Crystal-chemistry of the Ti 3 AlC 2 and Ti 4 AlN 3 layered carbide/ nitride phasesÐcharacterization by XPS S. Myhra a,b, * , J.A.A. Crossley a , M.W. Barsoum c a AEA Technology plc, Begbroke Business and Science Park, Sandy Lane, Oxford OX5 1PF, UK b School of Science, Grif®th University, Nathan, QLD 4111, Australia c Department of Materials Engineering, Drexel University, Philadelphia, PA 19104, USA Received 3 April 2000; accepted 25 September 2000 Abstract The M n11 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 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 396.9 eV, 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±454.7 eV, are comparable to that of Ti metal and TiC, while the energy of the Al 2p, 72.0 eV, is lower by ca. 0.8 eV than that for Al metal. The signature of the XTi 6 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 out-of-plane interactions. Results relevant to oxidation arising from exposure to air have been obtained. q 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction It has been shown recently that the layered carbide-like ternary compound Ti 3 SiC 2 has an unusual combination of electrical, mechanical and thermal properties [1±7]. The material is readily machineable, is resistant to thermal shock, and exhibits plasticity at high temperature. The phase is a member of a wider family of layered carbides and nitrides described by M n11 AX n with n  1, 2 where M is an early transition metal, A is an A-group element and X is C or N. Recently an n  3 member, Ti 4 AlN 3 , has also been synthesized [8], signaling the existence of a yet larger group, in analogy with the Ruddlesden±Popper family and the Auri- villius phases, which have multiple layers of perovskite blocks interleaved with planar sheets. Some of the layered high temperature superconductor compounds also exhibit structural similarities. The structures of the 3-1-2 (A  Si, X  C) and 4-1-3 (A  Al, X  N) members have been char- acterized by XRD and neutron diffraction [9±11]. Both are hexagonal and have the space group P6 3 /mmc, Fig. 1. The respective unit cell parameters are a  0.307 nm and c  1.769 nm, and a  0.2988 nm and c  2.3372 nm. The details of the 3-1-2 (A  Al, X  N) structure have yet to be documented, but are likely to be broadly similar to the other members of the family. Relatively little is known about the crystal-chemistry of the compounds. Likewise, surface properties have as yet not attracted much attention. The present study is intended to constitute a beginning on both endeavours. The principal objectives were to extend the data base to another recently synthesized member of the 3-1-2 group and to a member of the 4-1-3 group. Surface analytical investigations are particularly relevant to an explanation of the unusual tribo-properties of the 3-1-2 phase [12,13] and the observed resistance to high-temperature oxidation [14]. Journal of Physics and Chemistry of Solids 62 (2001) 811±817 0022-3697/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0022-3697(00)00268-7 www.elsevier.nl/locate/jpcs * Corresponding author. Tel.: 161-7-3875-7546; fax: 161-7- 3875-7656. E-mail address: s.myhra@sct.gu.edu.au (S. Myhra).