Bonding profiling of gapless ceramic V2GaC/N MAX phases: a spectroscopic and dual theoretical approach

Abstract

The family of MAX phases is growing faster than ever, resulting in more than 340 reported members so far. Even though the variety of different M- and A-elements is overwhelming, the X-site of MAX phases is still mainly dictated by solely carbide-based materials, while nitrogen-based materials, such as nitrides and (carbo)nitrides account for below 10% in total. It follows that more profound studies comparing those three classical MAX phase groups are rare in the literature, particularly in terms of combining computational considerations with high-resolution spectroscopic experiments. Here, we report the electronic properties of three vanadium-based MAX phases: V₂GaC, V₂GaN, and the (carbo)nitride phase V₂GaC_1−xN_x. This investigation is carried out for C/N K-edge and the V L-edge using soft X-ray absorption (XAS) and emission spectroscopy (XES). We determine the x value to be 0.6 in the (carbo)nitride V₂GaC_1−xN_x phase. Additionally, we determine V^2.2+ as the formal oxidation state in all three phases using ligand field multiplet theory (LFMT) calculations. Our density functional theory (DFT) calculations indicate the presence of carbon vacancies in the V₂GaC phase. DFT and LFMT theoretical methods confirm that the V₂GaC has a higher degree of covalency than the V₂GaN phase. Also, Gallium interactions are the weakest with C/N and are entirely metallic.