Structural phase transitions involving changes in spin and valency. A theory of the three phase transitions of LaCoO3. II

For pt.I see ibid., vol.9, p.3731 (1976). The thermodynamics of the two-sublattice model of LaCoO 3 is discussed. The model is described by a Hamiltonian including two trivalent and two charge-transfer states of cobalt ions. The ions interact effectively through coupling to fully symmetrical vibrations of octahedral ion-ligand complexes. In the molecular-field approximation a single order parameter is introduced, simultaneously related to the antidistortive order, the high-spin-low-spin order, as well as the charge order. Five zero-temperature phases are possible: the low-spin phase, the high-spin phase, the two-sublattice high-spin-low-spin phase, the mixed-valency-low-spin phase and, finally, the charge-order phase. A three-dimensional phase diagram for finite temperatures is obtained numerically.