Landau–Ginzburg free energy study of spin state phase transition in LaCoO3

LaCoO3 (LCO) undergoes a cubic to rhombohedral structural phase transition at ∼1600 K, a metal–insulator transition at ∼550 K, and a spin-state transition at ∼100 K. The thermodynamics of spin-state phase transitions for LCO have been studied using the 2-4-6 Landau–Ginzburg (LG) free energy functional (ΔG) developed in terms of mode amplitude of octahedral tilt (primary order parameter), symmetry-adapted strains, and pure electronic free energy (depicting gradual change of high-spin population fraction). The expansion coefficients of (ΔG) are determined using experimentally reported structural parameters and by employing first principle-based calculations. The spin crossover transition is predicted to be ∼75 K. The developed LG free energy functional model can successfully explain the temperature evolution of mode amplitude of primary order parameter (QR4+), symmetry-adapted strains ea (particularly characteristic hump ∼70 K) and e4, and population of high-spin state (n). The present study depicts the primary role played by octahedral tilt in the spin-state phase transition of LCO.