Pressure-induced intermediate-to-low spin state transition inLaCoO3

Synchrotron x-ray powder-diffraction experiments reveal that the transition from a magnetic intermediate spin (IS) state $t_{2g}^5{e}_g^1$ to a nonmagnetic low-spin ground state $t_{2g}^6$ in ${\mathrm{LaCoO}}_3$ normally observed when cooling manifests itself under pressure by an anomalously low bulk modulus of 150(2) GPa and an initially very large Co-O bond compressibility of $4.8\times {10}^{-3}$ ${\mathrm{GPa}}^{-1}$ which levels off near 4 GPa. The continuous depopulation of the IS state is driven by an increased crystal-field splitting resulting in an effective reduction of the size of the ${\mathrm{Co}}^{3+}$ cation.