Mendeleev Communications Electronic Version, Issue 4, 2001 (pp. 125–164) The electronic structure, chemical bonding and ionic conductivity of Li6MoN4 and Li6WN4 Veronika M. Zainullina,*a Vladlen P. Zhukova and Vladlen H. Tammb a Institute of Solid State Chemistry, Urals Branch of the Russian Academy of Sciences, 620219 Ekaterinburg, Russian Federation. Fax: +7 3432 74 4495, e-mail: Veronika@ihim.uran.ru b Institute of the High-Temperature Electrochemistry, Urals Branch of the Russian Academy of Sciences, 620219 Ekaterinburg, Russian Federation 10.1070/MC2001v011n04ABEH001447 The calculations of the electronic structure, chemical bonding and energy of transition for lithium ions in Li6MoN4 and Li6WN4 have been carried out using the LMTO method; a possible model of lithium transport for these crystals has been suggested.The binary nitrides of lithium and transition metals (Cr, Mo and W) possess a high ionic conductivity and stability against melted lithium. Therefore, they are promising materials for lithium cells. However, the transport properties of these compounds have been studied insufficiently. For a better understanding of ionic conductivity, we evaluated the electronic structure and chemical bonding for Li6MoN4 and Li6WN4 by the first-principle linear muffin-tin orbital method in the tight-binding representation (LMTO-TB)1 and the semiempirical extended Hückel method (EH).2 The results of calculations have been used to analyse the mechanism of lithium ion migration in the anti-fluorite structure of these nitrides.The calculations of the electronic structure and total energy of Li6MoN4 and Li6WN4 crystals have been carried out by the LMTOTB method. We have employed a tetragonal unit cell (space group P42/nmc, Z = 2) with 20 atoms per cell: Li12M2N6E10, where M = Mo, W; E are empty spheres. The LMTO-ASA method has a higher precision for closely packed crystals; therefore, in our calculations additional spheres (empty spheres) with an s-orbital basis have been introduced into interstitial positions.These spheres were located at the octahedral and tetrahedral interstitial positions. The experimental lattice constants a = 6.673 and 6.679 Å, c = 4.925 and 4.927 Å for Li6MoN4 and Li6WN4, respectively, have been used.3 The optimised lattice constant for pure Li6MoN4 was 4.1% bigger than the experimental value.The valence 2s-, 2p-states of lithium and nitrogen; the ns-, np-, (n – 1)d-states of Mo, W, with n = 5, 6 and the s-states of empty spheres E were included in the atomic orbital basis used to construct the Bloch functions of crystals. The calculations were fulfilled for 128 k-vectors in the first Brillouin zone (30 k-vectors in the irreducible wedge).We found that the electronic structures of Li6MoN4 and Li6WN4 are close to each other. The total and partial densities of states for a Li6MoN4 phase are presented in Figure 1. The separation of the electronic energy spectrum into four zones is observed. A low-energy band A at about –15.0 eV was attributed to the 2s-states of nitrogen. The next band B is a band of hybrid N 2p and Mo 4d-states with some contributions of Li 2s-, 2p-states.The calculations of the overlap population of crystalline orbitals for the Mo–N bond showed that the antibonding partner of the band B is located at the bottom of the conductivity zone, which is a band with an energy of ~1.6 eV. The valence states in the range from –5.2 to –2.6 eV (band C) are hybridised N 2p and 2s-, 2p-states of Li, with the admixtures of Mo 4d-states.The presence of a forbidden gap confirms a semiconducting character of conductivity detected experimentally for similar phases.4 The main characteristics of the electronic energy spectrum for Li6MoN4 and Li6WN4 phases are presented in Table 1. The ab initio band calculations of the electronic structure for binary lithium nitrides were not studied previously.However, these results agree in the main features with the band structure for LiN3 obtained by Blaha and co-authors5 using a linearised augmented plane wave (LAPW) method. There are differences in the positions of the N-2s and N-2p bands and in their widths. The band appears, which consists of hybrid N 2p and Mo 4d-states. We also calculated the indices of chemical bonding for the considered compounds using the extended Hückel approach (Table 1).The averaged overlap population of crystalline orbitals is rather high for the Mo–N and W–N bonds. The Li–N interactions are characterised by a high degree of ionicity and an insignificant contribution of covalency. The low covalency of the Li–N bond corresponds to the high lithium mobility in the anti-fluorite structure.The investigations of the energy of defect formation using the ab initio LMTO calculations allowed us to offer a model of 30 20 10 20 10 20 10 20 10 0 –15 –10 –5 0 5 E/eV A B CE F total 2s,2p-N 2s,2p-Li 4d-Mo N(E)/eV–1 Figure 1 The (a) total and (b)–(d) partial densities of states [N(E)] for a Li6MoN4 crystal. (a) (b) (c) (d) Table 1 The characteristics of the electronic structure and chemical bonding in the crystals of Li6MoN4 and Li6WN4.Characteristic Li6MoN4 Li6WN4 Width and centre of band A/eV 1.45; –14.62 1.79; –14.80 Width and centre of band B/eV 1.21; –6.07 1.68; –5.79 Width and centre of band C/eV 2.66; –3.95 2.30; –3.63 Width of a forbidden gap/eV 2.87 3.42 Averaged bond populations by Mulliken M–N, M=Mo, W Li–N 0.837 0.047 0.849 0.046 O T N Li2 Li1 Mo Li1 Li2 Li2 Li2 Figure 2 The structure of a perfect crystal of Li6MoN4.Arrows show possible transitions for lithium ions into octahedral interstitial position (O) and through structure tetravacancies (T).Mendeleev Communications Electronic Version, Issue 4, 2001 (pp. 125.164) the ionic transport in Li6MoN4 and Li6WN4.The transport of the lithium ions is possible through the octahedral and tetrahedral interstitial positions in the anti-fluorite structure of these phases. The scheme of possible transport of the two crystallographically nonequivalent lithium atoms (Li1, Li2) is given in Figure 2. It is known3 that the lithium atoms Li1 have no tetravacancies in the nearest environment. Therefore, the Frenkel defect formation energy was calculated for the transport of Li2 into a tetraposition only.This energy was determined as the difference between the total energy of a perfect crystal (when a lithium atom is in a normal position) and the energy of a defect crystal (the lithium atom is located in octahedral or tetrahedral interstitial positions). The calculated energies are presented in Table 2.They show that the jump of lithium ions into a tetrahedral position requires an energy smaller by about 1.3 eV, than the jump of a lithium ion into octahedral positions. For both phases, the energy of transition of Li2 from a normal position into an octahedral position is a little lower than that of Li1. These energies, averaged over Li1 and Li2 atoms, within the limit of the errors of the method (0.1 eV) are practically identical for the Li6MoN4 and Li6WN4 crystals. Thus, the ab initio calculations of the electronic structure and energy of transition allow us to suggest a possible mechanism of lithium transport for the above crystals.The migration of lithium ions is more probable through the tetrahedral position, whereas the migration through the octahedral interstitial positions should be excluded.References 1 (a) O.-K. Andersen and O. Jepsen, Phys. Rev. Lett., 1984, 53, 2571; (b) O.-K. Andersen, Z. Pawlowska and O. Jepsen, Phys. Rev. B, 1986, 34, 5253. 2 M.-H. Whangbo and R. Hoffmann, J. Am. Chem. Soc., 1978, 100, 6093. 3 A. Gudat, S. Haag, R. Kniep and A. Rabenau, Z. Naturforsch., 1990, 45b, 111. 4 N. N. Batalov, O. V. Zheltonozhko, S. N. Zarembo, T. M. Akhmetzyanov, O. V. Volkova, G. V. Zelutin, V. P. Obrosov and V. K. Tamm, Elektrokhimiya, 1995, 31, 394 (Russ. J. Electrochem., 1995, 31, 356). 5 P. Blaha, J. Redinger and K. Schwarz, Z. Phys. B. Condenced Matter, 1984, 57, 273. Table 2 The energy of transition for Li1 (.Eoct1) and Li2 (.Eoct2) into an octahedral position and that for Li2 into a tetrahedral interstitial position (.Etetr2) in Li6MoN4 and Li6WN4 crystals. Crystal .Eoct1/eV .Eoct2/eV .Eaverage .Etetr2/eV Li6MoN4 4.77 4.47 4.62 3.16 Li6WN4 4.99 4.21 4.60 3.37 Received: 5th March 2001; Com. 01/1773