Density Functional Study of Calcium Nitride: Refined Geometries and Prediction of High-Pressure Phases

The high-pressure behavior of Ca 3 N 2 is studied up 100 GPa using density functional theory. Evaluation of many hypothetical polymorphs of composition A 3 X 2 leads us to propose four high-pressure polymorphs for both α- and β-Ca 3 N 2: (1) an anti-Rh 2 O 3 −II structure at 5 GPa, (2) an anti-B-sesquioxide structure at 10 GPa, (3) an anti-A-sesquioxide structure at 27 GPa, and (4) a hitherto unknown hexagonal structure ( P 6 3 / mmc ), derived from the post-perovskite structure of CaIrO 3, at 38 GPa. The development of the density and bulk modulus under pressure has been examined.