Ab initio study of the band structures of different phases of higher manganese silicides
D. B. Migas,* V. L. Shaposhnikov, A. B. Filonov, and V. E. Borisenko
Belarusian State University of Informatics and Radioelectronics, P.Browka 6, 220013 Minsk, Belarus
N. N. Dorozhkin
Belarusian State University, Nezavisimosti Av. 4, 220050 Minsk, Belarus
Received 16 November 2007; revised manuscript received 14 January 2008; published 25 February 2008
By means of first principles calculations, we have investigated the band structures of different phases of
higher manganese silicides MnSi
x
with x ranging from 1.73 to 1.75. In this family, Mn
11
Si
19
, Mn
15
Si
26
, and
Mn
27
Si
47
have been found to behave like degenerate semiconductors and, at the same time, like metals because
the Fermi level stays partly in the energy gap and partly in the valence band close to its top. The spin-polarized
calculations have revealed that these phases can be also treated as half-metals displaying 100% spin polariza-
tion of holes at the Fermi energy. On the contrary, Mn
4
Si
7
is shown to be a semiconductor with the indirect
band gap of 0.77 eV. Its dielectric function possesses some anisotropy effects with respect to different light
polarizations. We have also discovered that the MnSi
1.75
stoichiometry provides semiconductor properties
without degeneracy. The role of stacking faults in the gap reduction of higher manganese silicides is discussed.
DOI: 10.1103/PhysRevB.77.075205 PACS numbers: 71.20.Nr, 78.20.Bh, 75.50.Pp
I. INTRODUCTION
Semiconducting silicides have attracted much attention
because of their prospects for optoelectronic and thermoelec-
tric applications.
1
Among them, iron disilicide -FeSi
2
is
one of the most promising and well studied both experimen-
tally and theoretically.
1
However, in the case of higher man-
ganese silicides HMSsMnSi
x
with x 1.73–1.75, also
indicated to have promising properties,
1
there are several is-
sues which are not fully understood and explored: the exis-
tence of different phases with similar crystal structures and
the value and the character of the gap in addition to the
reported degenerate semiconducting nature and observed
metallic behavior as a magnetic system. In fact, HMSs were
shown to have several phases with slightly different stoichi-
ometry see Table I such as Mn
4
Si
7
,
2
Mn
11
Si
19
,
3,4
Mn
15
Si
26
,
5
and Mn
27
Si
47
.
6
They all derived from the TiSi
2
structure
7
and
displayed the tetragonal crystal structure with almost equal a
lattice parameters and unusually long c lattice parameters
Table I. The corresponding unit cells of HMSs are shown in
Fig. 1. Moreover, HMSs are also characterized by the same
building principle as determined by the high-resolution elec-
tron microscopy study.
7
Thus, Mn atoms occupy Ti sites and
form the Mn sublattice, while Si atoms in a double-helical
arrangement form another sublattice by filling intersites in
the Mn sublattice. A commensurate match of the two sublat-
tices requires such a long c lattice parameter. However, it is
hard to distinguish between different phases of HMS.
7,8
From the other side, resistivity of HMS indicates the be-
havior usually observed for degenerate semiconductors: at
temperatures below 500 K, the resistivity increases with
temperature, while above 500 K, the resistivity starts de-
creasing exponentially indicating the gap of about 0.4 eV.
9,10
The Hall effect measurements identify holes to be the domi-
nant carriers in the whole temperature range,
9–11
and the hole
mobility is shown to be very small.
8–11
The band gaps for
bulk samples were found to scatter from 0.4 eV Ref. 9 to
0.7 eV Ref. 11 and to 0.9 eV.
12
In addition, the gaps of
0.40 and 0.42 eV have been obtained on thin film samples
according to resistivity and the Hall effect experiments,
respectively.
10
Bost and Mahan
13
by analyzing the absorption
coefficient data have shown that polycrystalline films of
HMS were characterized by the direct transition of 0.68 eV
with a strong free carrier absorption below 0.2 eV. The same
band gap was derived from their resistivity measurements.
13
Another optical investigation
14
carried out on a single layer
of HMS has determined the direct band gap between 0.78
FIG. 1. Color online The unit cells of HMS. The larger gray
blue balls stand for Mn atoms, while the smaller dark red balls
indicate Si atoms.
PHYSICAL REVIEW B 77, 075205 2008
1098-0121/2008/777/0752059 ©2008 The American Physical Society 075205-1