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

Abstract

Room temperature Co K edge extended x-ray absorption fine structure(EXAFS), x-ray absorption near edge structure (XANES), including thepre-edge structure, and x-ray diffraction (XRD) studies are carried out onLaCoO3 and PrCoO3. The Co–O, Co–La/Pr and Co–Co bond lengths are obtained from EXAFS analysis and comparedwith those obtained from XRD study. The EXAFS analysis of the data indicates that theCoO6 octahedron isdistorted in both LaCoO3 and PrCoO3.There are two Co–O bonds with bond length 1.863 (1.886) Å and four Co–O bonds with bond length 1.928(1.942) Å for LaCoO3 (PrCoO3). Such distortion is expected in orthorhombicPrCoO3 but not inrhombohedral LaCoO3. Thisdistortion in the CoO6 octahedron is attributed to Jahn–Teller activeCo3+ ions in an intermediate spin state in these compounds. Higher shell studiesreveal that Debye–Waller (DW) factors of Co–Pr and Co–Co bonds inPrCoO3 are greater in comparison with those of Co–La and Co–Co bonds inLaCoO3, indicating that these bonds are structurally more disordered inPrCoO3. The comparison of Co–Co bond lengths and corresponding DW factors indicates that thestructural disorder plays an important role in deciding the insulating properties of thesecompounds. XANES studies have shown changes in the intensities and positions ofdifferent near edge features. The comparison of experimental spectra with thecalculated ones—using the Co 4p density of states obtained from local densityapproximation calculations and matrix elements calculated using an atomic likecore state as the initial state and a confluent hypergeometric function as thefinal state—indicates that for orthorhombic structure, the intensities of differentfeatures are lower as compared to those for the cubic structure. The pre-edge peaksattributed to and transitions show the effects of hybridization of theeg orbitals with O 2p orbitals, and their relative intensities inPrCoO3 and LaCoO3, can be explained by using the average Co–O bond length obtained from theEXAFS.