Unraveling the symmetry of Al5C3N

TL;DR

This study investigates the crystal structure of Al5C3N, revealing it is better described by a centrosymmetric disordered structure in space group P63/mmc rather than the previously proposed non-centrosymmetric P63mc.

Contribution

The paper provides experimental and computational evidence that challenges the original structure assignment of Al5C3N, proposing a disordered centrosymmetric model instead.

Findings

The non-centrosymmetric P63mc structure is rejected based on diffraction analysis.

Refined occupancies indicate a disordered stacking sequence in P63/mmc.

DFT calculations show the disordered structure has the lowest energy.

Abstract

The high-temperature ceramic compound Al5C3N with promising application usage belongs to the scarcely studied Al-C-N system. It was originally reported as an ordered compound in the non-centrosymmetric space group P63mc and described as a nanolaminate with an -Al2C-AlN-Al2C2- stacking sequence. The recently reported structural disorder in the related compound Al4SiC4 led us to question this proposed structure for Al5C3N and investigate the possibility of a disordered structure in the centrosymmetric space group P63/mmc. In the present work, we employed different synthesis routes to maximize the yield and quality of the desired phase, and applied a variety of techniques to probe the Al5C3N crystal structure. Our single-crystal X-ray diffraction analysis clearly indicates that the non-centrosymmetric space group P63mc must be rejected. From a joint refinement of single-crystal X-ray and powder neutron diffraction data, the occupancies of C and N were refined at two sites in P63/mmc resulting in the stacking sequence -Al2C-Al(C/N)-Al2(C/N)2-. Furthermore, DFT calculations show that a centrosymmetric disordered structure described in a supercell has the lowest energy, 0.2 eV per formula unit, relative to the previously reported P63mc structure. The calculated band structure shows both direct and indirect band gaps which lead to implications for the physical properties. Finally, STEM analysis provides additional evidence that the crystal structure of Al5C3N is better described in the centrosymmetric space group P63/mmc.