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 K₆Ta_{6 + z} O₁₅F₆ (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 K₆Ta _6.27 ⁵⁺ O₁₅F_7.4, brown semitransparent partly reduced Ta⁺-containing bronze of the composition K₆Ta _6.33 ⁵⁺ Ta _0.55 ⁺ O₁₅F_8.2, and dark gray metal-like completely reduced Ta⁺-containing bronze of the composition K₆Ta ₆ ⁵⁺ Ta⁺O₁₅F₇ are studied experimentally. __________ Translated from Kristallografiya, Vol. 49, No. 1, 2004, pp. 75–91. Original Russian Text Copyright ? 2004 by Arakcheeva, Chapuis, Grinevich, Shamrai. Dedicated to the 80th Birthday of L.A. Shuvalov