title: Structures and phase transitions of B-Ta2O5 and Z-Ta2O5: two high-pressure forms of Ta2O5.
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) A, beta = 104.264 (2)degrees, V = 332.45 A3, Z = 4, space group C2/c; Z-Ta2O5, a = 5.2252 (1), b = 4.6991 (1), c = 5.8534 (1) A, beta = 108.200 (2)degrees, V = 136.53 A3, 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.
authors.affiliation: Institute of Crystallography, Russian Academy of Sciences, 117 333 Moscow, Russia.
authors.forename: I P
authors.initials: IP
authors.lastname: Zibrov
authors.forename: V P
authors.initials: VP
authors.lastname: Filonenko
authors.forename: M
authors.initials: M
authors.lastname: Sundberg
authors.forename: P E
authors.initials: PE
authors.lastname: Werner
mesh_terms: D008854:Microscopy, Electron; D008958:Models, Molecular; D015394:Molecular Structure; D010087:Oxides / Q000737:chemistry*; D044367:Phase Transition; D033362:Powder Diffraction; D011312:Pressure; D012680:Sensitivity and Specificity; D013635:Tantalum / Q000737:chemistry*
chemical_list: D010087:Oxides; D013635:Tantalum; C078151:tantalum oxide