Phonon renormalization in LaCoO₃ by inelastic neutron and X-ray scattering

LaCoO₃ exhibits two broad magnetic-electric transitions, a diamagnetic to paramagnetic spin-state transition at T_SS ∼ 100 K and a metal-insulator transition at T_MI ∼ 500 K. The spin transitions on heating are proposed to be as follows: from a homogeneous LS state to a mixed Low-Spin/High-Spin (LS/HS) state with strong spin-charge fluctuations at T=T_SS and, subsequently, into a homogeneous HS state at T=T_MI. The lattice participates in the state mixture by expansion of CoO₆ octahedra around the HS sites, while the ones around LS sites have a reduced size. The originally proposed static order [2] has not been observed experimentally and, hence, the ordering is expected to be dynamic and short-ranged. We investigated the lattice dynamical properties of LaCoO₃ using inelastic neutron scattering. Based on detailed ab-initio lattice dynamical calculations (performed in our institute), we aim for a comprehensive understanding of lattice dynamics in LaCoO₃. The above discussed crossovers and spin state order should be reflected in the lattice degrees of freedom via quasi elastic scattering and phonon renormalization effects.