High-Pressure Synthesis of H2 –xTa2O6 – 0.5x · 2/3H2O (0 ≤ x ≤ 2) with the Hexagonal Tungsten Bronze Structure

Abstract

Using high-pressure, high-temperature processing of a low water content amorphous tantalum hydroxide, TaO_0.5–2.0(OH)_4–1 · (1.0–2.5)H₂O, at p = 5.0–6.0 GPa and t = 800–900°C, we have obtained a mixture of two phases, Ta₂O₅ · 2/3H₂O (sp. gr. P6₃/mсm, а = 7.4736(2) Å, с = 7.6798(2) Å, Z = 3, V = 371.48(1) ų) and H₂Ta₂O₆ · 2/3H₂O (sp. gr. P6₃/mсm, а = 7.4998(2) Å, с = 7.6171(2) Å, Z = 3, V = 371.04(1) ų), both crystallizing in the hexagonal tungsten bronze (HTB) structure. According to thermogravimetric analysis results, the material contains 4.9(3)% water. After water removal, heating in air to 550°C causes the HTB phase to convert into δ-Ta₂O₅. X-ray diffraction data have been analyzed by the Rietveld method and the following reliability factors have been obtained: R_F = 0.0374 for Ta₂O₅ · 2/3H₂O and R_F = 0.0416 for H₂Ta₂O₆ · 2/3H₂O. We assume that the MO₃ stoichiometry of the basic HTB cell in these compounds is ensured by different mechanisms.