- Email: [email protected]
Ferromagnetism and half metallicity induced by oxygen vacancies in the double perovskite BaSrNiWO6: DFT study
Accepted Manuscript Ferromagnetism and half metallicity induced by oxygen vacancies in the double perovskite BaSrNiWO6: DFT study Y. Aharbil, H. Labrim, S. Benmokhtar, M. Ait Haddouch, L. Bahmad, A. Belhaj, H. EzZahraouy, A. Benyoussef PII:
S0254-0584(16)30682-4
DOI:
10.1016/j.matchemphys.2016.09.019
Reference:
MAC 19165
To appear in:
Materials Chemistry and Physics
Received Date: 29 March 2016 Revised Date:
25 July 2016
Accepted Date: 10 September 2016
Please cite this article as: Y. Aharbil, H. Labrim, S. Benmokhtar, M.A. Haddouch, L. Bahmad, A. Belhaj, H. Ez-Zahraouy, A. Benyoussef, Ferromagnetism and half metallicity induced by oxygen vacancies in the double perovskite BaSrNiWO6: DFT study, Materials Chemistry and Physics (2016), doi: 10.1016/ j.matchemphys.2016.09.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Ferromagnetism and Half Metallicity induced by Oxygen Vacancies in the Double Perovskite BaSrNiWO6: DFT Study. Y. Aharbil 1, H. Labrim 2, S. Benmokhtar 1, M. Ait Haddouch 1, L. Bahmad 3,* A. Belhaj 4, H. Ez-Zahraouy 3 and A. Benyoussef 3 1
RI PT
1
M AN U
SC
Laboratoire de Chimie Physique des Matériaux LCPM, Faculté des Sciences Ben M’Sik, Casablanca, Morocco. 2 Unité Science de la Matière/DERS/ Centre National de l’Energie, des Sciences et des Techniques Nucléaires (CNESTEN), Rabat, Morocco. 3 Mohammed V University in Rabat, Laboratoire de Magnétisme et Physique des Hautes Energies L.M.P.H.E. URAC-12, B.P. 1014, Rabat, Morocco. 4 LIRST, Département de Physique, Faculté Poly-disciplinaire, Université Sultan Moulay Slimane, Béni Mellal, Morocco.
Abstract:
Using the spin polarized density functional theory (DFT) and exploring the Plane-Wave SelfConsistent Field (PWscf) code implemented in Quantum-ESPRESSO package, we investigate the effect of the Oxygen vacancies (VO) and the Oxygen interstitial (Oi) on the double perovskite BaSrNiWO6. This deals with the magnetic ordering and the electronic structure in
TE D
such a pure sample exhibiting the insulating anti-ferromagnetic (AFM) state. This study shows that the presence of oxygen deficient defects converts the insulating to half metal with ferromagnetic or anti-ferromagnetic states. The magnetic ordering in BaSrNiWO6-δ depends on
EP
the position of the Oxygen vacancy in the unit cell. However, it has been shown that the Oxygen interstitial preserves the anti-ferromagnetic propriety. We have computed the formation energies of different positions of the Oxygen vacancy (VO) and the Oxygen
AC C
interstitial (Oi) in the BaSrNiWO6 compound. We showed that the formation of VO is easier and vice versa for the Oi formation. The obtained results reveal(VO) and the Oxygen interstitial (Oi) that the anti-ferromagnetic can be converted to ferromagnetic in the double perovskite BaSrNiWO6 induced by Oxygen vacancies VO. Keywords: Ferromagnetism; Half Metallicity; Ab-inito calculations; Oxygen vacancies; Oxygen interstitial; Double Perovskite BaSrNiWO6.
2
1
*) Corresponding author : [email protected]
ACCEPTED MANUSCRIPT
1. Introduction Recently, intensive efforts have been devoted to study the double perovskites of general formula A2B’B’’O6/A’A’’B’B’’O6 in connection with the activities elaborating materials with the ferromagnetic properties at room temperature [1-4]. It has been shown that the structure of double perovskite has different crystallographic structures which are cubic, tetragonal,
RI PT
orthorhombic, mono-clinic [5, 6]. It has been considered that the double perovskite with appropriate transition metals is an interesting candidate material for spintronic applications. This is due, not only to the half-metallic ferrimagnetic behavior with higher Curietemperatures, but also to new properties such as anti-ferromagnetic half-metallicity [7, 8]. In
SC
particular, the ferromagnetic double perovskites having chemical formulae like: A2FeMoO6 (A=Ba, Sr, Ca) have been investigated [8]. The corresponding half-metallic ferromagnetic
M AN U
behavior in Sr2FeMoO6 has been found. The calculated electronic structure of Sr2FeMoO6 shows a gap in the up spin channel, while for the down spin channel it is found that a strongly hybridized Iron (Fe) 3d(t2g), Molybdenum (Mo) 4d(t2g) and Oxygen (O) 2p states rise at the Fermi level. Such hybridization gives as result, the half-metallic behavior [9]. Similarly, the electronic structure in ferromagnetic insulator La2NiMnO6 and La2CoMnO6 have been studied using first principles density functional calculation [10].
TE D
On the other hand, it has been realized that point defects have an important effect on the electronic and magnetic properties of insulating and semiconducting materials [11-18]. Defects in semiconducting materials can be native or created by implantation or irradiation
EP
method [19-20].
The aim of this work is to contribute to these activities by studying the double perovskite
AC C
BaSrNiWO6 with and without point defects which are oxygen vacancies (VO) and Oxygen interstitial (Oi). Based on PWSCF (Plane-Wave Self-Consistent Field) code implemented in the Quantum Espresso package, relaying the generalized gradient approximation (GGA) [21] as well as the Hubbard U correction method (GGA+U) [22], we first show that some point defects (VO) can convert the ground state from anti-ferromagnetic to a ferromagnetic one. Then, we discuss the magnetism stability of BaSrNiWO6 without and with point defects. The half metallicity ferromagnetic state (FM) and the half metallicity anti-ferromagnetic state are also discussed.
ACCEPTED MANUSCRIPT
2. Computational methods In this section, we present the method that will be used through this paper. Our calculations are performed using the spin polarized density functional theory with the Plane-Wave SelfConsistent Field (PW SCF) code implemented in Quantum-ESPRESSO package [23]. It is
RI PT
recalled that the PW SCF code uses the pseudo-potentials and a plane-wave basis set in its calculations. We explore the generalized gradient approximation (GGA) [21] for the exchange correlation functional with the Hubbard U correction method (GGA+U) [22] in order to taking of the strong electronic correlations on the localized ‘d’ orbitals of Ni and W atoms.
SC
We use the ultra-soft pseudo-potentials [24], including the semi-core states of each element forming the double perovskite BaSrNiWO6 material. The plane wave basis energy cutoff and
M AN U
charge cutoff are 40 Ry and 160 Ry, respectively. We use a Gaussian smearing width of 1 mRy when sampling the Brillouin zone. For the BaSrNiWO6 material, the k-grid sampling of the Brillouin zone is 7×7×7 per formula unit.
It is worth recalling that the BaSrNiWO6 crystallize in a cubic system with the space group Fm-3m [25, 26], in which the Ba/Sr, Mg, W, and O atoms are ordered into positions 8c(1/4, 1/4, 1/4), 4b(1/2, 1/2, 1/2), 4a(0, 0, 0), and 24e(x, 0, 0) see Fig. 1. The electronic
TE D
configurations of these atoms are [Xe] 6s2, [Kr] 5s2, [Ar] 4s2 3d8, [Xe] 4f14 5d4 6s2 and [He] 2s22p4 for Ba, Sr, Ni, W and O, respectively.
Roughly speaking, we will use, in this work, the lattice parameter of BaSrNiWO6 obtained
AC C
EP
later by using the relaxation method.
Fig. 1: Geometry representing the double perovskite: BaSrNiWO6
ACCEPTED MANUSCRIPT It is well known that the double Perovskite BaSrNiWO6, is considered as a strongly correlated material due to the presence of transition metals which are Ni and W atoms. Hence we are forced to use DFT+U for studying such a system. The values of U for both elements Ni and W in BaSrNiWO6 are calculated by using the DFT+U method [27-29]. Such method is
RI PT
described below. In fact, it is a simple and efficient energy functional which aims to reintroduce the missed correlation energy within the standard XCF. Indeed, its Hamiltonian is derived from the atomic Hartree-Fock with renormalized Slater integrals. The DFT+U Hamiltonian take the following form:
SC
E LDA+U [n(r)]= E LDA [n(r)]+ E Hub [ } ]- Edc [{ }]
where
E LDA+U represents the resultant energy functional when taking in account the effect of U,
M AN U
•
(1)
E LDA is the standard energy functional,
•
E Hub is the energy functional describing the effect of U,
•
E dc is the doubled counted effected inside both E LDA and E Hub,
•
n(r) is the electronic density,
•
σ are the atomic-orbital occupations for the atom I experiencing the ”Hubbard” term.
TE D
•
It has been suggested in [30] a simplified rationally invariant scheme consisting of neglecting
EP
the non sphericity of the electronic interaction and the proper treatment of magnetic interactions. The on-site exchange interaction parameter J can be estimated by redefining an alternative way to describe the on-site Coulomb interaction U as: Ueff =U-J. The Hubbard
AC C
correction to the energy functional reads as: E U[{ } ]= E Hub {[ }}]- E dc {[ } } ]=
∑, Tr[ ( 1 − ) ]
(2)
where Tr represents the trace of the orbital occupation experiencing the Hubbard terme U can be then extracted via the linear response approach by extracting the self-consisting and bare response matrix functions χ : U=(χ - χ )
with
(3)
ACCEPTED MANUSCRIPT χ=
(4)
where α and n stands for the potential shift and the target orbital occupation respectively. The value of U is then calculated under no unconstrained U which misses its physical meaning. The self-consisting DFT+U have been introduced in order to correct this problem by
RI PT
extracting the value of Uscf. This is done via the extrapolation of the input values of Uin against the output one Uout . Using the linear response approach, Uscf can be written: Uout = Uscf -
(5)
SC
where m is an effective degeneracy of the perturbed geometry.
In this work, the value of potential shift ( α ) has been chosen in the interval of ±100 meV for
M AN U
the linear response approach, while for self-consistent approach the value of Uin lays in the interval of [1.00-5.00] eV for W and [2.00-10.00] eV for Ni. The fitting curves are plotted in Fig. 2. The calculated values of Uscf for Ni and W are 7.11 eV and 5.08 eV respectively.
AC C
EP
TE D
However, the value of U for O (4.1 eV) has been taken from literature [30].
Fig. 2: The calculation of the values of Uscf in the double Perovskite BaSrNiWO6for Ni and W.
3. Results and discussions In the BaSrNiWO6 sample, we study the effect of Oxygen vacancies (VO) and Oxygen interstitial (Oi) point defects on the magnetic properties and the electronic structure. First we
ACCEPTED MANUSCRIPT start by pure double perovskite. Then, we investigate the corresponding effect of such point defects.
3.1 Pure double perovskite BaSrNiWO6 Before studying the magnetic and electronic proprieties in pure double perovskite
RI PT
BaSrNiWO6 compound, we should first start by the relaxation method to estimate the equilibrium lattice parameters used later in the calculation. The total energy of such a material computed by DFT method depends on the nucleon positions. Hence, one can minimize the energy functional with respect to the positions. The geometry optimizations have been
SC
performed using two approaches which are the Murnaghan equation and Broyden–Fletcher– Goldfarb–Shanno algorithm (BFGS) in order to compare with the experimental results. The first one uses the fitting Murnaghan equation of state by computing the total energy of pure
M AN U
BaSrNiWO6 as a function of unit cell volume, without and with position relaxations. In the first case, only the stress is minimized. However, the stress and the effect of the atomic position relaxations are considered in our calculations. The second approach uses the algorithmic energy minimization by the BFGS algorithm implemented in the PWSCF code. At the end of each cycle of such an algorithm, two parameters which are Hellman-Feynman
TE D
forces and stress tensor are computed [31, 32]. In fact, the optimized final structure is obtained when a threshold is reached under 0.1 mRy per atom for Hellman-Feynman forces and 0.5 kBar for stress tensor. A last self-consistent field cycle is performed in order to validate the converged lattice parameter and the atomic position. The force and the stress
EP
which are computed may have a value above the threshold due to a several cycle cumulative errors. Fig. 3 illustrates the results of the two methods. It is observed that the fitting by
(a0).
AC C
Murnaghan equation shows a parabolic behavior of the energy in function of lattice parameter
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 3: Energy per unit cell versus lattice parameter, a0 and a’0 calculated by the relaxed and un-relaxed configuration.
TE D
As shown in Table 1, the value of the parameter a’0, obtained from un-relaxed configurations, is 7.8209 Å. This value is far from the value of 7.9398 Å obtained by the BFGS algorithm. The value of the relaxed configuration is in a good agreement with the experimental value of
(24e).
EP
7.9792 Å, see Fig. 3. The same finding is also valid for the oxygen general position parameter
AC C
With relaxation Without relaxation
a0(Å) uO a0(Å) uO
murnaghan 7.9499 0.2605 7.8209 0.2360
BFGS 7.9398 0.2395
Experimental 7.9792 0.2360
Table 1: Calculated lattice parameter (a) and oxygen parameter (uO).
It is interesting to note that the comparison with the experimental value is insufficient to validate these approaches. Then, the force and the stress must be the closer to zero at each converged configuration.
ACCEPTED MANUSCRIPT From the values given in Table 2, we show that the BFGS algorithm is the more appropriate one, with the value 0.077 mRy/atom. Indeed, the forces acting on Ba, Sr, W and Ni are null due to the symmetry. These atoms occupy special Wykoff sites acting on the oxygen atoms and -1.82 kBar for stress tensor.
With relaxation P (kbar) F (mRy/au) Without P (kbar) relaxation F (mRy/au)
BFGS -1.82 0.077 -1.82 0.077
RI PT
murnaghan -9.29 0.002 -7.36 271
SC
Table 2: Calculated Stress (P) and force (F).
The values obtained by using relaxed configurations are also acceptable (0.002 mRy/atom
M AN U
acting on the oxygen atoms and -9.29 kBar for stress tensor). However, the un-relaxed configuration is far away from the ground state, hence we can trust the bulk modulus and its derivative (153.2 GPa and 4.84 respectively) from relaxed configuration which reject the one from un-relaxed configuration (195.70 GPa and 4.75), see Table 3.
K0(Gpa) K'0 K0(Gpa) K'0
TE D
With relaxation Without relaxation
Murnaghan 153.20 4.84 195.70 4.75
EP
Table 3: Calculated bulk modulus (K0) and its derivative (K'0).
3.2 Double perovskite BaSrNiWO6 with point defects
AC C
3.2.1 Magnetic proprieties
The main objective here is to investigate the role of the oxygen vacancies (VO) and oxygen interstitial (Oi) on magnetic properties of BaSrNiWO6 material. More precisely, we discuss the magnetic transition between anti-ferromagnetic (AFM) and ferromagnetic (FM) states. This can be done by calculating the magnetic energy difference between the AFM state and FM state ∆ (
!"!")
=
!"
− !" (1)
ACCEPTED MANUSCRIPT This quantity determines the stabilization of the magnetic phase in the AFM using PWscf code implemented in the Quantum Espresso package. In particular, the negative value of this quantity corresponds to the fact that AFM phase is more stable than FM phase and vice versa. It is recalled that considerable experimental works have been done for the similar pure Ba2NiWO6 and Sr2NiWO6 compounds involving the anti-ferromagnetic behavior [33-35].
RI PT
Effectively, the performed calculation, presented in table 4, on magnetic energy difference of the pure BaSrNiWO6 shows that the AFM state is more stable of the FM state. This is in a perfect agreement with similar experimental results reported in [33-35].
Position
Total
BaSrNiWO6
x=0.2395
moment
Partial magnetic moment (µB) µNi1
µNi2
SC
Phase
µNi3
Pure
M AN U
(µB)
µNi4
∆Ε=Ε ΕAFM-Ε ΕFM (mRy)
0
1.7895
1.7894
-1.7895
-1.7895
-0.0286
Formation energy (Ry)
(0.248, 0.248,-0.039)
0
1.7809
1.7810
-1.7911
-1.7911
-0.1055
24.1676
VO1
(x,0,0)
0
1.7830
1.7845
-1.0414
-1.7898
-0.3316
-23.9746
VO2
(1-x,0,0)
2
1.7811
1.7843
1.0383
1.7836
0.1736
-23.9748
VO3
(0,x,0)
2
1.7812
1.7843
1.7837
1.0385
0.1733
-23.9748
VO4
(0,1-x,0)
VO5
(0,0,x)
VO6
(0,0,1-x)
TE D
Oi
1.7811
1.7843
1.7836
1.0384
0.1735
-23.9748
2
1.7848
1.0428
1.7840
1.7840
0.1899
-23.9748
2
1.7848
1.0417
1.7840
1.7840
0,1897
-23.9748
EP
2
AC C
Table 4: Magnetic proprieties and formation energies of the studied double perovskite with and without defects.
However, the studied sample without defects is anti-ferromagnetic behaviors, while those sharing point defects may become ferromagnetic. It follows from these results that we can convert AFM states to FM stable ones by introducing the oxygen vacancies in the special positions into the BaSrNiWO6 system. However, the over stoichiometric material (BaSrNiWO6+δ) involving the oxygen interstitial, keeps the anti-ferromagnetic behavior of pure sample. From these results, we show that, the magnetic order in such a material after
ACCEPTED MANUSCRIPT insertion of VO and Oi point defects can change the magnetic states. This leads to calculate the formation energies of such defects in BaSrNiWO6, in order to evaluate the possibility of their existence in such systems. The reactions related to the formation of VO and Oi point defects are respectively:
RI PT
1 $%&'()*+, → $%&'()*+. + + (2) 2 1 $%&'()*+, + + → $%&'()*+1 (3) 2
following equations:
SC
Based on these reactions, the formation energies of VO and Oi are calculated using the
M AN U
For VO: 3 (45 ) = 6 ($%&'()*+. ) + (+ ) − 6 ($%&'()*+, ) (4)
For Oi: 3 (45 ) = 6 ($%&'()*+1 ) − (+ ) − 6 ($%&'()*+, ) (5) In these equations, 6 ($%&'()*+, ) represents the total energy of the pure BaSrNiWO6 compound. 6 ($%&'()*+. ) is the total energy of BaSrNiWO5 and 6 ($%&'()*+1 )
TE D
corresponds to the total energy of BaSrNiWO7. These two last total energies correspond to the studied system with Oxygen vacancy and Oxygen interstitial respectively. However, (+ ) corresponds to the energy of Oxygen molecular. The energy quantity can be explored to analyze the possibility of existence of such point defects. Indeed, the negative value of this
EP
quantity indicates that the formation in BaSrNiWO6 of point defects in question is easy and vice versa. The calculations of formation energies of such point defects are presented in the
AC C
table 4. It is found that the formation of VO is easier and vice versa, for Oi in BaSrNiWO6. Thus, we can enhance ferromagnetic in double perovskite BaSrNiWO6 by creating the oxygen vacancy point defect.
This finding has been previously studied theoretically using the Korringa–Kohn–Rostoker coherent potential approximation method to explain the Origin of Magnetism from Native Point Defects in ZnO [16], and experimentally, while observing the room temperature ferromagnetism in t-ZrO2 caused by oxygen vacancies [36]. In what follows, we discuss the effect of the Oxygen vacancies (VO) on the electronic structure in the BaSrNiWO6 compound using DFT based on GGA+U approximation.
ACCEPTED MANUSCRIPT
3. 2. 2. Electronic structures In order to understand and explain the more stabilized ferromagnetism induced by oxygen vacancies (VO), placed at special positions in BaSrNiWO6 system, and the mechanism of exchange interaction considered in the ferromagnetic phase, we compute the density of state for the compound BaSrNiWO6 without and with the (VO) and the Oxygen interstitial (Oi)
RI PT
point defects.
In Figs. 4 (a,b), we present the total and the partial densities of state (DOS) of Steochiometric BaSrNiWO6 compound. From Fig. 4a, corresponding the total DOS, we find a perfect symmetry for the up and down spins. This leads to a null total moment, summarized in table
SC
4. Moreover, it is found that only the atoms of Nickel present a partial magnetic moment value 1.789 µB. This is in a good agreement with the theoretical value given in Ref. [37].
M AN U
This leads to the anti-ferromagnetic states found in this compound. Such results are in a good agreement with the DOS of Nickel partial magnetic moment, see Fig. 4b. Also we find a large value of the band gap. This means that the studied system belongs to isolating materials. On the other hand, this study shows that the pure double perovskite BaSrNiWO6 compound is the insulating anti-ferromagnetic behavior. This is in a good agreement with the similar experimental studies [33].
TE D
Moreover, in order to inspect the more stabilized ferromagnetism induced by (VO), in the BaSrNiWO6 compound, we plot in Figs. 5(a,b) the corresponding total and partial density of states (DOS). Indeed, the presence of the oxygen vacancies in the studied compound induces a broken symmetry between the up and down spins. Hence, the Fermi level is displaced
EP
towards the conduction band. The presence of the density of state at the Fermi level reveals the half metallicity of such compound. The origin of the half metallicity is caused exclusively
AC C
by the hybridization of the orbitals 6s in Ba atom, 5s in Sr atom, 5d in W atom and 2p in O atom. This hybridization phenomenon is located at the Fermi level with a band width, localized between -0.3 eV and +1.7 eV, conversely. However, the Ni atom does not play any notable role in this hybridization process because it maintains its t2g excited states in the conduction band. It confirms again its strong correlated character confirmed by the corresponding high values of Uscf.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 4: The total and the partial densities of state (DOS) of the steochiometric BaSrNiWO6
AC C
compound.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
Fig. 5: The total and the partial densities of state (DOS) of the BaSrNiWO6 compound with the oxygen vacancy (Vo).
4. Conclusion
In this work, we have investigated the properties of the double perovskite BaSrNiWO6 system with and without point defects. These point defects are Oxygen vacancies (VO) ad Oxygen interstitial (Oi). The calculations have been performed using density functional theory (DFT) with the PWscf code implemented in Quantum-ESPRESSO package. The generalized gradient approximation (GGA) for the exchange correlation functional with the Hubbard U
ACCEPTED MANUSCRIPT correction method (GGA+U) has been used in order to consider the strong electronic correlations on the localized ‘d’ orbitals of Ni and W atoms. In particular, we have discussed the stability of magnetism in the BaSrNiWO6 system with and without such point defects. This study has showed that the pure double perovskite BaSrNiWO6 compound is the insulating anti-ferromagnetic behavior. This is in a good agreement with the similar
RI PT
experimental studies, see Ref [33-35]. The same results are obtained in such a system with the Oxygen interstitial Oi. However, the presence of the Oxygen vacancies, in the all positions except one (x, 0, 0), can convert the ground state from the anti-ferromagnetic to half-metal ferromagnetic states. Moreover, the remaining Oxygen vacancy localized at (x, 0, 0) positions
SC
keeps the anti-ferromagnetic with the half-metal.
M AN U
References
[1] R.C. Sahooa, S.K. Giria, Papri Dasguptab, A. Poddarb, T.K. Natha, Journal of Alloys and Compounds, Volume 658, 15 February 2016, Pages 1003–1009
[2] M. Cernea, F. Vasiliu, C. Bartha, C. Plapcianu, I. Mercioniu, Ceramics International, Volume 40, Issue 8, Part A, (2014), Pages 11601–11609
2016, Pages 217–223
TE D
[3] M. Musa Saad H.-E,N. Rammeh, Physica B: Condensed Matter, Volume 481, 15 January
[4] P. Neenu Lekshmi, M. Vasundhara, Manoj Raama Varma, K.G. Suresh, M. Valant, Physica B: Condensed Matter, Volume 448, 1 September 2014, Pages 285–289 [5] P.W. Barnes, M.W. Lufaso, P.M. Woodward, Acta Crystallogr. B 62 (2006) 384.
EP
[6] J. B. Philipp · P. Majewski · L. Alff · A. Erb · R. Gross · T. Graf · M. S. Brandt · J. Simon · T. Walther · W. Mader · D. Topwal · D. D. Sarma, Phys. Rev. B 68, 144431 (2003)
AC C
[7] J. H. Park, S. K. Kwon, and B. I. Min, Phys. Rev. B 65, 174401 (2002) [8] R. P. Borges, R. M. Thomas, C. Cullinan, J. M. D. Coey, R. Suryanarayanan, L. Ben-Dor, L. Pinsard-Gaudart, and A. Revcolevschi, J. Phys. Condens. Matter 11, L445 (1999). [9] D. D. Sarma, Sugata Ray, Chem. Sci.113, 515 (2001). [10] Min Zhu, Yong Lin, Edward W. C. Lo, Qiong Wang, Zhengjie Zhao, Wenhui Xie, Appl. Phys. Lett. 100, 062406 (2012) [11] H. Z. Guo, J. Burgess, E. Ada, S. Street, and A. Gupta, M. N. Iliev A. J. Kellock, C. Magen, M. Varela, and S. J. Pennycook, Phys. Rev. B 77, 174423 (2008) [12] Andrew M. Ritzmann, Ana B. Muñoz-García, Michele Pavone, John A. Keith, and Emily A. Carter, Chem. Mater (2013), 25, 3011−3019
ACCEPTED MANUSCRIPT [13] Rohan Mishra, Oscar D. Restrepo, Patrick M. Woodward, and Wolfgang Windl, Chem. Mater., (2010), 22 (22), pp 6092–6102 [14] B. Khalil, H. Labrim, O. Mounkachi, B. Belhorma, A. Benyoussef, A. El Kenz and A. Belhaj, Journal of Superconductivity and Novel Magnetism, (2013) 26:151-156 [15] M. Boujnah, H. Labrim, K. Allam, A. Belhaj, A. Benyoussef, A. El Kenz1, B. Belhorma,
RI PT
A. El Bouari, Journal Superconductivity and Novel Magnetism, July 2013, Volume 26, Issue 7, pp 2429-2434.
[16] B. Khalil, H. Labrim, O. Mounkachi, B. Belhorma, A. Benyoussef, A. El Kenz and E. Ntsoenzok, Journal Superconductivity and Novel Magnetism 2012, Volume 25, Number 4,
SC
Pages 1145-1150
M AN U
[17] Jie Zheng, Xiao Tian, Xiao-Lin Wei, Lin Shao, Xiong-Ze Pan, Bi-Yu Tang; Materials Chemistry and Physics, Vol. 167, (2015) 70-76 [18] S. Mishra, B. Ganguli; Materials Chemistry and Physics, Vol. 173, (2016) 429-437 [19] H. Labrim, M.-F. Barthe, P. Desgardin, T. Sauvage, G. Blondiaux, C. Corbel, J.P. Piron, Applied Surface Science, Volume 252, Issue 9, 28, (2006), Pages 3262-3268 [20] M. F. Barthe, H. Labrim, A. Gentils, P. Desgardin, C. Corbel, S. Esnouf, J. P. Piron,
TE D
Phys. Stat. Sol. (c) 09/2007; 4(10):3627 - 3632.
[21] J. P. Perdew, A. Ruzsinszky, G. I. Csonka, O. A. Vydrov, G. E. Scuseria, L. A. Constantin, Phys. Rev. Lett. 100 (2008) 136406
EP
[22] A. I. Liechtenstein, V. I. Anisimov, J. Zaanen, Phys. Rev. B, 52, (1995) R5467.
AC C
[23] P. Giannozzi et al., J. Phys.: Condens. Matter 21 (2009) 395502. [24] K. F. Garrity, J. W. Bennett, K. M. Rabe and D. Vanderbilt, Comput. Mater. Sci. 81 (2014) 446.
[25] B. Manoun, A. Ezzahi, S. Benmokhtar, L. Bih, Y. Tamraoui, R. Haloui, F. Mirinioui, S. Addakiri, J.M. Igartua, P. Lazor, Journal of Molecular Structure, 1045, 6 (2013) 1-14. [26] A. Ezzahi, B. Manoun, A. Ider, L. Bih, S. Benmokhtar, M. Azrour, M. Azdouz, J.M. Igartua, P. Lazor, Journal of Molecular Structure, 985, Issues 2–3, (2011) 339-345. [27] K. F. Garrity, J. W. Bennett, K. M. Rabe and D. Vanderbilt, Comput. Mater. Sci. 81 (2014) 446.
ACCEPTED MANUSCRIPT [28] V. I. Anisimov, V. I. Solovyev, M. A. Korotin, M. T. Czyzyk, Sawatzky, G. A. Phys. Rev. B, 48, (1993)16929. [29] V. I. Anisimov, J. Zaanen, O. K. Andersen, Phys. Rev. B, 44, (1991)943 [30] H. J. Kulik, M. Cococcioni, D. A. Scherlis, N. Marzari, Phys. Rev. Let. 97 (2006), 103001, 00258.
RI PT
[31] R. P. Feynman, Phys. Rev. 56 (1939) 340. [32] H. Hellmann, “Einfûhurung in Die Quantenchemie”, Denticke, Vienna, 1937, p 285. [33] D. E. Cox, G. Shirane, B. C. Frazer, J. Appl. Phys. 38, (1967)1459.
SC
[34] Y. Todate, Journal of Physics and Chemistry of Solids, 60, Issue 8-9,(1999) 1173 – 1175. [35] S. Z. Tian, J. C. Zhan, C. D. Qiao, X. L. Ji, B. Z. Jiang, Materials Letters 60 (2006) 2747–2750.
M AN U
[36] Kumar, Sachin, and Animesh K. Ojha. Materials Chemistry and Physics 169 (2016) 1320.
[37] S. Naji, A. Belhaj,H. Labrim,M. Bhihi, A. Benyoussef,and A. El Kenz, J. Phys. Chem. C
AC C
EP
TE D
2014, 118, 4924−4929. dx.doi.org/10.1021/jp407820a.
ACCEPTED MANUSCRIPT Highlight: In this paper, we have obtained the following results:
• • •
We have studied the ferromagnetism and Half Metallicity in Double Perovskite BaSrNiWO6. We have applied the Ab-inito calculations using the DFT approach. We showed the effects induced by Oxygen Vacancies and Oxygen interstitial. We found that the magnetic ordering in BaSrNiWO6-δ depends on the position of the Oxygen vacancy in the unit cell.
RI PT
•
AC C
EP
TE D
M AN U
SC
Sincerely Yours.
S0254-0584(16)30682-4
DOI:
10.1016/j.matchemphys.2016.09.019
Reference:
MAC 19165
To appear in:
Materials Chemistry and Physics
Received Date: 29 March 2016 Revised Date:
25 July 2016
Accepted Date: 10 September 2016
Please cite this article as: Y. Aharbil, H. Labrim, S. Benmokhtar, M.A. Haddouch, L. Bahmad, A. Belhaj, H. Ez-Zahraouy, A. Benyoussef, Ferromagnetism and half metallicity induced by oxygen vacancies in the double perovskite BaSrNiWO6: DFT study, Materials Chemistry and Physics (2016), doi: 10.1016/ j.matchemphys.2016.09.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Ferromagnetism and Half Metallicity induced by Oxygen Vacancies in the Double Perovskite BaSrNiWO6: DFT Study. Y. Aharbil 1, H. Labrim 2, S. Benmokhtar 1, M. Ait Haddouch 1, L. Bahmad 3,* A. Belhaj 4, H. Ez-Zahraouy 3 and A. Benyoussef 3 1
RI PT
1
M AN U
SC
Laboratoire de Chimie Physique des Matériaux LCPM, Faculté des Sciences Ben M’Sik, Casablanca, Morocco. 2 Unité Science de la Matière/DERS/ Centre National de l’Energie, des Sciences et des Techniques Nucléaires (CNESTEN), Rabat, Morocco. 3 Mohammed V University in Rabat, Laboratoire de Magnétisme et Physique des Hautes Energies L.M.P.H.E. URAC-12, B.P. 1014, Rabat, Morocco. 4 LIRST, Département de Physique, Faculté Poly-disciplinaire, Université Sultan Moulay Slimane, Béni Mellal, Morocco.
Abstract:
Using the spin polarized density functional theory (DFT) and exploring the Plane-Wave SelfConsistent Field (PWscf) code implemented in Quantum-ESPRESSO package, we investigate the effect of the Oxygen vacancies (VO) and the Oxygen interstitial (Oi) on the double perovskite BaSrNiWO6. This deals with the magnetic ordering and the electronic structure in
TE D
such a pure sample exhibiting the insulating anti-ferromagnetic (AFM) state. This study shows that the presence of oxygen deficient defects converts the insulating to half metal with ferromagnetic or anti-ferromagnetic states. The magnetic ordering in BaSrNiWO6-δ depends on
EP
the position of the Oxygen vacancy in the unit cell. However, it has been shown that the Oxygen interstitial preserves the anti-ferromagnetic propriety. We have computed the formation energies of different positions of the Oxygen vacancy (VO) and the Oxygen
AC C
interstitial (Oi) in the BaSrNiWO6 compound. We showed that the formation of VO is easier and vice versa for the Oi formation. The obtained results reveal(VO) and the Oxygen interstitial (Oi) that the anti-ferromagnetic can be converted to ferromagnetic in the double perovskite BaSrNiWO6 induced by Oxygen vacancies VO. Keywords: Ferromagnetism; Half Metallicity; Ab-inito calculations; Oxygen vacancies; Oxygen interstitial; Double Perovskite BaSrNiWO6.
2
1
*) Corresponding author : [email protected]
ACCEPTED MANUSCRIPT
1. Introduction Recently, intensive efforts have been devoted to study the double perovskites of general formula A2B’B’’O6/A’A’’B’B’’O6 in connection with the activities elaborating materials with the ferromagnetic properties at room temperature [1-4]. It has been shown that the structure of double perovskite has different crystallographic structures which are cubic, tetragonal,
RI PT
orthorhombic, mono-clinic [5, 6]. It has been considered that the double perovskite with appropriate transition metals is an interesting candidate material for spintronic applications. This is due, not only to the half-metallic ferrimagnetic behavior with higher Curietemperatures, but also to new properties such as anti-ferromagnetic half-metallicity [7, 8]. In
SC
particular, the ferromagnetic double perovskites having chemical formulae like: A2FeMoO6 (A=Ba, Sr, Ca) have been investigated [8]. The corresponding half-metallic ferromagnetic
M AN U
behavior in Sr2FeMoO6 has been found. The calculated electronic structure of Sr2FeMoO6 shows a gap in the up spin channel, while for the down spin channel it is found that a strongly hybridized Iron (Fe) 3d(t2g), Molybdenum (Mo) 4d(t2g) and Oxygen (O) 2p states rise at the Fermi level. Such hybridization gives as result, the half-metallic behavior [9]. Similarly, the electronic structure in ferromagnetic insulator La2NiMnO6 and La2CoMnO6 have been studied using first principles density functional calculation [10].
TE D
On the other hand, it has been realized that point defects have an important effect on the electronic and magnetic properties of insulating and semiconducting materials [11-18]. Defects in semiconducting materials can be native or created by implantation or irradiation
EP
method [19-20].
The aim of this work is to contribute to these activities by studying the double perovskite
AC C
BaSrNiWO6 with and without point defects which are oxygen vacancies (VO) and Oxygen interstitial (Oi). Based on PWSCF (Plane-Wave Self-Consistent Field) code implemented in the Quantum Espresso package, relaying the generalized gradient approximation (GGA) [21] as well as the Hubbard U correction method (GGA+U) [22], we first show that some point defects (VO) can convert the ground state from anti-ferromagnetic to a ferromagnetic one. Then, we discuss the magnetism stability of BaSrNiWO6 without and with point defects. The half metallicity ferromagnetic state (FM) and the half metallicity anti-ferromagnetic state are also discussed.
ACCEPTED MANUSCRIPT
2. Computational methods In this section, we present the method that will be used through this paper. Our calculations are performed using the spin polarized density functional theory with the Plane-Wave SelfConsistent Field (PW SCF) code implemented in Quantum-ESPRESSO package [23]. It is
RI PT
recalled that the PW SCF code uses the pseudo-potentials and a plane-wave basis set in its calculations. We explore the generalized gradient approximation (GGA) [21] for the exchange correlation functional with the Hubbard U correction method (GGA+U) [22] in order to taking of the strong electronic correlations on the localized ‘d’ orbitals of Ni and W atoms.
SC
We use the ultra-soft pseudo-potentials [24], including the semi-core states of each element forming the double perovskite BaSrNiWO6 material. The plane wave basis energy cutoff and
M AN U
charge cutoff are 40 Ry and 160 Ry, respectively. We use a Gaussian smearing width of 1 mRy when sampling the Brillouin zone. For the BaSrNiWO6 material, the k-grid sampling of the Brillouin zone is 7×7×7 per formula unit.
It is worth recalling that the BaSrNiWO6 crystallize in a cubic system with the space group Fm-3m [25, 26], in which the Ba/Sr, Mg, W, and O atoms are ordered into positions 8c(1/4, 1/4, 1/4), 4b(1/2, 1/2, 1/2), 4a(0, 0, 0), and 24e(x, 0, 0) see Fig. 1. The electronic
TE D
configurations of these atoms are [Xe] 6s2, [Kr] 5s2, [Ar] 4s2 3d8, [Xe] 4f14 5d4 6s2 and [He] 2s22p4 for Ba, Sr, Ni, W and O, respectively.
Roughly speaking, we will use, in this work, the lattice parameter of BaSrNiWO6 obtained
AC C
EP
later by using the relaxation method.
Fig. 1: Geometry representing the double perovskite: BaSrNiWO6
ACCEPTED MANUSCRIPT It is well known that the double Perovskite BaSrNiWO6, is considered as a strongly correlated material due to the presence of transition metals which are Ni and W atoms. Hence we are forced to use DFT+U for studying such a system. The values of U for both elements Ni and W in BaSrNiWO6 are calculated by using the DFT+U method [27-29]. Such method is
RI PT
described below. In fact, it is a simple and efficient energy functional which aims to reintroduce the missed correlation energy within the standard XCF. Indeed, its Hamiltonian is derived from the atomic Hartree-Fock with renormalized Slater integrals. The DFT+U Hamiltonian take the following form:
SC
E LDA+U [n(r)]= E LDA [n(r)]+ E Hub [ } ]- Edc [{ }]
where
E LDA+U represents the resultant energy functional when taking in account the effect of U,
M AN U
•
(1)
E LDA is the standard energy functional,
•
E Hub is the energy functional describing the effect of U,
•
E dc is the doubled counted effected inside both E LDA and E Hub,
•
n(r) is the electronic density,
•
σ are the atomic-orbital occupations for the atom I experiencing the ”Hubbard” term.
TE D
•
It has been suggested in [30] a simplified rationally invariant scheme consisting of neglecting
EP
the non sphericity of the electronic interaction and the proper treatment of magnetic interactions. The on-site exchange interaction parameter J can be estimated by redefining an alternative way to describe the on-site Coulomb interaction U as: Ueff =U-J. The Hubbard
AC C
correction to the energy functional reads as: E U[{ } ]= E Hub {[ }}]- E dc {[ } } ]=
∑, Tr[ ( 1 − ) ]
(2)
where Tr represents the trace of the orbital occupation experiencing the Hubbard terme U can be then extracted via the linear response approach by extracting the self-consisting and bare response matrix functions χ : U=(χ - χ )
with
(3)
ACCEPTED MANUSCRIPT χ=
(4)
where α and n stands for the potential shift and the target orbital occupation respectively. The value of U is then calculated under no unconstrained U which misses its physical meaning. The self-consisting DFT+U have been introduced in order to correct this problem by
RI PT
extracting the value of Uscf. This is done via the extrapolation of the input values of Uin against the output one Uout . Using the linear response approach, Uscf can be written: Uout = Uscf -
(5)
SC
where m is an effective degeneracy of the perturbed geometry.
In this work, the value of potential shift ( α ) has been chosen in the interval of ±100 meV for
M AN U
the linear response approach, while for self-consistent approach the value of Uin lays in the interval of [1.00-5.00] eV for W and [2.00-10.00] eV for Ni. The fitting curves are plotted in Fig. 2. The calculated values of Uscf for Ni and W are 7.11 eV and 5.08 eV respectively.
AC C
EP
TE D
However, the value of U for O (4.1 eV) has been taken from literature [30].
Fig. 2: The calculation of the values of Uscf in the double Perovskite BaSrNiWO6for Ni and W.
3. Results and discussions In the BaSrNiWO6 sample, we study the effect of Oxygen vacancies (VO) and Oxygen interstitial (Oi) point defects on the magnetic properties and the electronic structure. First we
ACCEPTED MANUSCRIPT start by pure double perovskite. Then, we investigate the corresponding effect of such point defects.
3.1 Pure double perovskite BaSrNiWO6 Before studying the magnetic and electronic proprieties in pure double perovskite
RI PT
BaSrNiWO6 compound, we should first start by the relaxation method to estimate the equilibrium lattice parameters used later in the calculation. The total energy of such a material computed by DFT method depends on the nucleon positions. Hence, one can minimize the energy functional with respect to the positions. The geometry optimizations have been
SC
performed using two approaches which are the Murnaghan equation and Broyden–Fletcher– Goldfarb–Shanno algorithm (BFGS) in order to compare with the experimental results. The first one uses the fitting Murnaghan equation of state by computing the total energy of pure
M AN U
BaSrNiWO6 as a function of unit cell volume, without and with position relaxations. In the first case, only the stress is minimized. However, the stress and the effect of the atomic position relaxations are considered in our calculations. The second approach uses the algorithmic energy minimization by the BFGS algorithm implemented in the PWSCF code. At the end of each cycle of such an algorithm, two parameters which are Hellman-Feynman
TE D
forces and stress tensor are computed [31, 32]. In fact, the optimized final structure is obtained when a threshold is reached under 0.1 mRy per atom for Hellman-Feynman forces and 0.5 kBar for stress tensor. A last self-consistent field cycle is performed in order to validate the converged lattice parameter and the atomic position. The force and the stress
EP
which are computed may have a value above the threshold due to a several cycle cumulative errors. Fig. 3 illustrates the results of the two methods. It is observed that the fitting by
(a0).
AC C
Murnaghan equation shows a parabolic behavior of the energy in function of lattice parameter
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 3: Energy per unit cell versus lattice parameter, a0 and a’0 calculated by the relaxed and un-relaxed configuration.
TE D
As shown in Table 1, the value of the parameter a’0, obtained from un-relaxed configurations, is 7.8209 Å. This value is far from the value of 7.9398 Å obtained by the BFGS algorithm. The value of the relaxed configuration is in a good agreement with the experimental value of
(24e).
EP
7.9792 Å, see Fig. 3. The same finding is also valid for the oxygen general position parameter
AC C
With relaxation Without relaxation
a0(Å) uO a0(Å) uO
murnaghan 7.9499 0.2605 7.8209 0.2360
BFGS 7.9398 0.2395
Experimental 7.9792 0.2360
Table 1: Calculated lattice parameter (a) and oxygen parameter (uO).
It is interesting to note that the comparison with the experimental value is insufficient to validate these approaches. Then, the force and the stress must be the closer to zero at each converged configuration.
ACCEPTED MANUSCRIPT From the values given in Table 2, we show that the BFGS algorithm is the more appropriate one, with the value 0.077 mRy/atom. Indeed, the forces acting on Ba, Sr, W and Ni are null due to the symmetry. These atoms occupy special Wykoff sites acting on the oxygen atoms and -1.82 kBar for stress tensor.
With relaxation P (kbar) F (mRy/au) Without P (kbar) relaxation F (mRy/au)
BFGS -1.82 0.077 -1.82 0.077
RI PT
murnaghan -9.29 0.002 -7.36 271
SC
Table 2: Calculated Stress (P) and force (F).
The values obtained by using relaxed configurations are also acceptable (0.002 mRy/atom
M AN U
acting on the oxygen atoms and -9.29 kBar for stress tensor). However, the un-relaxed configuration is far away from the ground state, hence we can trust the bulk modulus and its derivative (153.2 GPa and 4.84 respectively) from relaxed configuration which reject the one from un-relaxed configuration (195.70 GPa and 4.75), see Table 3.
K0(Gpa) K'0 K0(Gpa) K'0
TE D
With relaxation Without relaxation
Murnaghan 153.20 4.84 195.70 4.75
EP
Table 3: Calculated bulk modulus (K0) and its derivative (K'0).
3.2 Double perovskite BaSrNiWO6 with point defects
AC C
3.2.1 Magnetic proprieties
The main objective here is to investigate the role of the oxygen vacancies (VO) and oxygen interstitial (Oi) on magnetic properties of BaSrNiWO6 material. More precisely, we discuss the magnetic transition between anti-ferromagnetic (AFM) and ferromagnetic (FM) states. This can be done by calculating the magnetic energy difference between the AFM state and FM state ∆ (
!"!")
=
!"
− !" (1)
ACCEPTED MANUSCRIPT This quantity determines the stabilization of the magnetic phase in the AFM using PWscf code implemented in the Quantum Espresso package. In particular, the negative value of this quantity corresponds to the fact that AFM phase is more stable than FM phase and vice versa. It is recalled that considerable experimental works have been done for the similar pure Ba2NiWO6 and Sr2NiWO6 compounds involving the anti-ferromagnetic behavior [33-35].
RI PT
Effectively, the performed calculation, presented in table 4, on magnetic energy difference of the pure BaSrNiWO6 shows that the AFM state is more stable of the FM state. This is in a perfect agreement with similar experimental results reported in [33-35].
Position
Total
BaSrNiWO6
x=0.2395
moment
Partial magnetic moment (µB) µNi1
µNi2
SC
Phase
µNi3
Pure
M AN U
(µB)
µNi4
∆Ε=Ε ΕAFM-Ε ΕFM (mRy)
0
1.7895
1.7894
-1.7895
-1.7895
-0.0286
Formation energy (Ry)
(0.248, 0.248,-0.039)
0
1.7809
1.7810
-1.7911
-1.7911
-0.1055
24.1676
VO1
(x,0,0)
0
1.7830
1.7845
-1.0414
-1.7898
-0.3316
-23.9746
VO2
(1-x,0,0)
2
1.7811
1.7843
1.0383
1.7836
0.1736
-23.9748
VO3
(0,x,0)
2
1.7812
1.7843
1.7837
1.0385
0.1733
-23.9748
VO4
(0,1-x,0)
VO5
(0,0,x)
VO6
(0,0,1-x)
TE D
Oi
1.7811
1.7843
1.7836
1.0384
0.1735
-23.9748
2
1.7848
1.0428
1.7840
1.7840
0.1899
-23.9748
2
1.7848
1.0417
1.7840
1.7840
0,1897
-23.9748
EP
2
AC C
Table 4: Magnetic proprieties and formation energies of the studied double perovskite with and without defects.
However, the studied sample without defects is anti-ferromagnetic behaviors, while those sharing point defects may become ferromagnetic. It follows from these results that we can convert AFM states to FM stable ones by introducing the oxygen vacancies in the special positions into the BaSrNiWO6 system. However, the over stoichiometric material (BaSrNiWO6+δ) involving the oxygen interstitial, keeps the anti-ferromagnetic behavior of pure sample. From these results, we show that, the magnetic order in such a material after
ACCEPTED MANUSCRIPT insertion of VO and Oi point defects can change the magnetic states. This leads to calculate the formation energies of such defects in BaSrNiWO6, in order to evaluate the possibility of their existence in such systems. The reactions related to the formation of VO and Oi point defects are respectively:
RI PT
1 $%&'()*+, → $%&'()*+. + + (2) 2 1 $%&'()*+, + + → $%&'()*+1 (3) 2
following equations:
SC
Based on these reactions, the formation energies of VO and Oi are calculated using the
M AN U
For VO: 3 (45 ) = 6 ($%&'()*+. ) + (+ ) − 6 ($%&'()*+, ) (4)
For Oi: 3 (45 ) = 6 ($%&'()*+1 ) − (+ ) − 6 ($%&'()*+, ) (5) In these equations, 6 ($%&'()*+, ) represents the total energy of the pure BaSrNiWO6 compound. 6 ($%&'()*+. ) is the total energy of BaSrNiWO5 and 6 ($%&'()*+1 )
TE D
corresponds to the total energy of BaSrNiWO7. These two last total energies correspond to the studied system with Oxygen vacancy and Oxygen interstitial respectively. However, (+ ) corresponds to the energy of Oxygen molecular. The energy quantity can be explored to analyze the possibility of existence of such point defects. Indeed, the negative value of this
EP
quantity indicates that the formation in BaSrNiWO6 of point defects in question is easy and vice versa. The calculations of formation energies of such point defects are presented in the
AC C
table 4. It is found that the formation of VO is easier and vice versa, for Oi in BaSrNiWO6. Thus, we can enhance ferromagnetic in double perovskite BaSrNiWO6 by creating the oxygen vacancy point defect.
This finding has been previously studied theoretically using the Korringa–Kohn–Rostoker coherent potential approximation method to explain the Origin of Magnetism from Native Point Defects in ZnO [16], and experimentally, while observing the room temperature ferromagnetism in t-ZrO2 caused by oxygen vacancies [36]. In what follows, we discuss the effect of the Oxygen vacancies (VO) on the electronic structure in the BaSrNiWO6 compound using DFT based on GGA+U approximation.
ACCEPTED MANUSCRIPT
3. 2. 2. Electronic structures In order to understand and explain the more stabilized ferromagnetism induced by oxygen vacancies (VO), placed at special positions in BaSrNiWO6 system, and the mechanism of exchange interaction considered in the ferromagnetic phase, we compute the density of state for the compound BaSrNiWO6 without and with the (VO) and the Oxygen interstitial (Oi)
RI PT
point defects.
In Figs. 4 (a,b), we present the total and the partial densities of state (DOS) of Steochiometric BaSrNiWO6 compound. From Fig. 4a, corresponding the total DOS, we find a perfect symmetry for the up and down spins. This leads to a null total moment, summarized in table
SC
4. Moreover, it is found that only the atoms of Nickel present a partial magnetic moment value 1.789 µB. This is in a good agreement with the theoretical value given in Ref. [37].
M AN U
This leads to the anti-ferromagnetic states found in this compound. Such results are in a good agreement with the DOS of Nickel partial magnetic moment, see Fig. 4b. Also we find a large value of the band gap. This means that the studied system belongs to isolating materials. On the other hand, this study shows that the pure double perovskite BaSrNiWO6 compound is the insulating anti-ferromagnetic behavior. This is in a good agreement with the similar experimental studies [33].
TE D
Moreover, in order to inspect the more stabilized ferromagnetism induced by (VO), in the BaSrNiWO6 compound, we plot in Figs. 5(a,b) the corresponding total and partial density of states (DOS). Indeed, the presence of the oxygen vacancies in the studied compound induces a broken symmetry between the up and down spins. Hence, the Fermi level is displaced
EP
towards the conduction band. The presence of the density of state at the Fermi level reveals the half metallicity of such compound. The origin of the half metallicity is caused exclusively
AC C
by the hybridization of the orbitals 6s in Ba atom, 5s in Sr atom, 5d in W atom and 2p in O atom. This hybridization phenomenon is located at the Fermi level with a band width, localized between -0.3 eV and +1.7 eV, conversely. However, the Ni atom does not play any notable role in this hybridization process because it maintains its t2g excited states in the conduction band. It confirms again its strong correlated character confirmed by the corresponding high values of Uscf.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 4: The total and the partial densities of state (DOS) of the steochiometric BaSrNiWO6
AC C
compound.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
Fig. 5: The total and the partial densities of state (DOS) of the BaSrNiWO6 compound with the oxygen vacancy (Vo).
4. Conclusion
In this work, we have investigated the properties of the double perovskite BaSrNiWO6 system with and without point defects. These point defects are Oxygen vacancies (VO) ad Oxygen interstitial (Oi). The calculations have been performed using density functional theory (DFT) with the PWscf code implemented in Quantum-ESPRESSO package. The generalized gradient approximation (GGA) for the exchange correlation functional with the Hubbard U
ACCEPTED MANUSCRIPT correction method (GGA+U) has been used in order to consider the strong electronic correlations on the localized ‘d’ orbitals of Ni and W atoms. In particular, we have discussed the stability of magnetism in the BaSrNiWO6 system with and without such point defects. This study has showed that the pure double perovskite BaSrNiWO6 compound is the insulating anti-ferromagnetic behavior. This is in a good agreement with the similar
RI PT
experimental studies, see Ref [33-35]. The same results are obtained in such a system with the Oxygen interstitial Oi. However, the presence of the Oxygen vacancies, in the all positions except one (x, 0, 0), can convert the ground state from the anti-ferromagnetic to half-metal ferromagnetic states. Moreover, the remaining Oxygen vacancy localized at (x, 0, 0) positions
SC
keeps the anti-ferromagnetic with the half-metal.
M AN U
References
[1] R.C. Sahooa, S.K. Giria, Papri Dasguptab, A. Poddarb, T.K. Natha, Journal of Alloys and Compounds, Volume 658, 15 February 2016, Pages 1003–1009
[2] M. Cernea, F. Vasiliu, C. Bartha, C. Plapcianu, I. Mercioniu, Ceramics International, Volume 40, Issue 8, Part A, (2014), Pages 11601–11609
2016, Pages 217–223
TE D
[3] M. Musa Saad H.-E,N. Rammeh, Physica B: Condensed Matter, Volume 481, 15 January
[4] P. Neenu Lekshmi, M. Vasundhara, Manoj Raama Varma, K.G. Suresh, M. Valant, Physica B: Condensed Matter, Volume 448, 1 September 2014, Pages 285–289 [5] P.W. Barnes, M.W. Lufaso, P.M. Woodward, Acta Crystallogr. B 62 (2006) 384.
EP
[6] J. B. Philipp · P. Majewski · L. Alff · A. Erb · R. Gross · T. Graf · M. S. Brandt · J. Simon · T. Walther · W. Mader · D. Topwal · D. D. Sarma, Phys. Rev. B 68, 144431 (2003)
AC C
[7] J. H. Park, S. K. Kwon, and B. I. Min, Phys. Rev. B 65, 174401 (2002) [8] R. P. Borges, R. M. Thomas, C. Cullinan, J. M. D. Coey, R. Suryanarayanan, L. Ben-Dor, L. Pinsard-Gaudart, and A. Revcolevschi, J. Phys. Condens. Matter 11, L445 (1999). [9] D. D. Sarma, Sugata Ray, Chem. Sci.113, 515 (2001). [10] Min Zhu, Yong Lin, Edward W. C. Lo, Qiong Wang, Zhengjie Zhao, Wenhui Xie, Appl. Phys. Lett. 100, 062406 (2012) [11] H. Z. Guo, J. Burgess, E. Ada, S. Street, and A. Gupta, M. N. Iliev A. J. Kellock, C. Magen, M. Varela, and S. J. Pennycook, Phys. Rev. B 77, 174423 (2008) [12] Andrew M. Ritzmann, Ana B. Muñoz-García, Michele Pavone, John A. Keith, and Emily A. Carter, Chem. Mater (2013), 25, 3011−3019
ACCEPTED MANUSCRIPT [13] Rohan Mishra, Oscar D. Restrepo, Patrick M. Woodward, and Wolfgang Windl, Chem. Mater., (2010), 22 (22), pp 6092–6102 [14] B. Khalil, H. Labrim, O. Mounkachi, B. Belhorma, A. Benyoussef, A. El Kenz and A. Belhaj, Journal of Superconductivity and Novel Magnetism, (2013) 26:151-156 [15] M. Boujnah, H. Labrim, K. Allam, A. Belhaj, A. Benyoussef, A. El Kenz1, B. Belhorma,
RI PT
A. El Bouari, Journal Superconductivity and Novel Magnetism, July 2013, Volume 26, Issue 7, pp 2429-2434.
[16] B. Khalil, H. Labrim, O. Mounkachi, B. Belhorma, A. Benyoussef, A. El Kenz and E. Ntsoenzok, Journal Superconductivity and Novel Magnetism 2012, Volume 25, Number 4,
SC
Pages 1145-1150
M AN U
[17] Jie Zheng, Xiao Tian, Xiao-Lin Wei, Lin Shao, Xiong-Ze Pan, Bi-Yu Tang; Materials Chemistry and Physics, Vol. 167, (2015) 70-76 [18] S. Mishra, B. Ganguli; Materials Chemistry and Physics, Vol. 173, (2016) 429-437 [19] H. Labrim, M.-F. Barthe, P. Desgardin, T. Sauvage, G. Blondiaux, C. Corbel, J.P. Piron, Applied Surface Science, Volume 252, Issue 9, 28, (2006), Pages 3262-3268 [20] M. F. Barthe, H. Labrim, A. Gentils, P. Desgardin, C. Corbel, S. Esnouf, J. P. Piron,
TE D
Phys. Stat. Sol. (c) 09/2007; 4(10):3627 - 3632.
[21] J. P. Perdew, A. Ruzsinszky, G. I. Csonka, O. A. Vydrov, G. E. Scuseria, L. A. Constantin, Phys. Rev. Lett. 100 (2008) 136406
EP
[22] A. I. Liechtenstein, V. I. Anisimov, J. Zaanen, Phys. Rev. B, 52, (1995) R5467.
AC C
[23] P. Giannozzi et al., J. Phys.: Condens. Matter 21 (2009) 395502. [24] K. F. Garrity, J. W. Bennett, K. M. Rabe and D. Vanderbilt, Comput. Mater. Sci. 81 (2014) 446.
[25] B. Manoun, A. Ezzahi, S. Benmokhtar, L. Bih, Y. Tamraoui, R. Haloui, F. Mirinioui, S. Addakiri, J.M. Igartua, P. Lazor, Journal of Molecular Structure, 1045, 6 (2013) 1-14. [26] A. Ezzahi, B. Manoun, A. Ider, L. Bih, S. Benmokhtar, M. Azrour, M. Azdouz, J.M. Igartua, P. Lazor, Journal of Molecular Structure, 985, Issues 2–3, (2011) 339-345. [27] K. F. Garrity, J. W. Bennett, K. M. Rabe and D. Vanderbilt, Comput. Mater. Sci. 81 (2014) 446.
ACCEPTED MANUSCRIPT [28] V. I. Anisimov, V. I. Solovyev, M. A. Korotin, M. T. Czyzyk, Sawatzky, G. A. Phys. Rev. B, 48, (1993)16929. [29] V. I. Anisimov, J. Zaanen, O. K. Andersen, Phys. Rev. B, 44, (1991)943 [30] H. J. Kulik, M. Cococcioni, D. A. Scherlis, N. Marzari, Phys. Rev. Let. 97 (2006), 103001, 00258.
RI PT
[31] R. P. Feynman, Phys. Rev. 56 (1939) 340. [32] H. Hellmann, “Einfûhurung in Die Quantenchemie”, Denticke, Vienna, 1937, p 285. [33] D. E. Cox, G. Shirane, B. C. Frazer, J. Appl. Phys. 38, (1967)1459.
SC
[34] Y. Todate, Journal of Physics and Chemistry of Solids, 60, Issue 8-9,(1999) 1173 – 1175. [35] S. Z. Tian, J. C. Zhan, C. D. Qiao, X. L. Ji, B. Z. Jiang, Materials Letters 60 (2006) 2747–2750.
M AN U
[36] Kumar, Sachin, and Animesh K. Ojha. Materials Chemistry and Physics 169 (2016) 1320.
[37] S. Naji, A. Belhaj,H. Labrim,M. Bhihi, A. Benyoussef,and A. El Kenz, J. Phys. Chem. C
AC C
EP
TE D
2014, 118, 4924−4929. dx.doi.org/10.1021/jp407820a.
ACCEPTED MANUSCRIPT Highlight: In this paper, we have obtained the following results:
• • •
We have studied the ferromagnetism and Half Metallicity in Double Perovskite BaSrNiWO6. We have applied the Ab-inito calculations using the DFT approach. We showed the effects induced by Oxygen Vacancies and Oxygen interstitial. We found that the magnetic ordering in BaSrNiWO6-δ depends on the position of the Oxygen vacancy in the unit cell.
RI PT
•
AC C
EP
TE D
M AN U
SC
Sincerely Yours.