Mendeleev Communications Electronic Version, Issue 1, 2001 (pp. 1.42) The electronic structure of the new cubic carbaboride NaB5C as compared to CaB6 and ¡®B4C2¡� by the full-potential LMTO method Alexander L. Ivanovskii* and Sergey V. Okatov Institute of Solid State Chemistry, Urals Branch of the Russian Academy of Sciences, 620219 Ekaterinburg, Russian Federation. Fax: + 7 3432 74 4495; e-mail: ivanovskii@ihim.uran.ru 10.1070/MC2001v011n01ABEH001393 The full-potential LMTO (FP LMTO) method was used for the first time to examine the electronic properties and chemical bonding of the new carbaboride NaB5C in comparison with isostructural and isoelectronic hexaboride CaB6 and ¡®B4C2¡�.Metal hexaborides (MB6) exhibit interesting physical and chemical properties. They are used as materials for high-temperature applications and are intensively studied both theoretically and experimentally.1,2 Extensive investigations of the modification of properties of binary hexaborides are under way.A conventional technique is the doping of the cation sublattice of MB6 with metal atoms (M' ). For example, ternary phases such as rare-earth metal.doped CaB6 (Ca1 .xLaB6,3 Ca1 . xEuxB6,4 etc.) are well known. An alternative way of changing the properties of binary hexaborides is the doping of their boron sublattice. A new group of ternary boron-rich compounds (NaB5C and KB5C) has been synthesised recently.5 They crystallise in a cubic (space group Oh 1.Pm3m) CaB6-like structure where carbon atoms statistically replace boron atoms in B6 octahedra.As distinct from metal-like NaB6 and KB6, these new compounds (carbaborides) are semiconductors.5 In this communication, we report the first findings of the electronic state investigations of a new phase, hexagonal NaB5C, and compare them with the electronic states of isostructural and isoelectronic phases of thoroughly studied CaB6,2,6.8 as well as with the hypothetical ¡®carbaboride¡� B4C2. The latter compound represents the structure model of a CaC6-like boride, in which the cation sublattice is ¡®empty¡� and one third of boron atoms in the anion sublattice is replaced by carbon atoms.The calculation of ¡®B4C2¡� makes it possible to follow the tendencies in changing the electronic spectra of hexaborides when the C/B ratio increases and to establish the role of cationic vacancies. It is well known that these lattice defects are present in some hexaborides.1,2 Recently, Mair et al.9 reported the synthesis of dilithium hexaboride. The structure of Li2B6 is of the CaB6 type with the incomplete occupancy of cationic positions.This favours the appearance of Li+-ionic conductivity. However, the role of cationic vacancies in the formation of the electronic spectra of hexaborides has not been studied until the present time.The electronic structure of CaB6, NaB5C and ¡®B4C2¡� was calculated by the self-consistent full-potential linear muffin-tin orbital method (FP LMTO)10,11 in the local electronic density approximation.12 The computational procedure was described in detail elsewhere.13 The lattice parameter corresponded to a(CaB6) = 7.8352 a.u.1 The energy bands and densities of states (DOS) of CaB6, NaB5C and ¡®B4C2¡� are given in Figures 1 and 2, and the parameters of electronic structures are listed in Table 1.The common valence band of CaB6 (width of ~1.07 Ry) contains 10 occupied energy bands. The lower quasi-core B 2slike band is separated from the group of nine hybrid B 2p,2sbands by a forbidden gap.The DOS profile of these bands has two maxima (B and B' in Figure 2) corresponding to hybrid states, which form covalent B.B sp-bonds inside and between B6 clusters. The first unoccupied band has a large wave-vector dependence, which reflects a delocalised character of the d-states of cations forming the lower edge of the conductivity band. This feature is typical of all CaB6-like hexaborides.2,6.8 The direct energy gap (.Eg) between the valence band and the conduction band (in the X point) is ~0.05 Ry (~0.7 eV).An essentially different type of the electronic spectrum was obtained for NaB5C (see Figures 1 and 2). Whereas the struc- 0.4 0.2 0.0 .0.2 .0.4 .0.6 .0.8 .1.0 .1.2 CaB6 0.4 0.2 0.0 .0.2 .0.4 .0.6 .0.8 .1.0 .1.2 NaB5C 0.4 0.2 0.0 .0.2 .0.4 .0.6 .0.8 .1.0 .1.2 ¡®B4C2¡� ¥Ã M X R ¥Ã Energy/Ry Energy/Ry Energy/Ry Figure 1 Energy bands of CaB6, NaB5C and ¡®B4C2¡�. Table 1 Electronic structure parameters of cubic CaB6, NaB5C and ¡®B4C2¡� (Ry).Parameter/ phase Forbidden gap (transition) Band widths Hybrid sp B 2s Energy gap (sp.B 2s) CaB6 0.048 (X ¢ç X) 0.664 0.080 0.328 NaB5C 0.120 (¥Ã ¢ç X) 0.792 0.056 0.304 ¡®B4C2¡� 0.125 (¥Ã ¢ç X) 0.992 0.041 0.261Mendeleev Communications Electronic Version, Issue 1, 2001 (pp. 1.42) ture of the lower edge of the conduction band of CaB6 and NaB5C remains generally the same (the replacement of a cation leads mainly to a decrease in the energy dispersion of the lower unoccupied band), the substitution of a carbon atom for a boron atom (in B6 clusters) radically alters the structure of the valence band.Figures 1 and 2 and Table 1 show that for NaB5C, as compared to CaB6, (i) the total width of the valence band and of the hybrid sp-band increases; (ii) the forbidden gap between B 2s- and sp-bands decreases; (iii) the widths of B 2s- and B sp-bands decrease; and (iv) new C 2s- (not shown in Figures 1 and 2) and C 2p-bands appear.The latter bands lie in the range from 0.8 to 0.4 Ry below the Fermi level (EF; peak A, Figure 2). The emergence of an anisotropic system of covalent B.B and B.C sp-bonds (inside and between B5C clusters, see Figure 3) disrupts the quasiatomic character of some bands. For example, the upper near-Fermi band becomes energetically dispersed along the ¥Ã.X direction (Figure 2). As a result, the type of interband transitions in CaB6 and NaB5C radically changes: instead of the direct gap in the hexaboride, the semiconducting state for NaB5C is characterised by an indirect gap [¥Ã ¢ç X transition; .Eg ~ 0.12 Ry (~1.63 eV)].The type of the electronic spectrum of the hypothetical ¡®B4C2¡� changes as well. The increase in the C/B ratio and the appearance of direct s.p C.C bonds (Figure 3) favour further growth of the total width of the valence band, near the lower edge of which isolated C 2p-like states (a group of DOS peaks in the range from 0.95 to 0.75 Ry below EF, Figure 2) emerge.The contribution of C 2p-states to the lower unoccupied band increases. The forbidden gap of ¡®B4C2¡� [indirect transition ¥Ã ¢ç X; .Eg ~ 0.13 Ry (~1.77 eV)] has an intermediate value between .Eg for the stable rhombohedral boron carbide B4C (with icosahedral B11C clusters) and the higher carbon-rich BC3 with a layered graphite-like structure.14 To draw a conclusion on the possible stabilization of ¡®B4C2¡�, it is necessary to solve correctly the equation of state for this phase.In summary, note that the imitation of the role of cation defects in the calculations of ¡®B4C2¡� with an ¡®empty¡� metallic sublattice makes it possible to arrive at the preliminary conclusion that no new occupied ¡®vacancy¡� states appear in the spectra of cation-deficient hexaborides (unlike, for example, nonstoichiometric transition metal carbides15).This can be seen in the electronic density maps of ¡®B4C2¡� (Figure 3).Qualitatively, this fact may be related to the distribution of valence states of anions (B, C), which are localised in the vicinity of nuclei and are not ¡®trapped¡� by the cation defect sphere. Hence, we can assume that the role of variable content of cation vacancies in metal hexaborides may be reduced to changes in electron concentration in the cell (i.e., to changes in the degree of near- Fermi band occupation as a futectable rearrangement of the energy spectrum structure.References 1 G. V. Samsonov, T. I. Serebrjakova and V. A. Neronov, Boridy (Borides), Atomizdat, Moscow, 1975 (in Russian). 2 G. P. Schveikin and A. L. Ivanovskii, Usp. Khim., 1994, 63, 751 (Russ. Chem. Rev., 1994, 63, 711). 3 T. Terashima, C.Terakura, Y. Umeda, N. Kimura, H. Aoki and S. Kunii, J. Phys. Soc. Jpn., 2000, 69, 2423. 4 S. Paschen, D. Pushin, M. Schlatter, P. Voltanthen, H. R. Ott, D. P. Young and Z. Fisk, Phys. Rev., 2000, B61, 4174. 5 B. Albert and K. Schmidt, Chem. Mater., 1999, 11, 3406. 6 H. Hasegawa and A. Yanase, J. Phys. C: Solid State Phys., 1979, 12, 5431. 7 H. Ripplinger, K. Schwarz and P.Blaha, J. Solid State Chem., 1997, 133, 51. 8 S. Massidda, A. Continenza, T. M. DePascale and R. Monnier, Z. Phys. B, Condens. Matter, 1997, 102, 83. 9 G. Mair, H. G. van Schering, M. Worle and R. Nesper, Z. Anorg. Allg. Chem., 1999, 625, 1207. 10 M. Methfessel, C. Rodriquez and O. K. Andersen, Phys. Rev., 1989, B40, 2009. 11 M. Methfessel and M. Scheffler, Physica B., 1991, 172, 175. 12 M. Methfessel, Phys. Rev., 1988, B38, 1537. 13 N. I. Medvedeva, D. L. Novikov, A. L. Ivanovskii, M. V. Kuznetzov and A. J. Freeman, Phys. Rev., 1998, B58, 16042. 14 A. L. Ivanovskii, Usp. Khim., 1997, 66, 511 (Russ. Chem. Rev., 1997, 66, 459). 15 V. A. Gubanov, A. L. Ivanovskii and V. P. Zhukov, Electronic Structure of Refractory Carbides and Nitrides, University Press, Cambridge, 1994. 100 80 60 40 20 0 100 80 60 40 20 0 100 80 60 40 20 0 A B B' EF C A EF EF (a) (b) (c) DOS (states/Ry cell) DOS (states/Ry cell) DOS (states/Ry cell) .1.2 .1.0 .0.8 .0.6 .0.4 .0.2 0.0 0.2 0.4 Energy/Ry Figure 2 DOS of (a) CaB6, (b) NaB5C and (c) ¡®B4C2¡�. B C Na Vac Na Na Na Vac Vac Vac C C (a) (b) Figure 3 Valence density distribution for (a) NaB5C and (b) ¡®B4C2¡�. Received: 31st October 2000; Com. 00/