Elsevier

Journal of Alloys and Compounds

Volume 759, 30 August 2018, Pages 32-43
Journal of Alloys and Compounds

Electronic and thermoelectric properties of the layered BaFAgCh (Ch = S, Se and Te): First-principles study

https://doi.org/10.1016/j.jallcom.2018.05.142Get rights and content

Highlights

  • Electronic and thermoelectric properties of BaFAgCh materials have been predicted.

  • BaFAgCh systems are direct band gap semiconductors .

  • Their electronic properties look like that of a natural superlattice .

  • Spin-orbit coupling effect is not negligible in the studied compounds.

  • p-type doping compounds are more favourable for thermoelectric properties than the n-type doping ones.

Abstract

By using the full potential linearized augmented plane wave (FP-LAPW) method, the electronic properties of the layered BaAgChF (Ch = S, Se, Te) were investigated. Both the standard GGA and the TB-mBJ potential were used to model the exchange-correlation potential. To evaluate the spin-orbit coupling (SOC) effect, both the scalar relativistic and full relativistic calculations were performed. The SOC effect is found to be not negligible in the title compounds. The FP-LAPW band structure and the semi-classical Boltzmann transport theory were used to study the charge-carrier concentration and temperature dependences of the thermoelectric parameters, including Seebeck coefficient, electrical conductivity, thermal conductivity and figure of merit. Our results show that the values of the thermoelectric parameters of the p-type compounds are larger than that of the n-type ones. The optimal p-type doping concentrations and temperatures that yield the maximum values of the figure of merit of the title compounds were calculated. These are important parameters to guide experimental works.

Introduction

The history of thermoelectricity backs to 1821, when Seebeck observed that if two dissimilar materials (copper and bismuth) are joined together and the junctions are held at different temperatures, T and T+ΔT, a voltage difference, ΔV, proportional to the temperature difference ΔT [1,2], i.e., ΔV=-SΔT, is developed. The ratio of the developed voltage to the temperature gradient, S=-ΔV/ΔT, which is related to an intrinsic property of materials, is called Seebeck coefficient (also known as thermopower). Generally, thermoelectricity, which is the direct conversion of heat into electricity, is the term that indicates the physical phenomena resulting from the motion of charge carriers under the action of temperature gradient [1].

Recently, thermoelectric materials (TE) attract a heightened interest because they would allow the fabrication of both efficient thermoelectric generators that transfer the lost heat energy into a useful electrical energy (Seebeck effect) and efficient refrigerators that utilize electricity for cooling (Peltier effect) [3]. The conversion of waste heat into electrical energy may play an important role in our current challenge to develop alternative energy technologies to reduce our dependence on fossil fuels and reduce greenhouse gas emissions. Efficient thermoelectric devices could be realized if high-efficient TE could be elaborated. The performance of TE can be quantified by the dimensionless figure of merit ZT given by Ref. ZT=S2σκT[2], where S, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity that includes both the electronic (κe) and lattice (κl) contributions, i.e., κ=κe+κl. A high ZT requires a combination of high electrical conductivity, high thermopower and low thermal conductivity. Though there is no theoretical upper limit of the ZT value, it is challenging to achieve higher values because the three conflicting transport parameters S, σand κ[4]. Therefore, a compromise has to be reached between S, σand κ to enhance ZT. Efficient TE materials can be obtained via two principal ways. The first one is by increasing the value of S2σ (known as power factor (PF)), which define the electrical property of materials, by engineering the electronic structure around the Fermi level [5,6]. The second way is by reducing the lattice thermal conductivity κl by introducing phonon scattering centres, nanostructuring and increasing the grain boundaries [[7], [8], [9]].

The layered quaternary LaOAgS-type compounds (also known as “1111” structure), such as oxychalcogenides and fluorochalcogenides, have recently received an increasing amount of interest [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]] because of their natural superlattice features, such as near two-dimensional electronic structures. They are potential candidates for a wide range of technological applications, such as p-type transparent semiconductors [10,24,26,30], thermoelectrics [[12], [13], [14],18,28], optoelectronic devises [10,31] and photovoltaics [24,32]. The present work focuses on the BaAgChF (Ch = S, Se, Te) compounds.

The quaternary barium silver fluoride chalcogenides BaAgChF (Ch = S, Se) have been synthetized and their structural parameters determined by Charkin and co-workers [16]. They crystallize in the tetragonal LaOAgS-type structure, symmetry group P4/nmm. They are constituted of an alternating quasi-two-dimensional blocks [BaF] and [AgCh] stacked along the c crystallographic axis in the sequence … [BaF]/[AgCh]/[BaF]/[AgCh] …, in another word, they are a natural superlattice structure. A 1 × 2 × 1 supercell of the BaAgSF crystal is shown in Fig. 1 for a better illustration of the layered structure of the considered compounds. The BaAgChF (Ch = S, Se, Te) compounds appear as interesting candidates for thermoelectric applications [12,18] owing to their layered structure. On the theoretical side, Bannikov and co-workers [20] calculated the electronic structures and optical spectra of BaAgSF and BaAgSeF employing the full-potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA). Boudiaf et al. [29] investigated the elastic properties of BaAgSF, BaAgSeF and BaAgTeF using the pseudopotential plane wave method with the GGA and their electronic and optical properties using the FP-LAPW method with the modified Beck-Johnson potential, which is more accurate than the GGA for the calculation of the energy band structure and optical spectra. Unfortunately, both previous studies [20,29] did not include the spin-orbit coupling effect that is not negligible in the case of the BaAgChF (Ch = S, Se and Te) systems. Therefore, the first objective of the present work is the calculation of the electronic properties, including band structure, charge-carrier effective masses, density of states and density of charge distribution, of the title compounds using the FP-LAPW method with the modified Beck-Johnson potential including the spin-orbit coupling. Additionally, the BaAgChF crystals have natural superlattice characteristics, which would result in very favourable electronic properties for thermoelectrics [12,14]. Therefore, the second objective is the prediction of the thermoelectric properties of the BaAgChF compounds as functions of charge-carrier concentration and temperature using the semi-classical Boltzmann theory in combination with the band structure obtained via the FP-LAPW method.

Section snippets

Computational methods

Calculation of the optimized structural parameters, including the lattice parameters (a and c) and atomic position coordinates, and the electronic properties of the BaAgChF (Ch = S, Se and Te) compounds were performed using the full-potential linearized augmented plane wave (FP-LAPW) method [33] based on the density functional theory (DFT) as implemented in the WIEN2k code [34]. For the structural properties, the electronic exchange and correlation effects were treated using the generalized

Structural properties

The BaAgChF (Ch = S, Se, Te) compounds crystallize in a layered tetragonal structure of the LaOAgS-type, space group P4/nmm (n.189) [[16], [17], [18], [19], [20],25]. This structure may be viewed as alternating blocks of [BaF] and [AgCh] stacked along the c crystallographic axis, as shown in Fig. 1. There are four inequivalent atomic positions in the conventional cell, which are Ba: 2c (1/4, 1/4, zBa), F: 2a (3/4, 1/4, 0), Ag: 2b (3/4, 1/4, 1/2) and Ch: (1/4, 1/4, zCh), where zBa and zCh are

Conclusion

In this work, we have investigated the structural, electronic and thermoelectric properties of the barium silver fluoride chalcogenides BaAgChF (Ch = S, Se, Te) using the full potential linearized augmented plane wave approach in the framework of density functional theory. The GGA-PBEsol optimized structural parameters of the title compounds are in excellent agreement with the available experimental data. Analysis of the calculated band structures using the GGA-PBEsol and TB-mBJ both with and

Acknowledgment

The authors (A. Bouhemadou and S. Bin-Omran) extend their appreciation to the International Scientific Partnership Program ISPP at King Saud University for funding this research work through JSPP# 0025.

References (57)

  • G.K.H. Madsen et al.

    BoltzTraP. A code for calculating band-structure dependent quantities

    Comput. Phys. Commun. Phys. Commun

    (2006)
  • X. Zhang et al.

    Influence of the elements (Pn = As, Sb, Bi) on the transport properties of p-type Zintl compounds Ba2ZnPn2

    Comput. Mater. Sci.

    (2017)
  • D.G. Cahill et al.

    Heat flow and lattice vibrations in glasses

    Solid State Commun.

    (1989)
  • P.S. Kireev

    Semiconductor Physics

    (1975)
  • T.M. Tritt et al.

    Thermoelectric materials, phenomena, and applications: a bird's eye view

    MRS Bulletin

    (2006)
  • D.A. Polvani et al.

    Large improvement in thermoelectric properties in pressure-tuned p-type Sb1.5Bi0.5Te3

    Chem. Mater.

    (2001)
  • T. Fang et al.

    Validity of rigid-band approximation in the study of thermoelectric properties of p-type FeNbSb-based half-Heusler compounds

    J. Electron. Mater.

    (2017)
  • Y. Takagiwa et al.

    Dopants effect on the band structure of PbTe thermoelectric material

    Appl. Phys. Lett.

    (2012)
  • T. Takabatake et al.

    Phonon-glass electron-crystal thermoelectric clathrates: Experiments and theory

    Rev. Mod. Phys.

    (2014)
  • A.J. Minnich et al.

    Bulk nanostructured thermoelectric materials: current research and future prospects

    Energy Environ. Sci.

    (2009)
  • O. Delaire et al.

    Giant anharmonic phonon scattering in PbTe

    Nat. Mater.

    (2011)
  • H. Yanagi et al.

    P-type conductivity in wide-band-gap BaCuQF (Q = S, Se)

    Appl. Phys. Lett.

    (2003)
  • H. Yanagi et al.

    Valence band structure of BaCuSF and BaCuSeF

    J. Appl. Phys.

    (2006)
  • A.K.F. Ul Islam et al.

    First principles study of electronic structure dependent optical properties of oxychalcogenides BiOCuCh (Ch=S, Se, Te)

    Indian J. Phys.

    (2017)
  • V.K. Gudelli et al.

    Electronic structure, transport, and phonons of SrAgChF (Ch=S, Se, Te): Bulk superlattice thermoelectrics

    Phys. Rev. B

    (2015)
  • D. Zou et al.

    Electronic structures and thermoelectric properties of layered BiCuOCh oxychalcogenides (Ch = S, Se and Te): first-principles calculations

    J. Mater. Chem. A

    (2013)
  • G. Liu et al.

    Thermal properties of layered oxychalcogenides BiCuOCh (Ch = S, Se, and Te): a first-principles calculation

    J. Appl. Phys.

    (2016)
  • A. Zakutayev et al.

    Electronic properties of BaCuChF (Ch = S, Se, Te) surfaces and BaCuSeF/ZnPc interfaces

    J. Appl. Phys.

    (2010)
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