Electronic and magnetic properties of Ba2

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Electronic and magnetic properties of Ba

2

CoWO

6

: First principal investigation

H. Bouhani benziane¹ ^*, O. Sahnoun¹, M. Sahnoun¹, K. Hebali¹ Email:

[email protected] [email protected]

1Laboratoire de Physique Quantique de la Matière et Modélisation Mathématique (LPQ3M), University of Mascara, Algeria

Abstract

We report an investigation of the structural; electronic and magnetic properties of a new double perovskite Ba2CoWO6 by means of density functional calculations (DFT); within the generalized gradient approximation (GGA). The lattice constants of Ba2CoWO6 was obtained and found to agree very well with published experimental reports.

The total and partial density of state are calculated and discussed. The results reveal that the Ba2CoWO6 has stable antiferromagnetic character. Our results predicts that Ba2CoWO6 have metallic nature.

Keywords

: Ab initio , FP-LAPW , Electronic structure, Double perovskite.

1. Introduction

In recent years, double perovskite have received much attention due to their wide applications in many fields magnetoresistance ; ferroelectricity and transport properties are commonly observed in this family; this materials are used as sensors and catalyst electrodes in certain types of fuel cells and candidates for memory devices and spintronics applications [1][2].

The double perovskite structure composition A2B’B’’O6 exihibits a rock-salt type ordering of the B’/B’’ cation transition metal ; also known as double perovskite are derived from a parent ABO3

structure by substituting a mixture of two cation B’ and B’’ on the octahedral B site.

In these A2B’B’’O6 double perovskites, the magnetic and electrical properties exhibit a strong dependence for their effects on particale size and the extent of B’/B’’ site ordering present in their structures or even hybridation of 5d electrons with 3d magnetic electrons of nearby ions [3][4][5].

For example in systeme like Sr2FeReO6 [6][7], minority spin conduction electrons have a mixed Fe⁺³(3d⁵) and Re⁺⁵(5d²) character and mediate a double exchange like ferromagnetic interaction between the lacalized (3d⁵) majority spin. In these materials which the ferrimagnetism is attributed to antiparallel ordering of Fe⁺³ and Re⁺⁵ moments. Other compounds of this sort show antiferromagnetic behavior, with rock-salt ordering where metal cation (B’,B’’) occupy alternate [111] layers. In these cases there is little doubt that the B`` ion is diamagnetic.

The Ba2CoWO6 call our attention as antiferromagnetic candidates due to their crystalline versatility.

Recently, Ba2CoWO6 has been synthesized by Hironoriet & al [8]. it was reported that Ba2CoWO6

adopt caction-ordered perovskite phases. The valence state of this compound was considered to be Co⁺² and W⁺⁶.

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Experimental data report that Ba2CoWO6 at a room temperature crystallize in the cubic space group Fm3m (225) with doubling of unit cell axes of the ideal perovskite and antiferromagnetic below 17 K° [9][10][11].

The present study, is the first theortical work of Ba2CoWO6, we have considering various possible antiferromagnetic states. Hence, in the our work have been made to calculate electronic properties and the order Co⁺² magnetic moments. No theoretical study, however, has been done for the compound.

2. Method and details of calculations

The calculations were performed by using the full-potential linearized augmented plan wave FP- LAPW method based on DFT, as implemented in the Wien2k code [12][13]. My aim focuses on the electronic and magnetic contributions with different anti-ferromagnetic configurations. We have in- vestigated four anti-ferromagnetic configurations the self-consistent calculations are performed using the generalized gradient approximation GGA [14]. The maximum value for partial waves used inside atomic spheres was l = 10 and the potential expansion up to lmax = 4. These parameters were chosen to set the self-consistent calculations where the convergence was taken with respect to the total charge of the system with a tolerance of 0.0001 electron charges. The plane-wave expansion with RMT × KMAX equal to 7, and k sampling with 10 × 10 × 8 k-points mesh in the Brillouin zone was used. The separation energy between inner electrons and valence electrons is −6.0 Ry in the calculation of the exchange-correlation energy.

In our calculation, to obtain better results of local DOS, the correlation parameter Coulomb repul- sion (U) between localized 3d electrons of Co was taken into account. Among the wide range values of U from 2 to 7 for Co, we used U= 6 eV, since it is adopted for Co in LaCoO3, these are taken from Refs[SAH].

3. Results and discussions

For Ba2CoWO6, crystallized in the cubic space group Fm3m(225), the Co ion sit on the 4a(0,0,0) and Co in 4b(0.5,0,0) sites respectively (figure 1)[15]. We used the lattice parameters 8.055 A°[15], witch was estimated from the experiment and the ideal value of internal parameter of O ions is x=0.25. On the figure 2, a arrows represent the magnetic moments of Co atoms. The moments are on the B sites, we have not shown the induced moments on the B’ sites figure 2. The studied states include the ferromagnetic (FM) state, and the three antiferromagnetic configurations, ie. AFM1, AFM2 and AFM3 (figure 2).

The structural properties are obtained by minimization of the total energy depending on the volumes of Ba2CoWO6 included four configurations in antiferromagnetic (AFM) and ferromagnetic (FM) states within GGA approximation. The Murnaghan equation of states [16] is used to obtain equilibrium lattice constant a0, the bulk modulus B(GPa) and its pressure derivative B0 were obtained for all AFM-Ba2CoWO6 phases, by fitting the total energy versus volume. The obtained results are presented in Table1 in comparison with experimental and available the structure prediction dia- gnostic software SPuDS[16] data.

The present calculations are also in a fair agreement with previous experimental calculations[8][15].

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Fig 1. Schematic representations of cubic double-perovskite structures of Ba2CoWO6 with AFM phase.

Fig 2. Schematic representations of the four hypothetical magnetic orderings correspond to FM(a) and AFM1(b), AFM2(c), AFM3 (d) in Ba2CoWO6. The moments are on the B’ sites, we have not shown the induced moments on the B” sites.

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From table1, it can be seen that the total energy from AFM2 is lower than the four magnetic configurations; this clearly indicates that AFM2 is most stable phase.

Table 1: Calculated structural constants for Ba2CoWO6 using GGA.

In Fig.3 we have presented the variation of the total energies versus lattice constant E = E(V), for most stable configurations AFM2 phase.

Fig.3: Total energies as function of volume constant.

Our calculations for the magnetization (table 2) give an average magnetic moment per, Co, and W atom of 3 and 0 μB respectively, for all magnetic configuration, with the Co atoms carrying the largest magnetic moment. In GGA+U, magnetic moment became 3.85 μB, which is in good agreement with the experimental value of 3.87 μB obtained from a neutron scattering study [11].

The smaller magnetic moment of W has been attributed to the direct W-O interaction, which oc- curs at these very short distances. After comparing the total energies, we find that the AFM2 configuration is more stable than the other configuration.

Phases a(A°) V(A°3) B(GPa) B` Atoms (x,y,z)

FM ^8.1822 ^547.7851 ^168.3259 ^3.6720 O(0.2615,0,0)

AFM1

8.0565 522.9247 159.2682 3.7154

O1(0.7385,0.2615,0) O2(0.0,0.0,0.2619) O3(0.2385,0.2385,0.5) O4(0.5,0.5,0.2385)

AFM2 ^8.1631 ^543.9579 ^166.4897 ^3.4801

O1(0.7380,0.7385,0) O2(0.0,0.0,02621) O3(0.2385,0.2387,0.5) O4(0.5,0.5,0.2385)

AFM3 8.0790 527.3182 160.5439 3.6982

O1(0.7385,0.7385,0) O2(0.0,0.0,0.7382) O3(0.2385,0.7615,0.5) O4(0.5,0.5,0.2385)

Exp Ref [8]

Ref [15]

Ref [17]

Ref [18]

8.1062 8.0310 8.1000

532.6622 517.9750 531.4410

O(0.2613,0,0) O(0.262,0,0)

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Equilibrium lattice parameters (a0) are used in order to calculate the electronic density of states. The total and partial density of states DOS and PDOS are presented in Fig.3. In these plots and in fol- lowing ones the energy reference along the x axis is with respect to the Fermi level (Ef).

As shown in fig.4, for the total density of states close to the Fermi level, we observe the metallic nature of Ba2CoWO6 compound, in order both GGA and GGA+U calculations.

In the calculation that all the states, except for Co (3d, W (5d), and O (2p), are located far

away from EF. The electronic configuration of tungsten 4f¹⁴5d⁰6s⁰ this indicate that W ions in the compound are nomagnetic, this is confirmed by the local moments are 0 µB (Table.2). On the other hand, the analysis of partial density of states many of the W (5d) states are empty and located from 0.5 eV to 2.5 eV above EF, while also a few states occupy the region below EF due to the strong hybridization with oxygen between -4 and -6 eV. As a result, although the W (5d) states crossing EF

are partly occupied.

Phases Etot(eV) Co(µB) W(µB)

FM ^1.905940 3.00041 -0.00316

AFM1 ^2.210000 3.00300 -0.00321

AFM2 ⁰ 3.00010 -0.00032

AFM3 ^0.899581 3.00010 -0.00032

Exp

Ref [11] 3.87000

Table 2 Calculated magnetic moments of different magnetic configurations of Co ions

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(a) (b)

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Fig 4 : GGA(a), GGA+U(b) total and atom-projected DOS in Ba2CoWO6 .

The majority O(2p) partial DOS is situated from -8 to 9 eV and their contribution emerge near EF is very little. From various partial DOS we can see that, the occupied of O (2p) and Co (3d) states, are almost located at the same region with dominant Co (3d) electron, which indicates the strong interaction between the O (2p) and Co (3d) orbitals. It can be seen in fig.3 those Co (3d⁷) electrons that are almost full have moved to oxygen atoms for stabilizing the ground state. The Co⁺² and W⁺⁶ ions are in states (3d⁷, S= 3/2) and (5d⁰, S=0) with magnetic moments 3 and 0 μB respectively and thus antiferromagnetic (AFM) coupling via oxygen between them.

On the other hand, the conduction band is mainly composed of Co (3d) character which hybridizes with the O(2p) and W (5d) states .

4. Conclusions

We have performed first principles total energy calculations to investigate the structural, electronic and magnetic properties of ordered double perovskite Ba2CoWO6. Crystallographic parameters were optimized by minimization of energy as a function of volume. Results are in agreement with experimental reports. The results reveal that the Ba2CoWO6 has an antiferromagnetic character where Co(3d) ions have great effect on the magnetic properties. The analysis of the calculated densities indicates there is a strong exchange interaction between the Co (3d) orbital and O (2p) orbital. About the conduction mechanism, oxygen plays an important role in the electronic conduction because the hybridization between Co (3d) and W (5d) is small; we conclude that the Ba2CoWO6 has a metallic character.

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