Cooperative Jahn-Teller Distortion in Mixed Chromites

Calculations of crystal distortions in Cu1−xNixCr2O4 and Fe1−xNixCr2O4 based on effective Hamiltonians in which the orbital states of the Jahn-Teller ions interact with bulk strains are presented. The derivation of the effective Hamiltonian from a microscopic Hamiltonian is discussed in some detail in the case of Cu1−xNixCr2O4. It is discussed that a local distortion accompanying each orbital state quenches the spin-orbit coupling whose role in determining the phase diagram was emphasized previously. The present calculations which neglect the spin-orbit coupling can reproduce almost quantitatively the observed phase diagrams of the mixed crystals. It is pointed out that the jump of | c/a−1 | at the cubic-to-tetragonal transition decreases when x approaches a critical concentration xc from both sides, which is supported by experiment.