3-SITE, 6-ELECTRON, EVEN-PARITY MODEL OF INTERMEDIATE VALENCE

In order to gain insight into the phenomenon of intermediate valency a three-site model is set up. At each site there is one localised orbital which can contain either one or two electrons. Zero occupancy is excluded. There are also three delocalised orbitals, with k-numbers 0, +or-2 pi /3. Occupation numbers of 0, 1 and 2 are allowed for these states. The system contains six electrons, and the model has two parameters, the energy of the localised levels relative to k=0, and the transfer energy between localised and delocalised levels, with the separation between k=+or-2 pi /3 and k=0 taken as unity. The resultant matrix eigenvalues are used to examine the specific heat, the magnetic susceptibility and the localised hole population as functions of temperature and parameters.