Mendeleev Communications Electronic Version, Issue 1, 1999 (pp. 1–44) Electronic structure and hybridization effects in hypothetical orthorhombic carbon oxynitride Alexander L. Ivanovskii,* Nadezhda I. Medvedeva and Gennady P. Shveikin Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 620219 Ekaterinburg, Russian Federation. Fax: +7 3432 74 4495; e-mail: ivanovskii@ihim.uran.ru The quantum-chemical examination of the electronic structure and hybridization of the valence states in the orthorhombic Si2N2O and hypothetical C2N2O compounds suggests that the cohesive properties can be improved by the substitution of carbon for silicon in a silicon oxynitride crystal. Sialons (SiAlONs) are solid solutions with the general formula Si6 – xAlxOxN8 – x.1,2 A large class of sialon ceramic materials, which are of crucial technological and commercial importance, is presently known.3–5 Intensive studies are carried out in the field of modification of unique properties of sialon materials combining hardness, high abrasive ability, wear resistance, thermal stability, chemical inertness, etc.1–5 A serious effort was made to search for optimum composition and microstructure of ceramics whose functional properties depend on the grain characteristics (SiAlONs) and are controlled by the morphology, structure and chemical composition of the grainboundary phase.3–5 As a preliminary, it is reasonable to consider the microscopic properties of simple compounds of this system such as AlN, b-Si3N4, Si2N2O.One of the possible ways to optimise the cohesive properties of the Si2N2O phase can be isovalent substitution of carbon atoms for silicon in its lattice.Recent theoretical results predicting extreme thermomechanical properties of the crystalline carbon nitride C3N4 with the basic structure b-Si3N4 allow this supposition (see ref. 6 for a review). In this work, we report on a quantum-chemical study of the electronic structure, chemical bonding and cohesive energy of the orthorhombic Si2N2O and the hypothetical isostructural phase C2N2O.The calculations were carried out by the self-consistent ab initio full-potential linear muffin-tin orbital method (FLMTO).7 The lattice parameters of Si2N2O corresponded to those given in ref. 8. The unit cell included four formula units.The FLMTO calculations were performed in a scalar relativistic version; Si 3s, 3p, 3d and (C,N,O) 2s, 2p orbitals were inserted into the basis as valent states. To take into account the lattice relaxation effect in C2N2O, which is associated with differences in the atomic radii of Si and C, the total energy (Etot) of the crystal was calculated as a function of the unit cell volume (V).The miminum Etot was obtained at V/V0 = 0.80 (V0 is the volume of a Si2N2O cell), which corresponds to the mean decrease in interatomic distances in the hypothetical compound C2N2O by ~7%. Total and local densities of states for Si2N2O and C2N2O are given in Figures 1 and 2. The energy spectrum of Si2N2O (Figure 1) contains three valence bands. The lower energy band is composed predominantly of O 2s states with an admixture of Si 3s states, and the next band includes the contributions of N 2s and Si 3s,3p states.The upper valence band exhibits a mixed character and contains hybridized p states of Si, N and O. The conduction band is made up of antibonding N 2p, O 2p and Si 3p,3d states. The three valence bands are separated by the forbidden gaps DEv, DE'v and DEg (between the valence band and the conduction band) (Table 1).The results of our FLMTO computations are in good agreement with the recent calculations of Si2N2O performed by the OLCAO method.9 The main differences in the electronic spectrum of C2N2O are attributed to a noticeable widening of the basic valence bands of this hypothetical compound, which reflects an increase in the degree of hybridization of p states of the constituting atoms (Figure 2).For the equilibrium volume V/V0 = 0.80, the forbidden gaps DEv and DE'v disappear; the DEg value decreases by ~37% (Table 1). Table 1 presents the cohesive energies (Ecoh) of silicon and carbon oxynitrides. The cohesive energy is calculated as Ecoh = = Etot – SEat, where Etot is the total energy of the crystal and Eat is the energy of free atoms constituting its sublattices.7 The maximum Ecoh value corresponds to Si2N2O.When lattice ‘compression’ occurs, Ecoh of C2N2O dramatically increases approaching the cohesive energy of silicon oxynitride at V/V0 = = 0.80 (Table 1). The Ecoh value is an integrated characteristic of the chemical bonding energy and describes the total effect of interatomic interactions.According to our computations, the chemical bonding in silicon and carbon oxynitrides is of a combined covalent-ionic aV is the cell volume of the calculated oxynitrides; V0 is the cell volume of the real Si2N2O phase.8 Table 1 Charges in the muffin-tin spheres (q, e), cohesion energies Ecoh (eV atom–1) and forbidden band gaps (eV) for Si2N2O and C2N2O.Si2N2O C2N2O C2N2 O V/V0 a 1.00 1.00 0.80 Si, C qs 0.62 1.19 1.02 qp 0.87 1.87 1.92 qd 0.34 — — N qs 0.85 0.88 0.68 qp 1.89 1.59 1.49 O qd 0.99 1.05 0.85 qp 2.39 2.18 1.98 –Ecoh 7.08 5.99 6.89 DEv 0.95 — — DE'v 2.83 0.64 — DEg 3.24 1.14 2.04 Total Si N O 100 0 10 0 20 0 20 0 –2 –1 0 1 E/Ry DOS/Ry–1 Figure 1 Total and local densities of states of the orthorhombic Si2N2O.Mendeleev Communications Electronic Version, Issue 1, 1999 (pp. 1–44) type. When substituting C for Si, the charges in the muffin-tin spheres of N and O decreased and those in the muffin-tin spheres of carbon increased as compared to silicon. The above effect of p–p hybridization strengthening (covalent bonding) for C2N2O is readily illustrated by the maps of valence charge densities (Figure 3).It can be seen that the degree of hybridization of C–N bond states increases in comparison with Si–N. It also follows from Figure 3 that the charge density distribution (CDD) is essentially non-spherical with different degrees of CDD delocalization along the directions of particular interatomic bonds (Si–N, Si–O, C–N, etc.). The channels (along the c axis) with low CDD are responsible for anisotropic properties of oxynitrides.In our calculations, the lattice relaxation for C2N2O was simulated by uniform compression of a cell of the initial Si2N2O crystal, i.e., all interatomic distances decreased simultaneously. However, in real situations, we usually have low concentrations of an impurity, which results in local distortions around the dopant atoms preserving the crystal structure as a whole.Such local displacements (relaxation) of the nearest atoms around the impurity will lower the total energy and, therefore, increase the absolute value of the cohesive energy. Based on the performed analysis of hybridization effects, we can anticipate that the cohesive energy of Si2N2O with a C dopant will increase owing to stronger hybridization of covalent C–N and C–O bond states.These tentative results of the quantum-chemical simulation of the hypothetical carbon oxynitride allow us to suppose that the doping of the orthorhombic Si2N2O structure will improve the cohesive properties of Si2 – xCxN2O as compared with the isoelectronic Si2N2O. We also suggest that partial or complete isovalent substitution of C for Si in the lattice of sialons can be an efficient way of modifying the functional properties of SiAlON-based ceramic materials, and theoretical and experimental studies in this field can be of great practical importance for the development of a new class of non-metallic ceramic materials based on the C–Al–O–N system.This work was supported by the Russian Foundation for Basic Research (grant no. 98-03-32512). References 1 T. Ekstroem and M. Nygren, J. Am. Ceram. Soc., 1992, 75, 259. 2 R. Metselaar, J. Eur. Ceram. Soc., 1998, 18, 183. 3 Y.-B. Cheng and J. Drennan, J. Am. Ceram. Soc., 1996, 79, 1314. 4 J. E. Gilbert and A. Mosset, Mater. Res. Bull., 1997, 32, 1441. 5 R. N. Katz, Industrial Ceram., 1997, 17, 3. 6 (a) D. M. Teter, MRS Bull., 1998, 1, 22; (b) B.I. Korsounskii and V. I. Pepekin, Usp. Khim., 1997, 66, 901 (Russ. Chem. Rev., 1997, 66, 1003). 7 M. Methfessel and M. Scheffler, Physica, 1991, B172, 175. 8 S. R. Srinivasa, L. Curtz, J. Jorgensen, T. G. Worlton, R. Beyerlein and M. Billy, J. Appl. Crystallogr., 1977, 10, 146. 9 Y.-N. Xu and W. Y. Ching, Phys. Rev., 1995, B51, 17379. Total C N O 100 0 10 0 20 0 20 0 –2 –1 0 1 E/Ry DOS/Ry–1 Figure 2 Total and local densities of states of the hypothetical orthorhombic carbon oxynitride C2N2O (for the ‘equilibrium’ cell volume V/V0 = = 0.80, see the text). 0.5 0.4 0.3 0.2 0.1 0.0 –0.1 –0.2 –0.3 –0.4 –0.5 –0.5 –0.4 –0.3 –0.2 –0.1 0.0 0.1 0.2 0.3 0.4 0.5 Si N N C 0.5 0.4 0.3 0.2 0.1 0.0 –0.1 –0.2 –0.3 –0.4 –0.5 –0.5 –0.4 –0.3 –0.2 –0.1 0.0 0.1 0.2 0.3 0.4 0.5 (b) (a) Figure 3 Maps of the valence charge density distribution in the plane (yz) of (a) Si2N2O and (b) C2N2O cells. FLMTO calculations. Received: Moscow, 3rd August 1998 Cambridge, 1st October 1998; Com. 8/06229C