The Perovskite Ghost

We have seen that in es.sentially all transition-metal compounds, a possible starting point for a calculation of the electronic structure is an ionic description. In the perovskites, however, it may be determined from Table 19-3 that perturbation theory runs into trouble. We may, for example, calculate the effective charge Z on oxygen, starting with a closed shell 0 . The perturbation of each oxygen pa 

33 from the. v states, leaving the oxygen positive. This unreasonable conclusion indicates that we must treat the coupling between atoms more earefully. 

To calculate the properties in terms of the perovskite gliosts, we will need a covalent energy, equal to the matrix element between a (x-oriented / state and a neighboring sp hybrid. 

These are listed for the four materials wc consider in Table 19-4. 

44 eV in SrTiO.,. We neglect this shift in treating each ghost like a bonding unit, that is, as independent. This is analogous to the use of the Bond Orbital Approximation in the covalent solids, and is the only approximation we shall make here. 

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Matrix elements associated with the u-oriented d states and oxygen, sp hybrids that make up the perovskite ghost. 

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