Electronic structure of tantalum subcarbides studied by XPS, XES, and XAS methods

X-ray photoelectron (XPS), emission (XES), and absorption (XAS) spectroscopy methods have been used to study the electronic structure of cubic TaCx and hexagonal Ta2Cy (y=2x) carbides, and also rhombohedral zeta-Ta4C3 and epsilon-Ta3C2 phases. The XPS core-level binding energies (BEs) and valence-band structures for TaCx over the range 0.36 less than or equal to x less than or equal to 0.98 have been determined. It was established that E(f) was generally lowered by the introduction of C-vacancies in cubic tantalum carbides. The results of the XPS and XES studies indicate that the layered subcarbides zeta-Ta4C3 and epsilon-Ta3C2 are individual phases and are by no means a simple mixture of the cubic TaCx and hexagonal Ta2Cy carbides. In the layered subcarbides zeta-Ta4C3 and epsilon-Ta3C2 the formation of new (shortened) M-M bonds between the tantalum atoms neighbouring the {111} plane of the carbon sublattice, where the C-vacancies concentrate, results in the appearance of an additional peak (E(BE) approximate to 2 eV below E(f)) in the XPS valence-band spectra. In the row TaCx --> zeta-Ta4C3 --> epsilon-Ta3C2 --> Ta2Cy the XPS Ta 4f and C 2s core-level BEs and main peaks of the XPS valence-band spectra shifted toward the Fermi level by about 0.5 eV, while the C 1s BEs remained constant (within the experimental error) for all studied subcarbides. In this row some shifts of the main maxima of the XES Ta 5d-like and C 2p-like bands toward E(f) are observed. In all tantalum carbides the charge transfers from Ta atoms to C atoms.