The Local Atomic Structure of La_(1-x)Sr_xCoO_3: Effects Induced by the Spin-State and Nonmetal to Metal Transitions

摘要

The pair density function (PDF) used in theanalysis of pulsed neutron diffraction data of La_(1-x)Sr_xCoO_3, revealed new structural effects which arecorrelated to the susceptibility and transport transitions. Thetransition in the spin configuration of the Co ions from the low-spin (LS) to the high-spin (MS) state in the Co perovskite oxidescan potentially induce structural distortions due to the couplingof the spin to the lattice and charge. The ground state of the purecompound, LaCoO_3, is in the Ls state and is non-magnetic. Atransition occurs to the MS state at approx 50 K as indicatedfrom the susceptibility measurements due to the thermalexcitation of electrons to the e_g level. The Co_(LS)O-6octahedra associated with the Co ions in the LS configurationare distinguished from the Co_(HS)O_6 octahedra with the Coin the HS configuration because the Co_(LS)-O bond length isshorter than the Co_(HS)-O distance due to the different size ofthe corresponding Co ions. Such bond lengths are clearlyidentified in the local structure between 15-300 K. This findingis in contrast to the average structure which shows only one typeof bond length in this temperature range but two types of bondlengths are suggested at considerably higher temperatures. Thissuggests that whereas the Ls and MS CoO_6-octahedra coexist,they are randomly distributed in the crystal lattice at lowertemperatures and become ordered at higher temperatures. Theintroduction of charge carriers in the structure does not eliminatethe coexistence of both the Ls and MS states, indicating thatwith the transition to the ferromagnetic metallic state, the spinconfiguration is not entirely of the MS character and structuralinhomogeneities are present.