We studied the melting of the charge order in the half-doped tetragonal perovskite ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoO}}_4$ by elastic neutron scattering. We found that diffuse peaks, corresponding to the breathing-type modulation of oxygen positions in the ${\mathrm{CoO}}_6$ octahedra, disappear above $T_c=825\mathrm{}\left(27\right)\mathrm{}\hspace{0.5em}\mathrm{K}.$ 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 $T_c.$ 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 ${\mathrm{Co}}^{3+}$ ion, which is favored by the spin entropy at high T.

• Received 22 May 2001

DOI:https://doi.org/10.1103/PhysRevB.64.195117