A Theory of the Cooperative Jahn-Teller Effect : Crystal Distortions in Cu_<1-x>Ni_xCr_2O_4 and Fe_<1-x>Ni_xCr_2O_4

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A theory of the cooperative Jahn-Teller effect is developed for the purpose of discussing the spontaneous crystal distortions in mixed chromites, Cu_<1-x>Ni_xCr_2O_4 and Fe_<1-x>Ni_xCr_2O_4. The static Jahn-Teller effect only is taken into account. The theory separates the self-energy of each ion having a degenerate orbital level from those energies which contribute to the cooperative phenomenon; the former energy represents the effect of a local distortion which persists in the cubic phase. The theory also distinguishes between the contributions of the Jahn-Teller coupling with the bulk strains and with relative displacements of ions. The theory explains successfully various features of the phase diagrams of the mixed chromites such as the concentration dependences of the transition temperatures and the magnitudes of the distortions, the appearance of an orthorhombic phase, etc. The temperature dependence of elastic moduli is also discussed to point out that it will yield additional informations, particularly on the contributions of relative displacements of ions to the transition temperatures.
社団法人日本物理学会の論文
1972-01-05