Local atomic structure of La(sub 1(minus)x)Sr(sub x)CoO(sub 3): Effects induced by the spin-state and non-metal to metal transitions

摘要

The pair density function (PDF) used in the analysis of pulsed neutron diffraction data of La(sub 1(minus)x)Sr(sub x)CoO(sub 3) revealed new structural effects which are correlated to the susceptibility and transport transitions. The transition in the spin configuration of the Co ions from the low-spin (LS) to the high-spin (HS) state in the Co perovskite oxides can potentially induce structural distortions due to the coupling of the spin to the lattice and charge. The ground state of the pure compound, LaCoO(sub 3), is in the LS state and is non-magnetic. A transition occurs to the HS state at (approximately) 50 K as indicated from the susceptibility measurements due to the thermal excitation of electrons to the e(sub g) level. The Co(sub LS)O(sub 6) octahedra associated with the Co ions in the LS configuration are distinguished from the Co(sub HS)O(sub 6) octahedra with the Co in the HS configuration because the Co(sub LS)-O bond length is shorter than the Co(sub HS)-O distance due to the different size of the corresponding Co ions. Such bond lengths are clearly identified in the local structure between 15--300 K. This finding is in contrast to the average structure which shows only one type of bond length in this temperature range but two types of bond lengths are suggested at considerably higher temperatures. This suggests that whereas the LS and HS CoO(sub 6)-octahedra coexist, they are randomly distributed in the crystal lattice at lower temperatures and become ordered at higher temperatures.