Spin-state-driven metal-insulator transition in (La,Sr) CoO3 under high-pressure

Using Co $K\ensuremath{\beta}$ x-ray emission spectroscopy (XES) we have investigated the effect of pressure on the Co spin state of a single crystal of metallic, ferromagnetic ${\mathrm{La}}_{0.82}{\mathrm{Sr}}_{0.18}\mathrm{Co}{\mathrm{O}}_{3}$. The analysis of the XES spectra up to $14.4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ at 300 and $34\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ clearly shows a gradual pressure-induced spin-state transition from a high- or intermediate-spin to a low-spin state. These results provide microscopic evidence that the recently observed pressure-induced metal-insulator transition in $(\mathrm{La},\mathrm{Sr})\mathrm{Co}{\mathrm{O}}_{3}$ is driven by a simultaneous spin-state transition. We further show that under high pressures the low-spin state is relatively stable against temperature, in contrast to that in the $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$ case. This is attributed to an enhancement of the crystal field splitting at high pressures.