Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts
In many oxides the outer d electrons are not localized, magnetic electrons but collective, metallic electrons either as a result of cation‐sublattice band formation or of covalent mixing of anion p and cationic d states. It is shown that cation‐sublattice band formation does not take place in oxides with ReO3 or perovskite structure and that they are therefore suitable for testing any covalency criterion for the magnetic vs metallic state. A parametric criterion is developed from physical arguments, and it is used to interpret systematically the magnetic and electric properties of oxides with the perovskite structure. The role of the Hartree—Fock intra‐atomic exchange correlations is found to be important, and the physical origin of the ferroelectric transitions is identified.
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