Strain interaction effects on the high-spin-low-spin transition of transition-metal compounds

The temperature-induced high-spin(HS)-low-spin(LS) transition of the 3d 4 -3d 7 transition-metal ions in crystals (oxides and sulphides) are investigated on a semiquantitative basis by taking into account the elastic interaction between HS (or LS) clusters produced in a LS (or HS) crystal. The critical temperature at which the discontinuous phase transition takes place is evaluated as a function of the ligand-field strength Delta in units of spin-pairing energy Pi . It is shown that the 'crossover range' where this critical temperature exists is restricted to a finite range of Delta / Pi around the crossover point. The crossover ranges are evaluated for the crystals MO, MS 2 , M 2 O 3 , and LaMO 3 where M represents transition-metal ions. The temperature variation of the equilibrium concentrations of HS and LS clusters in LS and HS crystals, respectively, is calculated for several values of Delta / Pi in perovskite LaMO 3 crystals. It is pointed out that two discontinuous transitions from HS to LS and LS to HS with decreasing temperature may be expected in the d 6 and the d 7 systems.