XPS, XES, and XAS studies of the electronic structure of substoichiometric cubic TaCx and hexagonal Ta2Cy carbides

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. The XPS core-level binding energies (BE's) and valence-band structures for TaCx over the range 0.36x0.98 have been determined. The XES bands and XAS edges in all studied compounds have been derived and compared in a single energy scale with the XPS valence-band spectra. In all tantalum carbides the Ta 5d-like and C 2p-like states are strongly hybridized, and the charge transfers from Ta atoms to C atoms. It was established that E1 was generally lowered by the introduction of C-vacancies in cubic tantalum carbides, and additional vacancy states appeared in the XPS valence-band spectra of these substoichiometric compounds. In the row TaC0.98→TaC0.75→TaC0.50→TaC0.36 the XPS Ta 4f and C 2s core-level BE's and main peaks of the XPS valence-band spectra shifted toward the Fermi level by 0.5 to 0.85 eV, while the C Is BE's remained constant (within the experimental error) for all studied substoichiometric tantalum carbides. In this row some shifts (0.6 to 0.9eV) of the main maxima of the XES Ta 5d-like and C 2p-like bands toward E1 are observed. In the mentioned row the intensity of the XPS valence-band spectra normalized on the peak intensity of the XPS Ta 4f7/2, 5/2 core-level spectra, decrease almost monotonously. The C Kα band half-width decreases by about 0.3 eV going from cubic TaCx carbides to hexagonal Ta2Cy carbides, but the C Kα band asymmetry index increases from 0.72 in TaC0.98 to about 1.75 in the carbides Ta2Cy. In all studied substoichiometric tantalum carbides the energy positions of the centres of gravity of the C 2p-like emission bands remain constant.