Thermodynamic phase diagrams are the cornerstones to develop synthesis of new materials. Zinc phosphide has evolved into a prospective semicontuctor for next generation solar cells, thanks to its abundance and functional properties. Here we derive an optimized Zn-P binary diagram, and compare it to two previously available assessments. We solve some of the artefacts and clarify the methodology to obtain the Gibbs free energy, reaching an accurate description of the phases. This work is important for the synthesis of zinc-phosphide in the form of thin film and nanostructures.