Photoemission and the Electronic Structure of Transition Metal Compounds

In this paper we study the valence electronic structure of 3d transition metal compounds in an Anderson impurity approximation. Using simplified models we arrive at a classification scheme (in terms of several parameters) for the nature of the band gaps in these materials. Mott-Hubbard insulators, in the simple sense, are special cases in this scheme. Another important class of insulators is characterized by light holes of ligand p character and heavy d-like electrons (charge-transfer semiconductors). Within the same theoretical basis we develop the theory describing the photoemission and inverse photoemission spectra from which one can obtain values for the parameters in the Anderson impurity Hamiltonian. Using an exact two particle Green's function formalism the photoemission of CuCl2 is studied and it is shown that this compound is a charge-transfer semi-conductor. For the more general case an approximate, easy to use method is developed and is applied to the (inverse) photoemission of NiO. It is shown that NiO can be characterized as being intermediate between the Mott-Hubbard and charge-transfer regimes.