, 2001, 11(4), 144–145
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.
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 semi-
empirical extended Hückel method (EH).2 The results of calcu-
lations have been used to analyse the mechanism of lithium ion
migration in the anti-fluorite structure of these nitrides.
Table 1 The characteristics of the electronic structure and chemical bond-
ing in the crystals of Li6MoN4 and Li6WN4.
Characteristic
Li6MoN4
Li6WN4
Width and centre of band A/eV
Width and centre of band B/eV
Width and centre of band C/eV
Width of a forbidden gap/eV
Averaged bond
1.45; –14.62
1.21; –6.07
2.66; –3.95
2.87
1.79; –14.80
1.68; –5.79
2.30; –3.63
3.42
M–N, M=Mo, W 0.837
0.849
0.046
populations by
Mulliken
Li–N
0.047
is observed. A low-energy band A at about –15.0 eV was attri-
buted 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 crystal-
line orbitals for the Mo–N bond showed that the antibonding
partner of the band B is located at the bottom of the conduc-
tivity 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 semi-
conducting 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 struc-
ture 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 dif-
ferences 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.
The calculations of the electronic structure and total energy of
Li6MoN4 and Li6WN4 crystals have been carried out by the LMTO-
TB 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 posi-
tions. 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 irreduc-
ible wedge). We found that the electronic structures of Li6MoN4
and Li6WN4 are close to each other. The total and partial den-
sities of states for a Li6MoN4 phase are presented in Figure 1.
The separation of the electronic energy spectrum into four zones
We also calculated the indices of chemical bonding for the con-
sidered 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.
EF
30
20
A
B
C
(a)
total
10
20
The investigations of the energy of defect formation using
the ab initio LMTO calculations allowed us to offer a model of
(b)
(c)
10
2s,2p-N
2s,2p-Li
20
10
N
Li2
Li1
Li2
T
20
O
Li1
(d)
Li2
10
0
4d-Mo
Mo
Li2
–15
–10
–5
E/eV
0
5
Figure 2 The structure of a perfect crystal of Li6MoN4. Arrows show pos-
sible transitions for lithium ions into octahedral interstitial position (O) and
through structure tetravacancies (T).
Figure 1 The (a) total and (b)–(d) partial densities of states [N(E)] for a
Li6MoN4 crystal.
– 144 –
Khripach
