The cluster variation method is used to calculate a phase diagram for a twodimensional Ising model representing the Cu20 plane of the high-T,, superconductor YBazCu30 ~. Both first (V1) and second-neighbor (V2) interactions are considered, with V2/Vt = _1. At high temperatures, the transition from the disordered (tetragonal) to the ordered (orthorhombic) phase is second-order. A tricritical point is found below which phase separation occurs. Fractional site occupancy and second-neighbor pair correlations are calculated as a function of temperature. The relevance of the model to the thermodynamics of ordering in the high-T c compound is discussed.