Interplay of metallic and ionic bonding in layered subnitrides AE2N (AE = Ca, Sr, or Ba) under high pressure

Layered alkaline earth-metal subnitrides AE(2)N (AE = Ca, Sr, or Ba) adopting the anti-CdCl2-type structure (space group R (3) over barm) were structurally investigated under pressures up to 40 GPa using diamond-anvil cells for pressure application and synchrotron radiation for collecting powder diffraction patterns. Anisotropic compressibility and structural changes under pressure were found to be consistent with the spatial separation of ionic and metallic bonding according to the formal description as (AE(2+))(2)N(3-)center dot e(-). Further increase of pressure was found to induce several phase transitions: in the case of Ba2N, in addition to a new layered anti-Cdl(2)-type modification (space group P (3) over bar ml), a cubic anti-Th3P4-type structure (I43d), and its distorted analogue were identified. The latter appears to be isostructural to the observed high-pressure phases of Ca2N and Sr2N.