Evidence for orbital ordering in ${\mathrm{LaCoO}}_3$

We present powder and single-crystal x-ray diffraction data as evidence for a monoclinic distortion in the low-spin $\mathrm{}(S=0\mathrm{})\mathrm{}$ and intermediate spin states $\mathrm{}(S=1\mathrm{})\mathrm{}$ of ${\mathrm{LaCoO}}_3.$ The alternation of short and long bonds in the ab plane indicates the presence of $e_g$ orbital ordering induced by a cooperative Jahn-Teller distortion. We observe an increase of the Jahn-Teller distortion with temperature in agreement with a thermally activated behavior of the ${\mathrm{Co}}^{3+}$ ions from a low-spin ground state to an intermediate-spin excited state.