Crystal-chemistry of the Ti3AlC2 and Ti4AlN3 layered carbide/nitride phases__characterization by XPS

The Mn+1AXn layered carbide/nitride-derived phases, where M is an early transition metal, A is an A-group element and X is N or C, have an unusual combination of mechanical, electrical and thermal properties. The surface and crystal-chemistries of two members with n=2 and 3 have been investigated by X-ray photoelectron spectroscopy. The results show that the constituent species are characterized by low binding energies, sometimes exceptionally so. The energies are 281.0-281.5 and 396.9eV, respectively, for C 1s and N 1s, both of which are at, or below, the lowest values measured for carbides and nitrides. Similarly the Ti 2p energies, in the range 454.0-454.7eV, are comparable to that of Ti metal and TiC, while the energy of the Al 2p, 72.0eV, is lower by ca. 0.8eV than that for Al metal. The signature of the XTi6 octahedral units in the stacking sequence is reminiscent of the corresponding units in TiN, and it is found that a decrease in Ti 2p binding energy is correlated with decrease in average X-Ti bond length. The low binding energies of the A-type species, Al and Si, in planar coordination suggest that binding and thus screening may be derived from outof-plane interactions. Results relevant to oxidation arising from exposure to air have been obtained.