Phase equilibria in the system Nd₂O₃|P₂O₅|H₂O at 500°C under 100 MPa and synthesis of NdP₅O₁₄ crystals

Phase equilibria in the system Nd₂O₃|P₂O₅|H₂O was studied under a hydrothermal condition at 500°C under 100 MPa by a quenching technique using encapsuled samples. Equilibria among Nd phosphates; NdP₅O₁₄, NdP₃O₉ and NdPO₄, and liquid phase could be attained within 24 h. The stability fields for liquid (L), NdP₅O₁₄+L, NdP₅O₁₄+NdP₃O₉+L, NdP₃O₉+L, NdP₃O₉+NdPO₄+L, NdPO₄+L and NdPO₄+Nd(OH)₃+H₂O have been determined. The composition of the liquid phase with which NdP₅O₁₄ and NdP₃O₉ or NdP₃O₉ and NdPO₄ could coexist was 3% Nd₂O₃-80% P₂O₅-17% H₂O or 4% Nd₂O₃-73% P₂O₅-23% H₂O respectively, at 500°C under 100 MPa. A phase diagram was constructed under this condition. Crystal growth of NdP₅O₁₄ were tried under the equilibrium condition. Diamond-shaped crystals 0.3 mm in size were grown at 500°C for 4 days under 100 MPa. Crystals tended to show aggregation and surface roughness at lower growth temperatures. The crystals with polyhedral shape and maximum 0.5 mm in size were grown at 650°C.