(NH ₄ )[B ₄ O ₅ (PO ₄ )] – A Boron Rich Three-Dimensional Borophosphate

(NH₄)[B₄O₅(PO₄)] was obtained as crystalline colorless powder via a solid state reaction in a closed system by using NH₄H₂PO₄ and B₂O₃ at 400 °C under vacuum conditions. Single-crystal X-ray diffraction revealed a new structure type (Pbca, no. 61, Z = 8, a = 8.1998(19), b = 13.3359(17), c = 13.4682(16) Å, R₁ = 0.0472, wR₂ = 0.1289, 2121 data, 149 parameters). It comprises a three-dimensional anionic framework built up by [PO₄], [BO₄] and [BO₃] units with charge balancing ammonium cations in the resulting one-dimensional channels. It represents only the third three-dimensional (3D) mixed-coordinated borophosphate revealing a new fundamental building unit. The titled compound is characterized by X-ray diffraction and electrostatic calculations. Besides, optical and thermal properties are investigated by FT-IR, UV-vis and thermogravimetry.