Theory of high-spin?low-spin transitions in transition metal compounds induced by the Jahn–Teller effect

Abstract

A model for high‐spin?low‐spin transitions in transition metal compounds is presented. The Jahn–Teller coupling between the d electrons and local distortion is considered as the driving force for the transition. The system built up of octahedrally coordinated Fe(II) complexes is studied microscopically on the basis of the ligand field theory. The first‐order phase transition, in which the order parameter is the local displacement Q, occurs more than once over a certain range of the coupling strength, a. When the second order coupling is introduced, in some cases all of the three possible patterns of the transition (Q≳0?Q=0, Q≳0?QQQ=0) appear in the system. The phase transitions occur not only between high‐spin and low‐spin states but also between the two high‐spin states split by the distortion Q. The calculated temperature dependences of effective magnetic moment μ_eff reproduce the essential feature of observed results for the case of discontinous high‐spin?low‐spin transition with reasonable values of E₀ and a, where E₀ is the energy separation between the high‐spin and low‐spin states in the undistorted phase. These values also give consistently reasonable values of axial ligand field splitting of the high‐spin state and the abrupt change of Fe–ligand bond length. It is suggested that the observed peculiar behavior of μ_eff at lower temperatures in the high‐spin compounds may be due to the first‐order phase transitions between the high‐spin states.