A theory of the pressure-induced high-spin—low-spin transition of transition-metal oxides

The pressure-induced high-spin—low-spin transition of the transition-metal ions in octahedral coordination is studied theoretically. The relation between the crossover point and the transition point is discussed and the formula to determine the transition point is given in terms of the crystal-field splitting Δ and the spin-pairing energy Π asΔ = αΠ. The numerical coefficient α is determined by the ratio of the change of the interatomic distances between the transition-metal ion and ligands. It is proved that α is generally less than 1 and takes a value of about 0.95-0.80 for the transition-metal oxides. Based on the discussions on the P–V relation of the low-spin oxides, transition pressures are estimated to be about 700–1300 kbar for MnO, CoO and Fe₂O₃, and about 250–400 kbar for FeO.The magnitude of the shifts of transition pressures due to entropy changes is roughly evaluated to be about 0.1 kbar K^?1 for transition-metal oxides. It is discussed whether the transition at high temperature takes place continuously or not. For MnO, FeO and Fe₂O₃, the high-spin—low-spin transition will take place gradually at temperatures higher than about 2000 K. For CoO, the gradual transition will take place at above 300 K.