Cr_5.7Si_2.3P₈N₂₄-A Chromium(+IV) Nitridosilicate Phosphate with Amphibole-Type Structure

The first nitridic analog of an amphibole mineral, the quaternary nitridosilicate phosphate Cr_5.7Si_2.3P₈N₂₄ was synthesized under high-pressure high-temperature conditions at 1400 °C and 12 GPa from the binary nitrides Cr₂N, Si₃N₄ and P₃N₅, using NH₄N₃ and NH₄F as additional nitrogen source and mineralizing agent, respectively. The crystal structure was elucidated by single-crystal X-ray diffraction with microfocused synchrotron radiation (C2/m, a=9.6002(19), b=17.107(3), c=4.8530(10) Å, β=109.65(3)°). The elemental composition was analyzed by energy dispersive X-ray spectroscopy. The structure consists of vertex-sharing PN₄-tetrahedra forming zweier double chains and edge-sharing (Si,Cr)-centered octahedra forming separated ribbons. Atomic resolution scanning transmission electron microscopy shows ordered Si and Cr sites next to a disordered Si/Cr site. Optical spectroscopy indicates a band gap of 2.1 eV. Susceptibility measurements show paramagnetic behavior and support the oxidation state Cr^+IV, which is confirmed by EPR. The comprehensive analysis expands the field of Cr-N chemistry and provides access to a nitride analog of one of the most prevalent silicate structures.