Synthesis and Crystal Structure of Cubic Ca₁₆Si₁₇N₃₄

Since the late 1960s, the exact structure of cubic calcium silicon nitride has been a source of debate. This paper offers evidence that the cubic phase CaSiN2 described in the literature is actually Ca16Si17N34. Presented here is a method for synthesizing single crystals of cubic-calcium silicon nitride from calcium nitride and elemental silicon under flowing nitrogen at 1500 °C. The colorless millimeter-sized crystals of Ca16Si17N34 with a refractive index (n25) = 1.590 were found to be cubic (a = 14.8882 Å) and belong to the space group F4̅3m (216). The synthesis of bulk, powdered cubic-Ca16Si17N34 from calcium cyanamide and silicon is also discussed. Ca16Si17N34 is a relatively air-stable refractory ceramic. In contrast to the orthorhombic phase of CaSiN2, in which Ca2+ sits in octahedral sites, this cubic phase has Ca2+ in cubic sites that makes it an interesting host for new phosphors and gives rise to unique crystal field splitting.