We report first-principles results obtained on Fe impurity incorporation into the SnO2 material. Different
impurity concentrations have been taken into consideration when computing structural, electronic and
magnetic properties of the material. DFT + U methodology within the GGA approach applied to a 96-atom
supercell allowed us to establish the equilibrium geometry of the system, which consists of six defectnearest
oxygens shifting towards the Fe impurity. Antiparallel magnetic alignment between the electrons
of the Fe 3d and impurity-neighbouring O 2p atomic orbitals forming the FeO6 complex has been found

Puchaicela, P.

Stashans, A.

SocioEconomic Challenges Journal (Sumy State University)

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ 
Vol. 7 No 2, 02032(3pp) (2015) Том 7 № 2, 02032(3cc) (2015) 
 
 
2077-6772/2015/7(2)02032(3) 02032-1  2015 Sumy State University 
Fe-doped SnO2: A Quantum-chemical Approach 
 
A. Stashans*, P. Puchaicela† 
 
Grupo de Fisicoquímica de Materiales, Universidad Técnica Particular de Loja, Apartado 11-01-608 Loja, Ecuador 
 
(Received 09 February 2015; revised manuscript received 30 April 2015; published online 10 June 2015) 
 
We report first-principles results obtained on Fe impurity incorporation into the SnO2 material. Differ-
ent impurity concentrations have been taken into consideration when computing structural, electronic and 
magnetic properties of the material. DFT + U methodology within the GGA approach applied to a 96-atom 
supercell allowed us to establish the equilibrium geometry of the system, which consists of six defect-
nearest oxygens shifting towards the Fe impurity. Antiparallel magnetic alignment between the electrons 
of the Fe 3d and impurity-neighbouring O 2p atomic orbitals forming the FeO6 complex has been found. 
 
Keywords: DFT+U, SnO2, Impurity doping, Microstructure, Magnetism. 
 
 PACS numbers: 61.72.S –, 61.72.U –, 71.15.Mb 
 
 
                                                                
* arvids@utpl.edu.ec 
† ppuchaicela@utpl.edu.ec 
1. INTRODUCTION 
 
Tin dioxide (SnO2) is a fascinating material with a 
lot of valuable applications in different fields. If doped 
with Fe impurities it becomes diluted magnetic semi-
conductor (DMS) showing permanent ferromagnetic 
behaviour. DMS, where transition metal (magnetic) 
impurities are doped into semiconductor material [1], 
has found increased interest in both scientific and in-
dustrial communities. In case of room temperature 
applications in spintronics, oxide-based DMS systems 
are gaining fast grow since such systems have not only 
high critical temperature but also large magnetic mo-
ments [2]. SnO2 is one of compounds displaying the 
mentioned features. 
In general, impurity doping in oxide crystals leads to 
a great deal of new properties in these materials [3]. 
These features sometimes are interrelated, i.e. atomic 
shifts in defective region and magnetism might be con-
nected [4] or strong coupling between different atomic 
orbitals (AOs) might influence electronic conductivity in 
a given sample [5]. That is why it is important to under-
stand as many properties as feasible in case of the im-
purity incorporation in SnO2 material. 
Iron is a typical magnetic atom and is ideal candi-
date to be used in order to understand better how a 
magnetic atom might influence the microstructure of 
SnO2 and its possible relation to the magnetism. Densi-
ty of states (DOS) have been analysed as well within the 
present work to find out the relationship between dif-
ferent electronic states at the equilibrium.  
 
2. OUTLINE OF THE METHOD 
 
Study is based on the density functional theory 
(DFT) [6] within the generalized gradient approxima-
tion (GGA) method [7]. VASP computer code [8] has 
been used throughout the work. The projector aug-
mented wave (PAW) method [9] serves to describe the 
interaction between the core electrons and the valence 
electrons. The following valence configurations have 
been employed in the present research: 4d105s25p2 for 
Sn, 3p63d74s1 for Fe and 2s22p4 for O atoms, respective-
ly. Perdew-Burke-Ernzerhof (PBE) developed [10] GGA 
functionals are utilised to describe the exchange corre-
lation interactions. Additionally, we make use of the so-
called U-term in order to correct some DFT deficiencies 
regarding the treatment of d-electrons. That means the 
intra-atomic interactions for the strongly correlated d-
electrons are taken into account by the unrestricted 
Hartree-Fock approximation leading to the DFT + U 
method. In the present work, we use a scheme of the 
rotationally invariant approach to the DFT + U pro-
posed by Dudarev et al. [11]. The corresponding values 
of U parameter were obtained previously: U  4.0 eV 
for the Sn 4d electrons [12] and U  3.8 eV for the Fe 
3d electrons [13]. 
A cut-off kinetic energy of 480 eV has been used, 
which is obtained by converging the total energy of 
SnO2 primitive unit cell to less than 1 meV atom – 1.  
Γ-centred Monkhorst-Pack (MP) grid with a 0.035 Å – 1 
separation has been applied corresponding to a k-point 
mesh of 6  6  8 for the 6-atom primitive unit cell of 
the tetragonal, D144h (P42/mnm) space group. The 
above-mentioned parameters were obtained through 
the atomic relaxation until all the forces were found to 
be  0.008 eV Å – 1 and the equilibrium state of the 
system was achieved. To study effects produced by the 
Fe dopants, 6-atom primitive unit cell was expanded 
sixteen times, 2  2  4 extension, resulting in a 96-
atom supercell. The k-point mesh of 3  3  2, maintain-
ing the same MP grid with a 0.035 Å – 1 separation in 
the reciprocal space, has been exploited for integrations 
over the Brillouin zone for the large supercell. Band 
structure features and lattice parameters of pure SnO2 
have been successfully reproduced and are described in 
detail elsewhere [12, 14]. 
 
3. RESULTS AND DISCUSSION 
 
Impurity-produced perturbation leads to atomic 
displacements in the region surrounding the Fe atom. 
Our computations show that only six defect-
neighbouring O atoms have non-negligible shifts from 
their original positions within the crystalline lattice. 
 A. STASHANS, P. PUCHAICELA J. NANO- ELECTRON. PHYS. 7, 02032 (2015) 
 
 
02032-2 
These movements are towards the Fe atom. The four 
defect-nearest oxygens situated in the basal plane move 
by 0.09 Å whereas the oxygens along the vertex relax 
around 0.07 Å. In our mind, the main reason of defect-
inward motions is the change in ionic radii due to the 
Fe  Sn substitution. Fe ion in six-fold neighbourhood 
has 0.61 Å radius while the host Sn ion has larger 
0.69 Å ionic radius [15]. Thus, the doping procedure 
produces some extra space in the defective region 
around the point defect and the neighbouring oxygens 
try to fill in this space by defect-inward shifts. 
A local magnetic moment occurs in the material be-
cause of the performed doping and it is equal to 2.00 B. 
The main contribution to this value is done by the Fe 
atom itself, 2.29 B, which is predominantly due to the 
Fe 3d electrons. Nevertheless, the six impurity-
neighbouring O atoms also contribute towards the 
magnetic moment of the system. These contributions 
are due to the O 2p electrons and come from the four 
basal oxygens, – 0.036 B from each atom, as well as 
from the two vertex oxygens, – 0.057 B from each at-
om. It has to be stated that computed magnetic mo-
ment, 2.29 B per Fe atom, is larger than in any other 
simple iron oxide. For instance, the net magnetic mo-
ment per iron in magnetite (Fe3O4) is 4/3  1.3 B, 
whereas that in YIG (Y3Fe5O12) material is 5/5  1.0 B 
[16]. Our results also show that the magnetic orienta-
tion of the O 2p electrons is antiparallel to that of the 
3d electrons of impurity. In the SnO2 structure, the 
impurity Fe atom has octahedral environment and the 
effect of environment on the Fe 3d electrons is deter-
mined by the amount of overlap between the Fe 3d and 
O 2p atomic orbitals (AOs). Obviously, the t2g orbitals 
(dxy, dxz and dyz) of iron are lowered in energy, while the 
eg orbitals (dz2 and dx2 – y2) are raised in energy. Thus, 
the Fe2+ ion exhibits high-spin configuration (Fig. 1) 
when doped in SnO2 crystal. In such a configuration 
the electrons first occupy each orbital before any orbital 
becomes doubly occupied. Thus due to the Pauli princi-
ple the orientation is antiparallel and that is a neces-
sary condition for the direct short range exchange in-
teraction. So, we think that direct exchange interaction 
[17, 18] might be responsible for the magnetism occur-
ring in the Fe-doped SnO2 material where we find anti-
parallel magnetic orientation between the electrons in 
the Fe 3d and O 2p AOs, respectively. We would like to 
add that low-spin configuration (Fig. 2), in which the 
 
 
 
Fig. 1 – Electronic configuration for the high-spin case of the 
Fe2+ ion in the octahedral environment of the SnO2 rutile 
structure 
 
 
Fig. 2 – Electronic configuration for the low-spin case of the 
Co2+ ion in the octahedral environment of the SnO2 rutile 
structure 
 
electrons doubly occupy lower energy orbitals before 
occupying the higher energy states leads to the parallel 
alignment between the Fe 3d and O 2p electrons as it 
was recently shown in Co-doped SnO2 [19].  
Iron incorporation into the SnO2 crystal produces 
changes upon the electronic band structure of the ma-
terial. The density of states (DOS) pattern (Fig. 3) 
shows a local unoccupied level within the band-gap 
region. This state is composed mainly of the Fe 3d and 
O 2p AOs. That is why one can assume a strong hybrid-
ization effect between the Fe 3d and O 2p within the 
forbidden energy region. Actually, hybridization be-
tween the Fe 3d and O 2p states are also observed 
within the upper valence band (VB) region; in particu-
lar, in the energy interval between – 6 eV and – 3 eV as 
it can be seen from Fig. 3. The effect of hybridization 
might occur due to the local microstructure in the de-
fective region, i.e. defect-inward shifts of the Fe-nearest 
oxygens. These distortions lead not only to the hybridi-
zation phenomenon but also to the spin polarization 
within the FeO6 complex and consequently to the mag-
netic features described above. 
 
 
 
Fig. 3 – DFT + U computed total DOS of the Fe-doped SnO2 
material. Observed local in-gap state is due to the Fe 3d and O 
2p AOs. Only upper valence band and conduction band is 
indicated. Vertical line denotes Fermi level 
 
Different impurity doping rates have been consid-
ered by incorporating the second and the third iron 
atom into the SnO2 crystalline lattice. It was found that 
defects tend to avoid each other and no clusterization 
effect takes place. Structural and magnetic properties 
of the SnO2 doped with iron at higher concentrations 
are practically identical to those for low impurity dop-
ing rate described above. Each Fe atom creates a local 
magnetic moment with its six octahedrally situated 
neighbouring O atoms displacing themselves towards the 
point defect. The alignment between the Fe 3d electrons 
 FE-DOPED SNO2: A QUANTUM-CHEMICAL APPROACH J. NANO- ELECTRON. PHYS. 7, 02032 (2015) 
 
 
02032-3 
 
 
Fig. 4 – DFT + U computed total DOS of two Fe impurities 
incorporated into the 96-atom SnO2 supercell. An empty band 
has been observed at the top of the upper VB. Vertical line 
denotes Fermi level 
 
and the O 2p electrons contributing to the local mag-
netic moment is always antiparallel. However, at high-
er doping rates we observe some changes into the DOS 
pattern. Especially, as one can notice from Fig. 4, the 
Fe 3d – O 2p hybridized band moves from the band-gap 
region to lower energies and now are situated at the top 
of the upper VB. We interpret these results stating that 
Fe impurity generates unoccupied band at the top of 
the upper VB since there is a notable energy between 
the Fermi level and the last state corresponding to the 
upper VB. This energy interval is populated by holes 
and, in principle; one might expect the occurrence of p-
type electrical conductivity within the crystal similar to 
that recently discovered in Sc-doped TiO2 crystals [20]. 
However, strictly speaking such a possibility is very 
low since these holes are not free to move through the 
lattice. Due to the hybridization phenomenon these 
holes are localized on the Fe atoms and we do not ex-
pect any changes upon the electrical properties of the 
material. Our results on electrical features are supported 
by the experimental spray pyrolysis technique meas-
urements [21] arguing that starting from 2 mol. % iron 
doping concentrations the resistivity of the SnO2 sample 
augments and the carrier concentration reduces. 
 
4. CONCLUSIONS 
 
Investigation of Fe-doped SnO2 materials carried out by 
the DFT + U methodology demonstrates that overlap 
between the Fe 3d and impurity-neighbouring O 2p 
states lead to the hybridization phenomenon between 
the mentioned states. As a result, spin polarization 
occurs close to the Fe site. Iron exhibits high spin state 
in all considered models independently of its concentra-
tion. Only six Fe-nearest O atoms are coupled to the 
impurity most probably via short-range direct exchange 
interactions occurring within the FeO6 complex. This 
complex is formed by the defect-inward dislocations 
which consist of four basal oxygen movements by about 
0.09 Å and two vertex oxygen displacements by approx-
imately 0.07 Å. According to the present study, atomic 
shifts in the defective region and magnetism are inter-
related and are responsible for antiparallel magnetic 
alignment between the Fe 3d and O 2p electrons within 
the FeO6 complex. 
 
 
REFERENCES 
 
1. H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, 
S. Katsumoto, Y. Iye, Appl. Phys. Lett. 69, 363 (1996). 
2. S.B. Ogale, R.J. Choudhary, J.P. Buban, S.E. Lofland, 
S.R. Shinde, S.N. Kale, V.N. Kulkarni, J. Higgins, 
C. Lanci, J.R. Simpson, N.D. Browning, S. Das Sarma, 
H.D. Drew, R.L. Greene, T. Venkatesan, Phys. Rev. Lett. 
91, 077205 (2003). 
3. A. Stashans, Int. J. Nanotechn. 1, 399 (2004). 
4. A. Stashans, K. Rivera, Mod. Phys. Lett. B 27, 1350145 
(2013). 
5. A. Stashans, S. Jácome, Comput. Mater. Sci. 81, 353 
(2014). 
6. W. Kohn, L.J. Sham, Phys. Rev. 140, A1133 (1965). 
7. J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, 
M.R. Pederson, D.J. Singh, C. Fiolhais, Phys. Rev. B 46, 
6671 (1992). 
8. G. Kresse, J. Furthmüller, Phys. Rev. B 54, 11169 (1996). 
9. G. Kresse, D. Joubert, Phys. Rev. B 59, 1758 (1999). 
10. J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77, 
3865 (1996). 
11. S.L. Dudarev, G.A. Botton, S.Y. Savrasov, C.J. Humphreys, 
A.P. Sutton, Phys. Rev. B 57, 1505 (1998). 
12. R. Rivera, F. Marcillo, A. Chamba, P. Puchaicela, 
A. Stashans, Transactions on Engineering Technologies, 
Lecture Notes in Electrical Engineering, Vol. 275 (Eds. by 
G.C. Yang, S.I. Ao, X. Huang, O. Castillo) (Springer Dor-
drecht: 2014).  
13. R. Rivera, H.P. Pinto, A. Stashans, L. Piedra, Phys. Scr. 
85, 015602 (2012). 
14. A. Stashans, P. Puchaicela, R. Rivera, J. Mater. Sci. 49, 
2904 (2014). 
15. CRC Handbook of Chemistry and Physics (Ed. by 
D.R. Lide) (CRC Press: Boca Raton: 2005). 
16. J.M.D. Coey, A.P. Douvalis, C.B. Fitzgerald, 
M. Venkatesan, Appl. Phys. Lett. 84, 1332 (2004). 
17. C. Kittel, Phys. Rev. 71, 270 (1947). 
18. J.H. Vanvleck, Phys. Rev. 78, 266 (1950). 
19. H. Wang, Y. Yan, Y.Sh. Mohammed, X. Du, K. Li, H. Jin, 
J. Magn. Magn. Mater. 321, 337 (2009). 
20. A. Stashans, Y. Bravo, Mod. Phys. Lett. B 27, 1350113 
(2013). 
21. M.-M. Bagheri-Mohagheghi, N. Shahtahmasebi, 
M.R. Alinejad, A. Youssefi, M. Shokooh-Saremi, Solid 
State Sci. 11, 233 (2009). 
 


Fe-doped SnO2: A Quantum-chemical Approach

Electronic Sumy State University Institutional Repository

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Vol. 7 No 2, 02032(3pp) (2015) Том 7 № 2, 02032(3cc) (2015)   2077-6772/2015/7(2)02032(3) 02032-1  2015 Sumy State University Fe-doped SnO2: A Quantum-chemical Approach  A. Stashans*, P. Puchaicela†  Grupo de Fisicoquímica de Materiales, Universidad Técnica Particular de Loja, Apartado 11-01-608 Loja, Ecuador  (Received 09 February 2015; revised manuscript received 30 April 2015; published online 10 June 2015)  We report first-principles results obtained on Fe impurity incorporation into the SnO2 material. Differ-ent impurity concentrations have been taken into consideration when computing structural, electronic and magnetic properties of the material. DFT + U methodology within the GGA approach applied to a 96-atom supercell allowed us to establish the equilibrium geometry of the system, which consists of six defect-nearest oxygens shifting towards the Fe impurity. Antiparallel magnetic alignment between the electrons of the Fe 3d and impurity-neighbouring O 2p atomic orbitals forming the FeO6 complex has been found.  Keywords: DFT+U, SnO2, Impurity doping, Microstructure, Magnetism.   PACS numbers: 61.72.S –, 61.72.U –, 71.15.Mb                                                                   * arvids@utpl.edu.ec † ppuchaicela@utpl.edu.ec 1. INTRODUCTION  Tin dioxide (SnO2) is a fascinating material with a lot of valuable applications in different fields. If doped with Fe impurities it becomes diluted magnetic semi-conductor (DMS) showing permanent ferromagnetic behaviour. DMS, where transition metal (magnetic) impurities are doped into semiconductor material [1], has found increased interest in both scientific and in-dustrial communities. In case of room temperature applications in spintronics, oxide-based DMS systems are gaining fast grow since such systems have not only high critical temperature but also large magnetic mo-ments [2]. SnO2 is one of compounds displaying the mentioned features. In general, impurity doping in oxide crystals leads to a great deal of new properties in these materials [3]. These features sometimes are interrelated, i.e. atomic shifts in defective region and magnetism might be con-nected [4] or strong coupling between different atomic orbitals (AOs) might influence electronic conductivity in a given sample [5]. That is why it is important to under-stand as many properties as feasible in case of the im-purity incorporation in SnO2 material. Iron is a typical magnetic atom and is ideal candi-date to be used in order to understand better how a magnetic atom might influence the microstructure of SnO2 and its possible relation to the magnetism. Densi-ty of states (DOS) have been analysed as well within the present work to find out the relationship between dif-ferent electronic states at the equilibrium.   2. OUTLINE OF THE METHOD  Study is based on the density functional theory (DFT) [6] within the generalized gradient approxima-tion (GGA) method [7]. VASP computer code [8] has been used throughout the work. The projector aug-mented wave (PAW) method [9] serves to describe the interaction between the core electrons and the valence electrons. The following valence configurations have been employed in the present research: 4d105s25p2 for Sn, 3p63d74s1 for Fe and 2s22p4 for O atoms, respective-ly. Perdew-Burke-Ernzerhof (PBE) developed [10] GGA functionals are utilised to describe the exchange corre-lation interactions. Additionally, we make use of the so-called U-term in order to correct some DFT deficiencies regarding the treatment of d-electrons. That means the intra-atomic interactions for the strongly correlated d-electrons are taken into account by the unrestricted Hartree-Fock approximation leading to the DFT + U method. In the present work, we use a scheme of the rotationally invariant approach to the DFT + U pro-posed by Dudarev et al. [11]. The corresponding values of U parameter were obtained previously: U  4.0 eV for the Sn 4d electrons [12] and U  3.8 eV for the Fe 3d electrons [13]. A cut-off kinetic energy of 480 eV has been used, which is obtained by converging the total energy of SnO2 primitive unit cell to less than 1 meV atom – 1.  Γ-centred Monkhorst-Pack (MP) grid with a 0.035 Å – 1 separation has been applied corresponding to a k-point mesh of 6  6  8 for the 6-atom primitive unit cell of the tetragonal, D144h (P42/mnm) space group. The above-mentioned parameters were obtained through the atomic relaxation until all the forces were found to be  0.008 eV Å – 1 and the equilibrium state of the system was achieved. To study effects produced by the Fe dopants, 6-atom primitive unit cell was expanded sixteen times, 2  2  4 extension, resulting in a 96-atom supercell. The k-point mesh of 3  3  2, maintain-ing the same MP grid with a 0.035 Å – 1 separation in the reciprocal space, has been exploited for integrations over the Brillouin zone for the large supercell. Band structure features and lattice parameters of pure SnO2 have been successfully reproduced and are described in detail elsewhere [12, 14].  3. RESULTS AND DISCUSSION  Impurity-produced perturbation leads to atomic displacements in the region surrounding the Fe atom. Our computations show that only six defect-neighbouring O atoms have non-negligible shifts from their original positions within the crystalline lattice.  A. STASHANS, P. PUCHAICELA J. NANO- ELECTRON. PHYS. 7, 02032 (2015)   02032-2 These movements are towards the Fe atom. The four defect-nearest oxygens situated in the basal plane move by 0.09 Å whereas the oxygens along the vertex relax around 0.07 Å. In our mind, the main reason of defect-inward motions is the change in ionic radii due to the Fe  Sn substitution. Fe ion in six-fold neighbourhood has 0.61 Å radius while the host Sn ion has larger 0.69 Å ionic radius [15]. Thus, the doping procedure produces some extra space in the defective region around the point defect and the neighbouring oxygens try to fill in this space by defect-inward shifts. A local magnetic moment occurs in the material be-cause of the performed doping and it is equal to 2.00 B. The main contribution to this value is done by the Fe atom itself, 2.29 B, which is predominantly due to the Fe 3d electrons. Nevertheless, the six impurity-neighbouring O atoms also contribute towards the magnetic moment of the system. These contributions are due to the O 2p electrons and come from the four basal oxygens, – 0.036 B from each atom, as well as from the two vertex oxygens, – 0.057 B from each at-om. It has to be stated that computed magnetic mo-ment, 2.29 B per Fe atom, is larger than in any other simple iron oxide. For instance, the net magnetic mo-ment per iron in magnetite (Fe3O4) is 4/3  1.3 B, whereas that in YIG (Y3Fe5O12) material is 5/5  1.0 B [16]. Our results also show that the magnetic orienta-tion of the O 2p electrons is antiparallel to that of the 3d electrons of impurity. In the SnO2 structure, the impurity Fe atom has octahedral environment and the effect of environment on the Fe 3d electrons is deter-mined by the amount of overlap between the Fe 3d and O 2p atomic orbitals (AOs). Obviously, the t2g orbitals (dxy, dxz and dyz) of iron are lowered in energy, while the eg orbitals (dz2 and dx2 – y2) are raised in energy. Thus, the Fe2+ ion exhibits high-spin configuration (Fig. 1) when doped in SnO2 crystal. In such a configuration the electrons first occupy each orbital before any orbital becomes doubly occupied. Thus due to the Pauli princi-ple the orientation is antiparallel and that is a neces-sary condition for the direct short range exchange in-teraction. So, we think that direct exchange interaction [17, 18] might be responsible for the magnetism occur-ring in the Fe-doped SnO2 material where we find anti-parallel magnetic orientation between the electrons in the Fe 3d and O 2p AOs, respectively. We would like to add that low-spin configuration (Fig. 2), in which the    Fig. 1 – Electronic configuration for the high-spin case of the Fe2+ ion in the octahedral environment of the SnO2 rutile structure   Fig. 2 – Electronic configuration for the low-spin case of the Co2+ ion in the octahedral environment of the SnO2 rutile structure  electrons doubly occupy lower energy orbitals before occupying the higher energy states leads to the parallel alignment between the Fe 3d and O 2p electrons as it was recently shown in Co-doped SnO2 [19].  Iron incorporation into the SnO2 crystal produces changes upon the electronic band structure of the ma-terial. The density of states (DOS) pattern (Fig. 3) shows a local unoccupied level within the band-gap region. This state is composed mainly of the Fe 3d and O 2p AOs. That is why one can assume a strong hybrid-ization effect between the Fe 3d and O 2p within the forbidden energy region. Actually, hybridization be-tween the Fe 3d and O 2p states are also observed within the upper valence band (VB) region; in particu-lar, in the energy interval between – 6 eV and – 3 eV as it can be seen from Fig. 3. The effect of hybridization might occur due to the local microstructure in the de-fective region, i.e. defect-inward shifts of the Fe-nearest oxygens. These distortions lead not only to the hybridi-zation phenomenon but also to the spin polarization within the FeO6 complex and consequently to the mag-netic features described above.    Fig. 3 – DFT + U computed total DOS of the Fe-doped SnO2 material. Observed local in-gap state is due to the Fe 3d and O 2p AOs. Only upper valence band and conduction band is indicated. Vertical line denotes Fermi level  Different impurity doping rates have been consid-ered by incorporating the second and the third iron atom into the SnO2 crystalline lattice. It was found that defects tend to avoid each other and no clusterization effect takes place. Structural and magnetic properties of the SnO2 doped with iron at higher concentrations are practically identical to those for low impurity dop-ing rate described above. Each Fe atom creates a local magnetic moment with its six octahedrally situated neighbouring O atoms displacing themselves towards the point defect. The alignment between the Fe 3d electrons  FE-DOPED SNO2: A QUANTUM-CHEMICAL APPROACH J. NANO- ELECTRON. PHYS. 7, 02032 (2015)   02032-3   Fig. 4 – DFT + U computed total DOS of two Fe impurities incorporated into the 96-atom SnO2 supercell. An empty band has been observed at the top of the upper VB. Vertical line denotes Fermi level  and the O 2p electrons contributing to the local mag-netic moment is always antiparallel. However, at high-er doping rates we observe some changes into the DOS pattern. Especially, as one can notice from Fig. 4, the Fe 3d – O 2p hybridized band moves from the band-gap region to lower energies and now are situated at the top of the upper VB. We interpret these results stating that Fe impurity generates unoccupied band at the top of the upper VB since there is a notable energy between the Fermi level and the last state corresponding to the upper VB. This energy interval is populated by holes and, in principle; one might expect the occurrence of p-type electrical conductivity within the crystal similar to that recently discovered in Sc-doped TiO2 crystals [20]. However, strictly speaking such a possibility is very low since these holes are not free to move through the lattice. Due to the hybridization phenomenon these holes are localized on the Fe atoms and we do not ex-pect any changes upon the electrical properties of the material. Our results on electrical features are supported by the experimental spray pyrolysis technique meas-urements [21] arguing that starting from 2 mol. % iron doping concentrations the resistivity of the SnO2 sample augments and the carrier concentration reduces.  4. CONCLUSIONS  Investigation of Fe-doped SnO2 materials carried out by the DFT + U methodology demonstrates that overlap between the Fe 3d and impurity-neighbouring O 2p states lead to the hybridization phenomenon between the mentioned states. As a result, spin polarization occurs close to the Fe site. Iron exhibits high spin state in all considered models independently of its concentra-tion. Only six Fe-nearest O atoms are coupled to the impurity most probably via short-range direct exchange interactions occurring within the FeO6 complex. This complex is formed by the defect-inward dislocations which consist of four basal oxygen movements by about 0.09 Å and two vertex oxygen displacements by approx-imately 0.07 Å. According to the present study, atomic shifts in the defective region and magnetism are inter-related and are responsible for antiparallel magnetic alignment between the Fe 3d and O 2p electrons within the FeO6 complex.   REFERENCES  1. H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, Y. Iye, Appl. Phys. Lett. 69, 363 (1996). 2. S.B. Ogale, R.J. Choudhary, J.P. Buban, S.E. Lofland, S.R. Shinde, S.N. Kale, V.N. Kulkarni, J. Higgins, C. Lanci, J.R. Simpson, N.D. Browning, S. Das Sarma, H.D. Drew, R.L. Greene, T. Venkatesan, Phys. Rev. Lett. 91, 077205 (2003). 3. A. Stashans, Int. J. Nanotechn. 1, 399 (2004). 4. A. Stashans, K. Rivera, Mod. Phys. Lett. B 27, 1350145 (2013). 5. A. Stashans, S. Jácome, Comput. Mater. Sci. 81, 353 (2014). 6. W. Kohn, L.J. Sham, Phys. Rev. 140, A1133 (1965). 7. J.P. 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Fe-doped SnO2: A Quantum-chemical Approach

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