Temperature dependent phase separation in the Emery model

We report on calculations for the Emery model of the carriers in the copper oxygen planes of HTSC materials using the variational principle of Bogoljubov. A strong coupling ansatz optimized for small values of the ratio hopping constant to Coulomb correlation gives the lowest free energy in the neutral case and in the case of moderate doping. From the curves chemical potential vs. doping concentration large regions of thermodynamic instability and phase separation respectively are determined using a Maxwell construction. The temperature and parameter dependence of these coexistence regions with respect to para-, ferro- and antiferromagnetic order is given