High-pressure synthesis of H2Ta2O6 · H2O with a defect pyrochlore structure

Abstract

Using high-pressure heat treatment of amorphous low-hydrated tantalum hydroxide of composition TaO_0.5–2.0(OH)_4–1 · 1.0–2.5H₂O at p = 5.0 GPa and t = 600–750°C, we obtained a mixture of two phases: H₂Ta₂O₆ · H₂O with a defect pyrochlore structure (sp. gr. Fd3m, a = 10.5956(1) Å, Z = 8, V = 1189.53(3) ų) and TT-Ta₂O₅ (δ) (α-UO₃ structure, sp. gr. P3 m1, a = 3.6368(2) Å, c = 3.8943(5) Å, Z = 1, V = 44.61 ų). The X-ray diffraction data for these compounds were analyzed by the Rietveld method. The agreement factors obtained are R_F = 0.0541 for H₂Ta₂O₆ · H₂O and R_F = 0.0885 for TT-Ta₂O₅. These phases are assumed to form an intergrowth structure along certain crystallographic planes. We have proposed a mechanism of intergrowth structure formation in the [111] direction of the pyrochlore structure and in the [001] direction of TT-Ta₂O₅.