Crystal-chemistry from XPS analysis of carbide-derived Mn+1AXn (n=1) nano-laminate compounds

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 several recently synthesized n = 1 members 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 C 1s energies are in the lower end of the range for carbides, at 281.1-282.0 eV. The M-species-Ti, V, Nb and Hf-have binding energies at or below those corresponding to the elemental metallic state. The binding energies of the A-species in apparent planar coordination-In, Ge, As and Al-are quite exceptional, being 0.5-2 eV below those corresponding to the elemental state. Those results suggest that screening of the A-group species is derived from out-of-plane interactions, while the XPS signatures of the species associated with the MX blocks are reminiscent of those obtained from the relevant carbide phases. (C) 2002 Elsevier Science Ltd. All rights reserved.