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

A sample of Ta2O5, ditantalum pentaoxide, heat-treated in a 'toroid'-type h igh-pressure chamber at P = 8 GPa and T = 1470 K, was studied by X-ray powd er 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 wer e 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) Angstrom, beta = 104.264 (2 )degrees, V = 332.45 Angstrom(3), Z = 4, space group C2/c; Z-Ta2O5, a = 5.2 252 (1), b = 4.6991 (1), c = 5.8534 (1) Angstrom, beta = 108.200 (2)degrees , V = 136.53 Angstrom(3), Z = 2, space group C2. The Z-Ta2O5 modification i s 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 intergrowt h and a transformation between them possible. An idealized model of the int ergrowth structure has been given. The HRTEM study showed defect-rich B-Ta2 O5 crystals, which could be interpreted as an intergrowth between the B-Ta2 O5 and Z-Ta2O5 phases.