The origin of the p-type conductivity has been investigated using hybrid density functional theory. The Materials Project phase diagram (calculated using GGA DFT) of CaCuP was used to identify potential stable limiting phases. Experimental crystal structures of these phases were relaxed with the PBE0 functional, using a converged k-point mesh and the same plane-wave energy cut-off as the CaCuP defect calculations. Figure shows the thermodynamic stability region of CaCuP, bound by the formation of CaCu, Ca 5 P 8 , CaCu 5 , CaP, CaCu 4 P 2 and CaCu 2 P 2 . The most ptype growth conditions are predicted to be found at the metalpoor (specifically Ca-poor), phosphorus-rich limit, while the least p-type growth conditions are predicted to occur at the metal-rich, phosphorus-poor limit.