The Crystal Structure of Al₄SiC₄ Revisited

Al₄SiC₄ is a ternary wide-band-gap semiconductor with a high strength-to-weight ratio and excellent oxidation resistance. It consists of slabs of Al₄C₃ separated by SiC layers with the space group of P6₃mc. The space group allows Si to occupy two different 2a Wykoff sites, with previous studies reporting that Si occupies only one of the two sites, giving it an ordered structure. Another hitherto unexplored possibility is that Si can be randomly distributed on both 2a sites. In this work, we revisit the published ordered crystal structure using experimental methods and density functional theory (DFT). Al₄SiC₄ was synthesized by high-temperature sintering at 1800 °C from a powder mixture of Al₄C₃ and SiC. Neutron diffraction confirmed that Al₄SiC₄ crystallized with the space group of P6₃mc, with diffraction patterns that could be fitted to both the ordered and the disordered structures. Scanning transmission electron microscopy, however, provided clear evidence supporting the latter, with DFT calculations further confirming that it is 0.16 eV lower in energy per Al₄SiC₄ formula unit than the former. TEM analysis revealed Al vacancies in some of the atomic layers that can introduce p-type doping and direct band gaps of 0.7 and 1.2 eV, agreeing with our optical measurements. Finally, we propose that although the calculated formation energy of the Al vacancies is high, the vacancies are stabilized by entropy effects at the high synthesis temperature. This indicates that the cooling procedure after high-temperature synthesis can be important in determining the vacancy content and the electronic properties of Al₄SiC₄.