The local atomic structure of La_(1-x)Sr_xCoO_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_(1-x)Sr_xCoO_3 revealed new structural effects which are correlated to the susceptibility and transport transitions. The transition in the spin configuration of the Co ions frm 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, LaCo_3, is in the LS state and is non-magnetic. A transition occurs to the HS state at approx 50 K as indicated from the susceptibility measurements due to the thermal excitation of electrons to the e_g level. The Co_(LS)O_6 octahedra associated with the Co ions in the LS configuration are distinguished from the Co_(HS)O_6 octahedra with the Co in the HS coonfiguration because the CO_(LS)-O bound length is shorter than the Co_(HS)-O distance due to the different size of the corresponding Co ions. Such bond lenths are clearly identified in the local structure between 15-300 K. This finding is in contrast to the average structure which lshws 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_4-octahedra coexist, they are randomly distributed in the crystal lattice at lower temperatures and become ordered at higher temperatures. The introduction of charge carriers in the structure does not eliminate the coexistence of both the LS and HS states, indicating that with the transition to the ferromagnetic metallic state, the spin configuration is not entirely of the HS character and structural inhom,ogeneities are present.