Phase relations, solubility and growth of potassium titanyl phosphate, KTP

Potassium titanyl phosphate is an unusually good nonlinear optical material whose use is constrained because of the difficulty of growing crystals. Promising new hydrothermal conditions for growth have been discovered. The yield of the reaction: KH₂PO₄+TiO₂ → KTiOPO₄+H₂O is used to deduce the hydrothermal phase diagram KPO₃-TiO₂-H₂O between 500 and 600°C at 10 kpsi in the presence of the mineralizer, 1m K₂HPO₄. It is shown that KTiOPO₄, "KTP", is congruently saturating above 600°C and crystallizes as the sole solid in an excess of KPO₃ below 600°C. Solubility is appreciable above 600°C. Phase equilibria and solubility results are used to find growth conditions (2m K₂HPO₄, 70% fill, 375-425°C) with rates of KTP crystallization from a new mil/day to ∼27 mil/day (∼0.68 mm/day). Good quality crystal growth takes place at the lower rates. These results hold promise of practical growth at mild conditions where low carbon steel, comparatively inexpensive autoclaves can be used.