PHYSICAL REVIEW B 74, 205102 (2006) 1 Electronic structure and chemical bonding in Ti 4 SiC 3 investigated by soft x-ray emission spectroscopy and first principle theory M. Magnuson 1 , M. Mattesini 1,4 , O. Wilhelmsson 2 , J. Emmerlich 3 , J. -P. Palmquist 2 , S. Li 1 , R. Ahuja 1 , L. Hultman 3 , O. Eriksson 1 and U. Jansson 2 1 Department of Physics, Uppsala University, P. O. Box 530, S-751 21 Uppsala, Sweden. 2 Department of Materials Chemistry, The Ångström Laboratory, Uppsala University, P.O. Box 538 SE-75121 Uppsala. 3 Department of Physics, IFM, Thin Film Physics Division, Linköping University, SE-58183 Linköping, Sweden. 4 Departamento de Física de la Tierra, Astronomía y Astrofísica I, Universidad Complutense de Madrid, E-28040, Spain Abstract The electronic structure in the new transition metal carbide Ti 4 SiC 3 has been investigated by bulk- sensitive soft x-ray emission spectroscopy and compared to the well-studied Ti 3 SiC 2 and TiC systems. The measured high-resolution Ti L, C K and Si L x-ray emission spectra are discussed with ab initio calculations based on density-functional theory including core-to-valence dipole matrix elements. The detailed investigations of the Ti-C and Ti-Si chemical bonds provide increased understanding of the physical properties of these nanolaminates. A strongly modified spectral shape is detected for the buried Si monolayers due to Si 3p hybridization with the Ti 3d orbitals. As a result of relaxation of the crystal structure and the charge-transfer from Ti (and Si) to C, the strength of the Ti-C covalent bond is increased. The differences between the electronic and crystal structures of Ti 4 SiC 3 and Ti 3 SiC 2 are discussed in relation to the number of Si layers per Ti layer in the two systems and the corresponding change of materials properties. 1 Introduction Ternary carbides and nitrides, also referred to as MAX-phases, denoted M n+ 1AX n , where n=1, 2 and 3 that we will refer to as 211, 312 and 413, respectively, have recently been the subject to intense research [1, 2, 3]. Here, M denotes an early transition metal, A is a p-element, usually belonging to the groups IIIA and IVA, and X is either carbon or nitrogen [4]. These nanolaminated-layered materials exhibit a unique combination of metallic and ceramic properties, including high strength and stiffness at high temperatures, resistance to oxidation and thermal shock, as well as high electrical and thermal conductivity [5]. The macroscopic properties are closely related to the underlying electronic structure and the structural properties of the constituent atomic layers. The MAX-phase family of compounds (over 50 variants are energetically stable) has a hexagonal structure with near close-packed layers of the M-elements interleaved with square-planar slabs of pure A-elements, where the X-atoms are filling the octahedral sites between the M-atoms. The A-elements are located at the center of trigonal prisms that are larger than the