Work

Unraveling the symmetry of Al 5 C 3 N

Year: 2026

Type: article

Abstract: High-temperature ceramic compound Al 5 C 3 N belongs to the scarcely studied Al–C–N system. It was originally reported as an ordered compound in noncentrosymmetric space group P 6 3 mc and described as a nanolaminate with an –Al 2 C–AlN–Al 2 C 2 – stacking sequence. The recently reported structural disorder in the related compound Al 4 SiC 4 led us to question this proposed structure for Al 5 C 3 N and investigate the possibility of a disordered structure in the centrosymmetric space group P 6 3 / 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 Al 5 C 3 N crystal structure. Our single-crystal X-ray diffraction analysis clearly indicates that noncentrosymmetric space group P 6 3 mc must be rejected. From a joint refinement of single-crystal X-ray and neutron powder diffraction data, the occupancies of C and N were refined at two sites in P 6 3 / mmc resulting in the stacking sequence –Al 2 C–Al(C/N)–Al 2 (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 P 6 3 mc structure. The calculated band structure shows both direct and indirect bandgaps, which leads to implications for the physical properties. Finally, STEM analysis provides additional evidence that the crystal structure of Al 5 C 3 N is better described in centrosymmetric space group P 6 3 / mmc .

Source: Acta Crystallographica Section B Structural Science Crystal Engineering and Materials

DOI: 10.1107/s2052520626002544