First-principles predictions of the structural, electronic, optical and elastic properties of the zintl-phases AE3GaAs3 (AE = Sr, Ba)
Graphical abstract
Introduction
Many experimental and theoretical studies have recently been devoted to the crystal chemistry and physical properties of ternary pnictides in the systems AE–Tr–Pn, especially in the “3-1-3” composition, where AE = alkaline-earth/rare-earth, viz. Ca, Sr, Ba, Eu, Yb; Tr = triel elements, viz. Al, Ga, In; Pn = pnictogen, viz. P, As, Sb. The many possibilities for modifying the chemical composition of the AE–Tr–Pn systems by changing their constituents pave the way for the development of a large number of isomorphic materials providing possibilities to tune their properties to meet the requirements of technological applications. Interest to these systems is because of their outstanding wide range of technologically useful chemical and physical properties [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]]. Almost exclusively, the AE–Tr–Pn systems can be classified according to the concept of Zintl. Named after the German chemist Eduard Zintl [22], classic Zintl phases are a subset of salt-like intermetallic compounds resulting from the reaction of alkali/alkaline-earth metals with transition/post-transition/rare-earth metals and metalloids/semimetals. The first Zintl phases, which were synthetized and structurally characterized by Zintl and co-workers, are binary intermetallic compounds, where the constituent elements are alkali or alkaline-earth elements as cations and group 13–15 elements as anions [24,25]. The compositions of these compounds are simple ratios of their oxidation states, indicating the oxidation states of constituent elements obey the octet rule. Besides the ‘classical’ binary Zintl phases, numerous ternary and quaternary compounds, amongst them transition metal containing ones, obeying the Zintl rules, have been synthetized [26], such as Ca11Sb10, K4Pb9, Na8Si46, Ca14AlSb1, KBa2InAs3 Yb14MnSb11, Pr4MnSb9, Eu10Mn6Sb13 and Cs13Nb2In6As10 [22]. It is conventionally assumed that the electropositive alkali/alkaline-earth metals; the cations, donate their valence electrons to the more electronegative elements (anions) to have stable electronic configurations; similar to ionic salt-like compounds. In cases where the total number of electrons transferred from the cations is insufficient to satisfy the octet rule for anions, the anions complete their closed-shell configuration by forming covalent bonds within polyanionic substructure [27,28]. Thus, in Zintl compounds both ionic and covalent bonding are observed. In the Zintl concept, the transfer of electrons between the cations and the electronegative elements (anions) is typically considered to be ‘‘complete’’; all constituent atoms achieve a filled valence configurations.
In the last decade, Zintl phases have attracted more and more attention from both fundamental scientific and technological application viewpoints and consequently a large number of new Zintl compounds of various complex crystal structures and compositions have been synthesized. These new synthetized Zintl phases have a wide variety of interesting physical properties, such as metallic/semi-metallic and semiconductor natures, thermoelectricity, superconductivity, magnetic order, colossal magnetoresistance, photovoltaics, energy storage, energy conversion, energy-related applications and mixed-valence, among others [[29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49]]. In addition, some Zintl phases have been identified as topological insulators and Dirac semimetals; materials of importance for their quantum properties [22]. Recently, Stoyko et al. [10] reported on the synthesis and crystal structures of the ternary gallium-arsenide Zintl phases Sr3GaAs3 and Ba3GaAs3. A preliminary ab initio calculation of the band structure of Ba3GaAs3 using the tight-binding linear muffin-tin orbital (TB-LMTO) method with the local density approximation reveals the semiconducting nature of this compound [10]. As far as we are aware of, expect [10], there are no prior reports on in-depth theoretical or experimental studies of the basic physical properties of the AE3GaAs3 (AE = Sr, Ba) compounds, such as elastic constants, charge carrier effective masses, optical properties and anisotropy of physical properties. Thus, the present study is devoted to a compressive study of the structural, electronic, optical and elastic properties of the Zintl phases Sr3GaAs3 and Ba3GaAs3 via DFT-based first-principles calculations, for both fundamental physics and application perspectives. Note that one of the main roles of computational physics is to determine materials worthy of experimental research and to assess perspectives of potential applications.
This paper is divided into four sections. The state-of-the-art and aims of the present investigation has been given in Section 1. Calculation methods and settings are provided in Section 2. Detailed analyses and discussions of the results regarding the structural, elastic, electronic and optical properties of the studied compounds are the subject of Section 3. A summary of the main results of the present work is presented in the last section; Section 4.
Section snippets
Calculation methods and settings
DFT-based pseudopotential plane-wave (PP-PW) method with the GGA-PBEsol functional to treat the exchange-correlation effects [50], as implemented in the CASTEP package [51], is used to perform the required quantum mechanics calculations to determine the equilibrium crystalline structure parameters and elastic constants. Vanderbilt's ultrasoft pseudopotentials [52] are used to describe Coulomb's interactions between the valence electrons; As: 4s24p1; Ga: 3d104s24p1; Sr: 4s23p2; Ba: 5s25p66s2,
Ground-state crystal structure parameters
According to Ref. [10], the isotypic ternary gallium-arsenide Zintl-phases AE3GaAs3 (AE = Sr, Ba) crystalize stably in the orthorhombic space group Pnma; no. 62, (Ba3GaSb3-type structure), with eight formula units per unit-cell; Z = 8. A schematic representation of the Sr3GaAs3 unit-cell is given in Fig. 1(a). The prominent structural feature in the AE3GaAs3 structure is that the Ga and As atoms form GaAs4 tetrahedra (Fig. 1(a)), which by connecting via two common edges form isolated [Ga2As6]
Conclusion
In summary, the equilibrium crystal structure, elastic modules and related properties, electronic structures and linear optical properties of the ternary Zintl phases Sr3GaAs3 and Ba3GaAs3 are studied using DFT-based first-principles calculations. It is found that the PP-PW method with the GGA-PBEsol products accurate equilibrium lattice and positional parameters that are in excellent agreement with experimental data. The monocrystalline and polycrystalline elastic constants and related
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgment
We warmly thank the General Directorate of Scientific Research and Technological Development-Algeria for its sponsorship. The author Saad Bin-Omran extends his appreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group no RG-1440-106.
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