Local distortion in LaCoO3 and PrCoO3:: extended x-ray absorption fine structure, x-ray diffraction and x-ray absorption near edge structure studies

Room temperature Co K edge extended x-ray absorption fine structure (EXAFS), x-ray absorption near edge structure (XANES), including the pre-edge structure, 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 study. The EXAFS analysis of the data indicates that the CoO6 octahedron is distorted in both LaCoO3 and PrCoO3. There are two Co-O bonds with bond length 1.863 (1.886) angstrom and four Co-O bonds with bond length 1.928 (1.942) angstrom for LaCoO3 (PrCoO3). Such distortion is expected in orthorhombic PrCoO3 but not in rhombohedral LaCoO3. This distortion in the CoO6 octahedron is attributed to Jahn-Teller active Co3+ ions in an 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 greater in comparison with those of 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. The comparison of experimental spectra with the calculated ones-using the Co 4p density of states obtained from local density approximation calculations and matrix elements calculated using an atomic like core state as the initial state and a confluent hypergeometric function as the final state-indicates that for orthorhombic structure, the intensities of different features are lower as compared to those for the cubic structure. The pre-edge peaks attributed to Co 1s -> e(g)(up arrow) and e(g)(down arrow) transitions show the effects of hybridization of the e(g) orbitals with O 2p orbitals, and their relative intensities in PrCoO3 and LaCoO3, can be explained by using the average Co-O bond length obtained from the EXAFS.