The discontinuous volume change in transition-metal compounds due to the pressure-induced spin-state transition

Abstract

Several transition-metal compounds change their volume abruptly under high pressure through their electronic spin-state transition. The pressure-induced spin-state transitions in transition-metal compounds are studied on the basis of the ligand-field theory by using the model in which the coupling of transition-metal ions with crystal distortions is taken into account. The various types of transitions occur due to the crystal distortion induced by pressure, that is, the continuous and discontinuous high-spin (HS) to low-spin (LS) transitions with increasing pressure in the weak-crystal-field compounds, the continuous and discontinuous LS to HS transitions and the multiple LS to HS to LS to HS transition in the strong-crystal-field compounds. The pressure against volume curve shows various features of changes due to the crystal distortion associated with the spin-state transitions. The observed pressure against volume curve of haematite Fe₂O₃ is well explained by the discontinuous HS to LS transition of Fe³⁺ ions.