  We studied the melting of the charge order in the half-doped tetragonal
perovskite La1.5Sr0.5CoO4 by elastic neutron scattering. We found that diffuse
peaks, corresponding to the breathing-type modulation of oxygen positions in
the CoO6 octahedra, disappear above Tc = 825(27) K. This melting of the diffuse
superstructure is reversible (no change in the correlation lengths upon
annealing is observed) and accompanied by a large nonlinear thermal expansion
along the tetragonal c-axis, with a cusp at Tc. We conclude that quenched
disorder of the doped Sr ions is at the origin of this anisotropic charge glass
state. We also suggest that its melting is driven by the transition from the
intermediate- to the high-spin state of Co{3+} ion, which is favored by the
spin entropy at high T.
