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

Abstract

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 R-and -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 (P6 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.

Group II element nitrides M3N2 under pressure: a comparative density functional study

“…It is the only compound for which an anti-post-perovskite highpressure phase is found (γ-Ba 3 N 2 ). However, the final highpressure phase found for Ba 3 N 2 is the same as for Ca 3 N 2 [13] and Sr 3 N 2 . They all adopt the hexagonal phase (space group P6 3 /mmc) derived from the post-perovskite structure.…”

“…1). Note that this follows the patterns of Be 3 N 2 , Mg 3 N 2 and Ca 3 N 2 [13]. Consequently, this hypothetical modification of Sr 3 N 2 -which still awaits its experimental realization -is termed α-Sr 3 N 2 .…”

“…In a previous work we have reported on high-pressure polymorphs of Ca 3 N 2 [13]. Compared to Mg 3 N 2 , Ca 3 N 2 exhibits two more high-pressure phases.…”

“…α-Mg 3 N 2 and β-Mg 3 N 2 exhibit the same B 0 of 109 GPa, whereas the bulk modulus of γ-Mg 3 N 2 is slightly smaller (104 GPa). B 0 of Ca 3 N 2 phases ranges from 45 GPa (λ-Ca 3 N 2 ) to 76 GPa (ε-Ca 3 N 2 ) [13], placing the bulk moduli of Ca 3 N 2 on average 35 GPa lower than those of the Mg 3 N 2 phases. The ambient-pressure bulk modulus for Sr 3 N 2 starts at 51 GPa for α-Sr 3 N 2 , increases to 55 GPa for β-Sr 3 N 2 , then goes down to 52 GPa for γ-Sr 3 N 2 and 32 GPa for γ′-Sr 3 N 2 , respectively, and finally adopts a value of 34 GPa for δ-Sr 3 N 2 .…”

“…Recently, we have reported on high-pressure phases of Ca 3 N 2 , calculated within density functional theory [13]. The enthalpy-pressure phase diagram revealed a phase sequence of anti-bixbyite, anti-Rh 2 O 3 -II (5 GPa), anti-B-sesquioxide (10 GPa), anti-A-sesquioxide (27 GPa) and a hexagonal structure with space group P6 3 /mmc, derived from the anti-post-perovskite structure (38 GPa).…”