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

摘要

Using high-pressure heat treatment of amorphous low-hydrated tantalum hydroxide of composition TaO0.5-2.0(OH)(4-1) center dot 1.0-2.5H(2)O at p = 5.0 GPa and t = 600-750A degrees C, we obtained a mixture of two phases: H2Ta2O6 center dot H2O with a defect pyrochlore structure (sp. gr. Fd3m, a = 10.5956(1) , Z = 8, V = 1189.53(3) (3)) and TT-Ta2O5 (delta) (alpha-UO3 structure, sp. gr. P3 m1, a = 3.6368(2) , c = 3.8943(5) , Z = 1, V = 44.61 (3)). 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 center dot 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.