Group II element nitrides M 3 N 2 under pressure: a comparative density functional study

The high‐pressure behavior of group II element nitrides M 3 N 2 (M=Be, Mg, Sr, and Ba) was studied up to 100 GPa and beyond. Evaluating a manifold of hypothetical polymorphs of composition A 3 X 2 leads to proposing a new high‐pressure polymorph of Be 3 N 2 with an anti‐A‐sesquioxide structure appearing at 82 GPa. Two high‐pressure phases were found for Mg 3 N 2 : first an anti‐B‐sesquioxide‐type structure should appear at 21 GPa followed by an anti‐A‐sesquioxide‐type structure at 65 GPa. While structure and true nature of Sr 3 N 2 and Ba 3 N 2 are not yet experimentally determined, we identified an anti‐bixbyite structure to be the ground state structure of Sr 3 N 2 and a distortion variant of the anti‐A‐sesquioxide‐type structure as lowest energy modification for Ba 3 N 2 . For Sr 3 N 2 the sequence of high‐pressure phases are (1) an anti‐Rh 2 O 3 ‐II structure appearing at 3 GPa, (2) an anti‐B‐sesquioxide structure becoming most stable at 12 GPa and (3) a hexagonal P 6 3 / mmc structure favored at 26 GPa. The development of the c / a ‐ratio of anti‐A‐sesquioxide Ba 3 N 2 under pressure was examined, revealing a gradual reduction under pressure. Three high‐pressure polymorphs are then further proposed for Ba 3 N 2 : (1) an anti‐Rh 2 O 3 ‐II structure at 2 GPa, (2) an anti‐CaIrO 3 structure at 32 GPa and (3) a hexagonal P 6 3 / mmc structure at 52 GPa. The results for all group II element nitrides M 3 N 2 were compared. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)