Three quinary compounds are formed in the In–H–O–P–N system. (NH 4 )In(OH)PO 4 crystallizes in the tetragonal structure with the lattice parameters a = 941.6 ± 0.2, c = 1,115.9 ± 0.3 pm and a calculated density of 3.288 g cm−3 (Mao et al. 2002a). It was synthesized under mild hydrothermal conditions. A mixture of H3BO3 (0.155 g), 85% H3PO4 (1 mL), 38% (NH4)OH (1 mL), and In (0.115 g) dissolved in 1 mL of 18% HCl (molar ratio of In/B/P = 1:2.5:8) was added to 1.0 mL of distilled water. The mixture was sealed in a glass tube. The filling of the solution was about 30% of the tube volume. The glass tube was placed in an oven and the temperature was increased slowly to 135°C and reacted for 10 days before cooling to room temperature. The reaction was under autogeneous pressure. The colorless ball-shaped crystals of the title compound were grown.