The X-ray diffraction lines of β-Ta2O5of relative intensities I ≫ 4 indicate an orthorhombic unit cell of the dimensions a = 6.20 A ̊, b = 3.66 A ̊ and c = 3.89 A ̊ with tantalum, atoms near the positions (0,0,0) ( 1 2, 1 2,0), two oxygen atoms at about (0,0, 1 2) ( 1 2, 1 2, 1 2) and the other three oxygen atoms in the plane z = o of the tantalum atoms. The average deviations of the tantalum atoms from the positions above are α = 0.04, β = 0.03 and γ = 0.03. The arrangement of the tantalum atoms, and that of the oxygen atoms in the plane z = 1 2, is almost hexagonal ( a b = 1.69 vs. √3 = 1.73) as might be expected from strong electrostatic repulsion forces among the tantalum atoms in the plane z = o and among the oxygen atoms in the plane z = 1 2. Presence of many weak lines of relative intensities I ≤ 4 indicates a complicated superlattice structure which has not been fully elucidated. This structure and the distortion from hexagonality appear to result from the difficulties of accommodating three oxygen atoms per two tantalum atoms in the z = o plane. © 1964.
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