Theory of Pressure-Induced High-Spin\( \)Low-Spin Transition in Compounds of Iron

The pressure-induced spin state transitions in Fe(II) compounds are studied on the basis of the ligand field theory by using the model in which the couplings of Fe ion with a molecular distortion and a lattice strain are taken into account. The calculated pressure dependences of the low-spin content reproduce the essential features of the observed several types of transitions: The continuous high-spin (HS) →low-spin (LS) transition with increasing pressure arises from a smooth increasing of the lattice strain with or without a slight change of the molecular distortion. The discontinuous HS →LS transition is due to an abrupt decreasing of the molecular distortion. The LS →HS transition occurs in the case where the lattice strain couples dominantly with Fe ion at lower pressure but the intramolecular coupling overwhelms the strain coupling at higher pressure through an increase of the former induced by pressure.