Mixed Tin Valence in the Tin(II/IV)-Nitridophosphate Sn₃P₈N₁₆

Sn₃P₈N₁₆ combines the structural versatility of nitridophosphates and Sn within one compound. It was synthesized as dark gray powder in a high-pressure high-temperature reaction at 800 °C and 6 GPa from Sn₃N₄ and P₃N₅. The crystal structure was elucidated from single-crystal diffraction data (space group C2/m (no. 12), a=12.9664(4), b=10.7886(4), c=4.8238(2) Å, β=109.624(1)°) and shows a 3D-network of PN₄ tetrahedra, incorporating Sn in oxidation states +II and +IV. The Sn cations are located within eight-membered rings of vertex-sharing PN₄ tetrahedra, stacked along the [001] direction. A combination of solid-state nuclear magnetic resonance spectroscopy, ¹¹⁹Sn Mössbauer spectroscopy and density functional theory calculations was used to confirm the mixed oxidation of Sn. Temperature-dependent powder X-ray diffraction measurements reveal a low thermal expansion of 3.6 ppm/K up to 750 °C, beyond which Sn₃P₈N₁₆ starts to decompose.