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

Abstract

Layered alkaline earth-metal subnitrides AE 2 N ( AE = Ca, Sr, or Ba) adopting the anti- CdCl 2 -type structure (space group R <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mover> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>-</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> </mml:math> m ) 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– • e – . Further increase of pressure was found to induce several phase transitions: in the case of Ba 2 N, in addition to a new layered anti- CdI 2 -type modification (space group P <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mover> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>-</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> </mml:math> m 1), a cubic anti- Th 3 P 4 -type structure ( I <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mover> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>-</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> </mml:math> 3 d ), and its distorted analogue were identified. The latter appears to be isostructural to the observed high-pressure phases of Ca 2 N and Sr 2 N. .