Cr 5.7 Si 2.3 P 8 N 24 -A Chromium(+IV) Nitridosilicate Phosphate with Amphibole-Type Structure.

The first nitridic analog of an amphibole mineral, the quaternary nitridosilicate phosphate Cr 5.7 Si 2.3 P 8 N 24 was synthesized under high-pressure high-temperature conditions at 1400 °C and 12 GPa from the binary nitrides Cr 2 N, Si 3 N 4 and P 3 N 5 , using NH 4 N 3 and NH 4 F 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 PN 4 -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.