Electronic and thermoelectric properties of the layered BaFAgCh (Ch = S, Se and Te): First-principles study
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 , a voltage difference, , proportional to the temperature difference [1,2], i.e., , is developed. The ratio of the developed voltage to the temperature gradient, , 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. [2], where S, , T and are the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity that includes both the electronic () and lattice () contributions, i.e., . 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, and [4]. Therefore, a compromise has to be reached between S, and to enhance ZT. Efficient TE materials can be obtained via two principal ways. The first one is by increasing the value of (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 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 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 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.
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