Structure studies of solid solutions of oxygen in electrolytic niobium

The crystal structures (cubic system, sp. gr. Im3m) of solid solutions of oxygen in niobium of the compositions NbOx<0.01 (I), NbO0.150(6) (II) and NbO0.107(5) (III) were studied for the first time by single-crystal X-ray diffraction analysis (Syntex PI diffractometer, lambdaMoK(alpha) radiation). Crystals I (a = 3.2969(8) Angstrom) and 11 (a = 3.3082(6) Angstrom) were prepared by the electrolysis of salt melts. Crystal III (a = 3.3114(4) Angstrom) was obtained after partial dissolution of crystal I in a solution of HNO3, HCl, and HF resulting in its saturation with oxygen. The residual electron density maps for crystal I revealed no 0 atoms. Localization of the 0 atoms in the tetrahedral cavities of crystal 11 and in the tetrahedral and octahedral cavities of crystal III indicates that the arrangement of the interstitial positions of oxygen atoms depends on the procedure of preparing solid solutions. In crystals 11 and 111, the amplitudes of thermal vibrations of the Nb atoms are smaller and the anharmonicity of these vibrations is higher than those in crystal I virtually devoid of oxygen. The reasons for the substantially higher oxygen content (up to 17 at. %) in cubic crystals of the solid solutions of 0 in Nb prepared by the electrolytic method than in the analogous phase obtained upon heating (lower than 7 at. %) are discussed. (C) 2002 MAIK "Nauka/Interperiodica".