Computational Materials Science

(1)

Effects of Fe substitution by Nb on physical properties of BaFeO

₃

: A DFT + U study

Imène Cherair

^a

, Nadia Iles

^b,^⇑

, Lyacine Rabahi

^c,d

, Abdelhafid Kellou

^a

aTheoretical Physics Laboratory, Faculty of Physics, USTHB, BP 32 El Alia, Bab Ezzouar, Algiers, Algeria

bLaboratory of Thin Films Physics and Materials for Electronics, Oran 1 University, Oran, Algeria

cMaterial Physics Laboratory, Faculty of Physics, USTHB, BP 32 El Alia, Bab Ezzouar, Algiers, Algeria

dResearch Center in Industrial Technologies (CRTI), BP 64, Cheraga, Algiers, Algeria

a r t i c l e i n f o

Article history:

Received 11 March 2016

Received in revised form 1 October 2016 Accepted 10 October 2016

Keywords:

BaFe1 xNbxO3perovskites Structural properties Magnetic moments Density Functional Theory GGA + U

Quasi-harmonic Debye model

a b s t r a c t

The structural, electronic, magnetic and thermal properties of BaFe1 xNbxO3perovskites oxides are investigated using the Density Functional Theory (DFT). The Generalized Gradient Approximation (GGA) and on-site Hubbard potential corrections (GGA+U) are considered. According to the formation energies and phonon spectra, the stoichiometric BaFeO3and BaNbO3oxides have a stable cubic phase. TheGGA +Ucalculations show a half metallic behavior of BaFeO3with a large exchange splitting, in agreement with previous experimental and theoretical works. The Fe substitution by Nb forx= 0.5 leads to a surpris- ing insulating ground state. The values of the band gap is 0.40 eV and 1.84 eV usingGGAandGGA+U, respectively. Forx= 0.875 andx= 1, the corresponding alloys are metallic and non-magnetic, while for x= 0.5 andx= 0.625 the antiferromagnetic ground state is found usingGGA+U. For the remaining Nb compositions a half metallic character is noticed with a ferromagnetic state. The quasi-harmonic Debye model is successfully applied to study the temperature evolution of lattice parameters and bulk moduli for different Nb compositions.

1. Introduction

The ABO₃perovskites have gained a growing interest in recent years thanks to their promising physical properties such as ferro- electricity[1]and ferromagnetism[2]. These oxides have attractive applications in many technological fields such as data storage, energy and sensing devices[3]. New functionalities with peculiar physical properties appear when the B element is substituted by another element B⁰ leading to complex perovskite oxides (ABxB⁰1 xO3). For example, Bormanis et al. [4] have evidenced capacitors with tunable permittivity in PbMg1/3Nb2/3O3 and PbZn1/3Nb2/3O3solid solutions. Setter et al.[5]have reported that the substitution of Ti by Zr in PbTiO3(PZT) gives rise to piezoelec- tricity. This property is exploited in transducers and actuators devices. Moreover, the substitution of a magnetic element in ABO₃perovskites affects considerably their magnetic and transport properties [6]. In this context, BaFeO3 is one of the few oxides which contain Fe as a ferromagnetic element in a high valence state (Fe⁴⁺). A stoichiometric BaFeO₃ with a cubic structure (Pm3m) has been obtained by Hayashi et al.[7]. This cubic symme-

try is preserved to at least 8 K. BaFeO3 has a A-type spiral spin order below 111 K and changes to a ferromagnetic order under a small magnetic field with a magnetic moment of 3.5

l

B/Fe. These features were investigated theoretically by Li et al.[8]. They have found that the A-and G-type helical orders in BaFeO3are almost degenerate with small energy difference as compared with the ferromagnetic spin state. A transition from helical order to ferromagnetic order is found when the lattice constant is reduced[9]. Very recently, Rahman et al.[10]have shown that the ground state of cubic BaFeO3is ferromagnetic. The transition from a ferromagnetic to an antiferromagnetic state is yielded only if a displacement of Fe and O ions is considered. However, an experimental work on a thin film BaFeO3deposited by pulse laser technique highlighted a ferromagnetic insulating behavior of this oxide with an optical gap of 1.8 eV[11].

It was also shown that the cubic structure of BaFeO₃facilitates the oxygen vacancies formation making BaFeO3 d(BF) an interest- ing material for innovative electrodes with high activity for oxygen reduction reaction (ORR)[12–14]. This feature is improved when the B-site is substituted by another appropriate element. Among the wide range of B-site dopant that stabilizes the oxygen defi- ciency in BaFeO₃ d, Nb⁵⁺is a stable cation. Accordingly, BaFe_{1 x}- NbxO3 d is recently exploited as a cathode material for

⇑ Corresponding author.

E-mail address:n_ilesdz@yahoo.fr(N. Iles).

Computational Materials Science 126 (2017) 491–502

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