V. F. Shamrai

Structure type of hexagonal tantalum bronzes with variable composition K6Ta6 + ZO15F6(F, O)y: Ta(5 − δ)+ bronzes and Ta5+ compounds

The structure type of hexagonal compounds with the variable composition K6Ta6 + zO15F6 (F, O)y, where Z ≤ 1 and y ≤ 3 (sp. gr. P6/m, a ∼ 13.12 Å, c ∼ 3.86 Å) has been studied. Based on the structural data for a crystal of the Ta5+-containing compound and two crystals of Ta(5 − δ)+-containing bronzes formed at the cathode during electrolysis of salt melts containing Ta+ cations, it was established that the bronze formation is associated with the interstitial defects of intercalated Ta cations. The scheme of reduction of Ta cations at the cathode is suggested, and the formulas of the compounds are obtained with due regard for partial Ta reduction to various integral oxidation degrees. The crystals of the colorless transparent Ta5+-containing compound of the composition K6Ta6.275+O15F7.4, brown semitransparent partly reduced Ta+-containing bronze of the composition K6Ta6.335+Ta0.55+O15F8.2, and dark gray metal-like completely reduced Ta+-containing bronze of the composition K6Ta65+Ta+O15F7 are studied experimentally.

Evolution of the calcium hydroxyapatite crystal structure under plasma deposition and subsequent reducing treatment

The structure of hydroxyapatite plasma coatings on a titanium substrate has been investigated by the X-ray Rietveld method. The hydroxyapatite crystal structure in plasma-deposited samples is characterized by strong distortions of its main element (tetrahedral PO4 cluster) and coordination calcium polyhedra, as well as calcium deficit in the Ca2 site; however, these features do not change the main motif of the hydroxyapatite structure. The bond distortions in PO4 clusters are estimated by the Bauer method. It is shown that hydrothermal treatment leads to the almost complete recovery of the hydroxyapatite structure.

Specific features of the crystal structure and magnetic properties of KTaO3 produced by electrolysis of melts

The magnetic susceptibility χ(T) at 4.2 K < T < 293 K; the dependence of the magnetic moment on the magnetic field strength, M(H), at 4.2, 77, and 293 K; and the electrical resistivity ρ(T) at 4.2 K < T < 293 K are studied for samples of perovskite-phase KTaO3 obtained by both solid-phase synthesis (KTaO3s) and deposition on a cathode during electrolysis of melts (KTaO3e). Yellowish white KTaO3s powders are diamagnetic and reveal dielectric properties. Dark polycrystalline KTaO3e samples with metallic luster are characterized by the dependence ρ(T) typical of metals and additional paramagnetic contribution to the paramagnetic susceptibility as compared with KTaO3e. Changes in the properties of KTaO3 during electrocrystallization are attributed to partial reduction of tantalum. They are revealed in the structural features of KTaO3e (excess of tantalum as compared to the stoichiometric composition of KTaO3e, deficiency of the oxygen sublattice, and clearly pronounced anharmonicity of atomic vibrations). A change of the cation-anion-cation interactions, occurring owing to the overlapping of oxygen p orbitals with tantalum t2g orbitals and the formation of impurity levels near the conduction band, leads to the generation of free carriers, which make a paramagnetic contribution to the magnetic susceptibility.

Crystal structure and resistivity characteristics of new tantalum bronze K6Ta6.5O15 + xF6 + y

The structure of K6Ta6.5O15 + xF6 + y (I) with partly reduced tantalum has been determined by the methods of X-ray diffraction analysis (SYNTEX P1 diffractometer, λMoKα radiation, 808 independent reflections): sp. gr. P 6/m, a = 13.118(4), c = 3.862(1) Å, RF = 0.0292. The resistivity of metal-like crystals I is almost independent of temperature in the temperature range T = 4–300 K. The physical properties studied allow one to relate the crystals to the group of tantalum bronzes. The structure of compound I is compared with that of the well-known crystals of the transparent dielectric K6Ta6.5O14.5F9.5 (Ia) containing only pentavalent tantalum not belonging to the group of bronzes. The I and Ia structures have the same basic frameworks but different distributions of additional Ta atoms located along the channel axes and different degrees of delocalization as well as the numbers and degrees of delocalization of the surrounding anions. The relation between these structural features and the resistivity characteristics of the two compounds is also discussed.