Phase relationships in the system were determined in air in the range between and , using solid state and quenching methods. Three congruently melting compounds exist, , , and . The orthophosphate has a sluggish, reversible inversion at 942° and melts at 1060°±8°C. The pyrophosphate undergoes a rapid, reversible inversion at 132° and melts at 1017°C. The irreversible inversion in the metaphosphate takes place in air in the range between 600° and 700°C, depending on the rate of heating of the sample. The low temperature form of the metaphosphate cannot be obtained by heating the high temperature form in air, but glass of the metaphosphate composition will yield a mixture of low and high temperature forms if held at 550°C for 16 hr. melts at 872°C. Simple eutectic relationships exist between and , and , and and .