2004 Volume 73 Issue 5 Pages 1244-1250
In order to get a realistic view of the origin of the antiferroorbital (AFO) order in LaMnO3, MnF3 and KCuF3, both the cooperative Jahn–Teller effect (CJTE) and the orbital-dependent exchange interaction (OEI) are taken into account for the dγ electron–lattice system in compounds with the perovskite-type structure. The phonon-mediated orbital interaction in the CJTE and the dγ electron-mediated one in the OEI are treated by use of the mean-field approximation with preserving the symmetry of the crystal, and are joined to give an effective Hamiltonian. It is stressed that the CJTE favors any of the FO and AFO orders, while the OEI does only the AFO order. The present theory is consistent with the observed fact that the AFO order is characterized by an arrangement of alternate mixed orbitals of dx2−y2 and d3z2−r2, the AFO phase survives even at sufficiently high temperatures in the paramagnetic phase, and the AFO order is always accompanied by the ferrodistortion and the antiferrodistortion.
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