system is found for the first time: space group P4/nmm, a = 5.9352(2) Å, c =8.5487(5) Å, V = 301.14(2) Å3, Z = 2, R1 = 0.0095. Parameters of the orthorhombic noncentrosymmetric structure of the α-phase, previously described by Poeppelmeier et al. (J. Am. Chem. Soc., 129, 13963–13969 (2007)), are refined and a comparative analysis of both structures is performed. The structures are characterized by the complete ordering of oxygen and fluorine atoms, the Nb-O distance in the α-phase (1.738(1) Å) being noticeably longer than that in the β-phase (1.709(2) Å). In β-KNaNbOF5, alternating NbOF5 and NaOF5 octahedra share vertices, while in α-KNaNbOF5, they share both vertices and edges. The existence of the non-polar centrosymmetric β-modification of KNaNbOF5 cancels the assumption of the substantial contribution of potassium cations to the polar structure of α-KNaNbOF5.

VOLATILE AMMONIUM FLUOROTITANATE

Single crystals of NH4TiO x F5−2x (x = 0.4) have been synthesized by gaseous product condensation at 300–400°C during the thermal decomposition of crystalline (NH4)2 Ti(OH) x F6−x (x = 0.6). The compound consists of kinked parallel infinite chains of cis-corner-connected TiF6 octahedra along the [1 0 1]-direction separated from each other by NH4 +-ions. The crystal parameters of this compound are as follows: monoclinic P21/n, a = 14.683 (1), b = 6.392 (1), c = 20.821 (2) Å, β = 110.538 (2)°, V = 1829.9 (3) Å3, Z = 16. The presence of oxygen was proved by chemical analyses, IR and NMR spectroscopy. Oxygen atoms must be randomly distributed in the crystal lattice. Fluorine and ammonium diffusion above 480 K can be the evidence of high ionic conductivity of this complex.

On the identification of oxygen and fluorine atoms in disordered inorganic oxyfluoride compounds

It is considered practically impossible to differentiate between oxygen and fluorine atoms by X-ray diffraction in disordered structures of oxyfluoride compounds due to the similarity of their ionic radii and diffusion factors. Indeed, many transition metal oxyfluoride compounds containing polar pseudo-octahedral MOxF6–x (x = 1-3) anions form crystal structures without any fluorine-oxygen (F/O) ordering owing to a large number of local anion configurations. Because of this static disorder, it is impossible to determine the positions of O and F atoms and find the real geometry of the polyhedron. However, this becomes possible in the case of dynamic disorder of oxyfluoride anions when the central atom is displaced from the center of the octahedron toward a vertex, edge, or face (depending on the number of oxygen atoms in the polyhedron), which enables the identification of O and F atoms owing to inherent differences between M–O and M–F bonding. On cooling, such compounds undergo phase transitions of the order–disorder type with substantial changes in the entropy. The examples of static and dynamic orientational disorder in oxyfluoride compounds of d0 transition metals are given.