Phonons Reflect Dynamic Spin-State Order in LaCoO 3

We investigate lattice dynamics in LaCoO3 using inelastic neutron and x-ray scattering over T ¼ 2 − 650 K, spanning the spin-state crossover at T1 ≈ 100 K and the insulator-metal transition at T2 ≈ 550 K. Comparison with quasiharmonic ab initio lattice-dynamical calculations helps reveal anomalous softening of a ≈ 10-meV oxygen phonon, confined to the temperature interval T1 ≤ T ≤ T2 and localized in momentum space at qSSO ¼ ð½; ½; ½Þc . This wave vector is characteristic of the spin-state-ordering pattern originally proposed by Goodenough [An interpretation of the magnetic properties of the perovskite-type mixed crystals La1−x Sr x CoO3−λ, J. Phys. Chem. Solids 6, 287 (1958)]. Our results provide momentum-resolved evidence for dynamic correlations of high-spin and low-spin Co3þ states characterized by qSSO ¼ ð½; ½; ½Þc, demonstrating that phonons are sensitive probes of fluctuating spin-state correlations in LaCoO3.