The Si–P system at high temperatures up to 6 wt%P was investigated. Silicon–phosphorus alloys were prepared through the melting of silicon–phosphorus mixtures in closed silica tubes. Microstructural studies indicated two phases in the alloys, i.e., a solid solution of P in Si and the intermediate compound SiP. Thermal analysis technique was applied to study the phase transformations in the alloys at elevated temperatures. It was observed that SiP is melted at 1139 ± 2 °C. A eutectic reaction in the system, in which liquid Si–P alloy is transformed to SiP and a solid solution of P in Si was observed at 1129 ± 2 °C. Moreover, the liquidus and solidus on the silicon-rich part of the phase diagram were determined as:
Solidus: TS = −75.03 CP + 1414 (°C) 0.35 < CP < 1 wt%P
Liquidus: TL = −6.74 CP + 1414 (°C) CP < 6 wt%P
The distribution coefficient of P in Si was calculated as k0 = 0.09 using the obtained solidus and liquidus at low concentrations.