Unraveling the symmetry of Al₅C₃N

High-temperature ceramic compound Al₅C₃N belongs to the scarcely studied Al-C-N system. It was originally reported as an ordered compound in noncentrosymmetric 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 noncentrosymmetric space group P6₃mc 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 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 bandgaps, which leads to implications for the physical properties. Finally, STEM analysis provides additional evidence that the crystal structure of Al₅C₃N is better described in centrosymmetric space group P6₃/mmc.