Acta Crystallographica Section B

Structural Science

Volume 56, Part 4 (August 2000)

research papers

Acta Cryst. (2000). B56, 659-665 [ doi:10.1107/S0108768100005462 ]

Structures and phase transitions of B-Ta₂O₅ and Z-Ta₂O₅: two high-pressure forms of Ta₂O₅

I. P. Zibrov, V. P. Filonenko, M. Sundberg and P.-E. Werner

Abstract: A sample of Ta₂O₅, ditantalum pentaoxide, heat-treated in a `toroid'-type high-pressure chamber at P = 8 GPa and T = 1470 K, was studied by X-ray powder diffraction and high-resolution transmission electron microscopy (HRTEM). Two high-pressure modifications of Ta₂O₅, isostructural with B-Nb₂O₅ and Z-Nb₂O₅, were identified from the X-ray powder pattern. Both structures were refined by the Rietveld method from the X-ray diffraction data: B-Ta₂O₅, a = 12.7853 (4), b = 4.8537 (1), c = 5.5276 (2) Å, [beta] = 104.264 (2)°, V = 332.45 Å³, Z = 4, space group C2/c; Z-Ta₂O₅, a = 5.2252 (1), b = 4.6991 (1), c = 5.8534 (1) Å, [beta] = 108.200 (2)°, V = 136.53 Å³, Z = 2, space group C2. The Z-Ta₂O₅ modification is new. The Ta atoms are six-coordinated in B-Ta₂O₅ and seven-coordinated in Z-Ta₂O₅. The two structures are closely related, which makes an intergrowth and a transformation between them possible. An idealized model of the intergrowth structure has been given. The HRTEM study showed defect-rich B-Ta₂O₅ crystals, which could be interpreted as an intergrowth between the B-Ta₂O₅ and Z-Ta₂O₅ phases.

Keywords: Structure; Phase transitions; High-pressure forms.