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

The M(n+1)AX(n) 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 investiga ted by X-ray photoelectron spectroscopy. The results show that the constitu ent species are characterized by low binding energies, sometimes exceptiona lly so. The energies are 281.0-281.5 and 396.9 eV, respectively, for C 1s a nd N 1s, both of which are at, or below, the lowest values measured for car bides and nitrides. Similarly the Ti 2p energies, in the range 454.0-454.7 eV, are comparable to that of Ti metal and TIG, while the energy of the Al 2p, 72.0 eV, is lower by ca. 0.8 eV than that for Al metal. The signature o f 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 bindin g energy is correlated with dt:crease in average X-Ti bond length. The low binding energies of the A-type species, Al and Si, in planar coordination s uggest that binding and thus screening may be derived from out-of-plane int eractions. Results relevant to oxidation arising from exposure to air have been obtained. (C) 2001 Elsevier Science Ltd. All rights reserved.