COMPETITION OF VARIOUS SPIN STATES OF LACOO3

The spin-state transition in perovskite LaCoO3 has been a controversia l topic ever since its discovery. To obtain a better physical understa nding of its origin we have studied various magnetically ordered state s of an enlarged double cell, including the low-spin state, intermedia te-spin state, high-spin state, as well as all combinations among thes e three states. The calculations were done within the unrestricted Har tree-Fock approximation and the real-space recursion method. The densi ty of states and stability of various states are analyzed as functions of model parameters. Our results show that the sound state of the sys tem depends on the competition between the crystal-field splitting Dq and the Hund's coupling j. For a fixed j, the ground state of the syst em changes from the low-spin state to the low-spin high-spin ordered s tate (LS-HS) and finally to the high-spin high-spin antiferromagnetica lly ordered state as Dq is weakened. Our study suggests that the spin- state transition at 90 K probably takes place from the low-spin state to the LS-HS ordered state. [S0163-1829(98)03414-6].