Electron-density distribution in ilmenite-type crystals. III. Nickel(II) titanium(IV) trioxide, NiTiO3.

Abstract

The electron-density distribution in a crystal of NiTiO3 has been investigated by means of the single crystal X-ray diffraction method. A small amount of partial disorder was observed in the cation arrangement of the examined crystal. The difference electron-density maps showed an aspherical distribution of 3d electrons of the Ni2+ ion in the octahedral crystal field. Electron populations of the a, e(t2g) and e(eg) orbitals of Ni2+ were refined on the basis of the 3(C3) crystal field, employing aspherical scattering factors for the cation. The refinement gave the populations 1.84(9), 4.00(10) and 2.16(7) for the respective orbitals. The Ti4+ ion lies in a negative region of the difference Fourier map after the spherical atom refinement, and a positive peak exists on the threefold axis at a position 0.49 A apart from the Ti4+ ion towards the Ni2+ ion. These residual densities around Ti4+ are supposed to be caused by positive charge of the neighbouring Ni2+ ion. A residual peak is also observed at the middle of the neighbouring Ni2+ and Ti4+ along the threefold axis, which might be an indication of a metal-metal bonding. Refinements with anharmonic thermal parameters for the cations, up to the fourth order, revealed that anharmonicity is small at the room temperature in the present crystal.