β‐Ca3N2, a Metastable Nitride in the System Ca–N

A new page in the phase book: The metastable nitride β‐Ca3N2, the synthesis, crystal structure (see picture), and physical properties of which are reported, is an isotype of corundum (α‐Al2O3). Vacancies in the crystal structure of β‐Ca3N2 are discussed by using full‐potential local orbital methods and taking into account both the physical properties and the density of states. magnified image The structural chemistry of calcium nitrides is represented by just two different intermediate phases: cubic Ca3N2 (referred to as α‐Ca3N2, Mn2O3 type structure) and Ca2N (CdCl2 type structure). Other binary phases (“γ‐Ca3N2”, “Ca11N8”) have been proven to represent cyanamide nitrides, Ca4N2[CN2] and Ca11N6[CN2]2, respectively. The metastable nitride β‐Ca3N2 is prepared either by reaction of freshly distilled calcium metal with nitrogen at 700 K, or by reaction of Ca2N with N2 at 500 K. β‐Ca3N2 transforms monotropically (Ttrans≈810 K) to cubic α‐Ca3N2. The crystal structure of β‐Ca3N2 (rhombohedral R3 c, no. 167, a=619.884(3) pm, c=1662.958(10) pm) is an isotype of the corundum structure (α‐Al2O3) and comprises a network of NCa6/4 octahedra that share common faces and edges. Magnetic susceptibility and electrical resistivity measurements confirm that β‐Ca3N2 is a diamagnetic semiconductor.