Phase equilibria in the system Nd₂O₃-P₂O₅-H₂O and growth of NdP₅O₁₄ single crystals under hydrothermal conditions

Phase equilibria in the system Nd₂O₃-P₂O₅-H₂O were determined under hydrothermal conditions at 500 and 350°C under 100 MPa by quenching techniques. There exist three types of neodymium phosphates: NdPO₄, NdP₃O₉ and NdP₅O₁₄, and a liquid phase of highly condensed phosphoric acid at both temperatures. The composition of the liquid phase with which NdP₅O₁₄ and NdP₃O₉ or NdP₃O₉ and NdPO₄ could coexist at 500°C was 3%Nd₂O₃-80%P₂O₅-17%H₂O or 4%Nd₂O₃-74%P₂O₅ -22%H₂O respectively, while that at 350°C changed to 2%Nd₂O₃-78%P₂O₅-20%H₂O or 2%Nd₂O₃-75%P₂O₅-23%H₂O respectively. These results indicate that the solubility curve of NdP₅O₁₄ had a positive slope against temperature, which allowed us to apply a temperature gradient method for the crystal growth of NdP₅O₁₄. Polyhedral crystals of 0.5 mm in maximum size were grown at the upper part of a gold capsule (T{reversed tilde equals}400°C) through the transportation due to ΔT=100°C after 10 days. These crystals had flat and smooth surfaces and contained less bubble-shaped inclusions than crystals synthesized under isothermal conditions. Crystals obtained at the lower part of the gold capsule (T=500°C) were polyhedral 1 mm in maximum size. Growth experiments with ΔT=50°C yielded no crystals at the upper part in the capsule (T{reversed tilde equals}450°C), probably due to an insufficient transportation.