Based on available experimental data on orthorhombic-to-rhombohedral phase transition and melting curve of black phosphorus , a tentative phase diagram of phosphorus was proposed in 1987 (Fig. ). This phase diagram is limited by 5 GPa and 1000°C and describes only two solid phosphorus allotropes i.e. orthorhombic and rhombohedral. Later the first-order liquid-liquid phase transition in phosphorus has been discovered which was attributed to the pressure-induced transition of a dense molecular fluid (liquid 1) into a polymeric liquid (liquid 2) . The simplified version of the phase diagram of phosphorus up to 5 GPa has been proposed , however it differs from the previous one only by the presence of the liquid 1 -liquid 2 transition line with negative slope, without specifying the position of the liquid 1 -liquid 2 -orthorhombic solid triple point.