  We study the thermally driven spin state transition in a two-orbital Hubbard
model with crystal field splitting, which provides a minimal description of the
physics of LaCoO3. We employ the dynamical mean-field theory with quantum
Monte-Carlo impurity solver. At intermediate temperatures we find a spin
disproportionated phase characterized by checkerboard order of sites with small
and large spin moments. The high temperature transition from the
disproportionated to a homogeneous phase is accompanied by vanishing of the
charge gap. With the increasing crystal-field splitting the temperature range
of the disproportionated phase shrinks and eventually disappears completely.
