The structures of niobium-hydride alloys

Abstract

A series of niobium-hydrogen alloys with compositions in the two-phase and single-phase hydride fields has been examined at room temperature by X-ray diffractometer techniques. The closeness of the hydride structure to the cubic body-centred form allows simple formulae to be used to represent each complex diffraction profile in terms of a group of elementary components. The results are explained if the unit cells are slightly tetragonal and if there is a small departure from 90° of the angle opposite the c axis; thus the characteristics are comparable with those of the related tantalum hydride, though the c a ratios are much closer to unity. The symmetry of this arrangement is orthorhombic, the corresponding face-centred cell having sides a 0, b 0, C 0 (= c) where a 0 and b 0 approximate to √2 C 0, and a volume twice that of the bodycentred cell. The edge parameters and the angular distortion vary across the single-phase field, and the latter also varies to a smaller extent across the two-phase field. It is suggested that this abnormal behaviour may be the result of lack of equilibrium or of an exceptional sensitivity of the niobium-hydrogen system to the presence of small amounts of impurity.