Electronic structure of cubic and rhombohedral tantalum carbide phases: DFT calculations and X-ray photoelectron and X-ray emission spectroscopy measurements

Abstract

Transition metal (TM) carbides are generally nonstoichiometric compounds crystallized within high-symmetry FCC and HCP structures. Carbides based on refractory metals of the IVb - VIb groups are of great importance for scientists and engineers because of their possibility to form intermediate phases in the region of compounds between MC phases (which crystallize in the FCC structure) and M₂C phases with HCP structure. Such intermediate carbide phases are superstructures. Unit cells of these superstructures are described in hexagonal or rhombohedral (trigonal) systems and are characterized by large values of hexagonal lattice parameter ratio [1]. In the present work we study from both experimental and theoretical approaches the electronic structure of cubic TaCₓ phases (x = 0.98 and 0.75) and rhombohedral Ta₄C₃ (C/M=0.74) and Ta₃C₂ (C/M=0.67) phases synthesized following the technique [2]. In particular, we have measured energy distributions of the C 2p- and Ta 5d-electronic states and determined experimentally the valence-band structures employing X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPS) measurements, respectively. Furthermore, the present XES and XPS data are verified theoretically by first-principles calculations of total and partial densities of states that are made using the Quantum Espresso software package. We have achieved good correspondence between the experimental and theoretical data in the present work. The present results indicate that, in the sequence TaC₀.₉₈→TaC₀.₇₅→Ta₄C₃→Ta₃C₂ the main maximum of the XPS valence-band spectra shift toward the Fermi energy. In all studied tantalum carbides, a strong hybridization between Ta 5d- and C 2p-electronic states is detected and the charge transfer occurs from tantalum atoms to carbon atoms.