Local distortion in LaCoO3 and PrCoO3: EXAFS, XRD and XANES studies

Room temperature Co K-edge extended x-ray absorption fine structure (EXAFS), x-ray absorption near edge structure (XANES) including pre-edge and x-ray diffraction (XRD) studies are carried out on LaCoO3 and PrCoO3. The Co-O, Co-La/Pr and Co-Co bond lengths are obtained from EXAFS analysis and compared with those obtained from XRD. The EXAFS analysis of data indicates that CoO6 octahedron is distorted in both LaCoO3 and PrCoO3. Such distortion is expected in orthorhombic PrCoO3 but not in rhombohedral LaCoO3. This distortion in CoO6 octahedron is attributed to Jahn-Teller active Co3+ ion in intermediate spin state in these compounds. Higher shell studies reveal that Debye-Waller (DW) factors of Co-Pr and Co-Co bonds in PrCoO3 are more in comparison to Co-La and Co-Co bonds in LaCoO3 indicating that these bonds are structurally more disordered in PrCoO3. The comparison of Co-Co bond lengths and corresponding DW factors indicates that the structural disorder plays an important role in deciding the insulating properties of these compounds. XANES studies have shown changes in the intensities and positions of different near edge features.Comment: 22 pages, 8 figures, 2 tables. To appear in J. Phys.: Condens. Matte

Interpretation of Pre-Edge Fine Structures (PEFS) of K Edge of Titanium in Rutile TiO2

A method based on the local density approximation and on the multiple scattering wave calculation taking into account screened X-ray hole potential for the final state has been used to analyze the experimental XANES spectra of the TiO2 rutile. The effect of cluster size on the computations has been analyzed concluding that taking into account atoms forming all neighboring octahedra lead to a reasonable agreement between theoretical and experimental spectra. In the crystal structure containing only slightly distorted regular octahedra in ground state, only two peaks corresponding to 1 s -> t2g and eg transitions are obtained. In contrast, due to the effect of the core hole potential calculated self consistently, the locally projected 3d density (DOS) is shifted down by about 3 eV giving rise to quadrupole transition. The 3d DOS arising from neighboring octahedra is not shifted by the core hole potential and contains non-centrosymmetric wave functions arising from solid state effect giving rise to dipole transitions. Temperature vibrations of atoms decrease the symmetry of the system thus making dipolar transitions to t2g and eg of absorbing octahedron allowed, this increases the intensities of the two first peaks A1 and A2 in the pre-edge region