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Structural Aspects of Skutterudites
Published in Ctirad Uher, Thermoelectric Skutterudites, 2021
Skutterudites are often viewed as Zintl phases. What do I mean by that statement? Zintl phases (after a German chemist Eduard Zintl) are electronic structures where an electropositive metallic element reacts with a nonmetallic element or metalloid by transferring its valence electrons, and the electronegative element accepts as many of them as fully fill its valence shell. The process is similar to ionic salts like NaCl, except that rather than achieving an electronic octet as isolated species, the anions bond together to form a polyanion to fulfill the octet rule. Precise electron counting is at the heart of the Zintl concept and is useful for identifying new thermodynamically stable semiconducting structures. In the case of skutterudites, the polyanion is [X4]4− and the binary skutterudite can be equivalently written as M3+4[X4]4−, reflecting the Zintl concept. Such a binary skutterudite has 24 electrons per formula unit (or, as I noted in Section 1.1, a VEC of 72 per one-half of the unit cell).
2-Related 1–2–2 Zintl Phases
Published in Zhifeng Ren, Yucheng Lan, Qinyong Zhang, Advanced Thermoelectrics, 2017
An outstanding example of a Zintl phase, which has aroused significant interest in TE studies, is the layered CaAl2Si2-type Zintl phases.9 The highest ZT of ~1.3 achieved for p-type in this 1–2–2 Zintl family10 is competitive with other Zintl categories (e.g., Yb14MnSb1111) and even with other good p-type skutterudites12 and half-Heuslers13 within 873 K. Even a higher ZT of ~1.5 has been realized in n-type Mg3Sb2-based materials. In this chapter, an overview of what is known of the structure–property relationships of this particular layered Zintl phases is presented. The fundamental structural and chemical bonding characteristic, and band structures, which underpin both electronic and phonon transport properties, will be discussed. Also proposed are the approaches for the significant improvement in the figure of merit and the prospect outlined for this promising Zintl phase as efficient TEs for power conversion application in the middle temperature range.
Electronic and thermoelectric properties of the layered Zintl phase CaIn2P2: first-principles calculations
Published in Philosophical Magazine, 2020
N. Guechi, A. Bouhemadou, Y. Medkour, Y. Al-Douri, R. Khenata, S. Bin-Omran
Zintl phases, in particular the layered ones, have recently attracted the attention of thermoelectricity researchers because of their high thermoelectric performance [1–4]. Generally, Zintl phases are narrow band-gap semiconductors with complex crystalline structures. The layered Zintl phases can be seen as an alternating cationic and polyanionic layers, stacked along a crystallographic direction [5–12]. These characteristics meet the requirements of high-performance thermoelectric (TE) materials [13–18]. A high-performance TE material must simultaneously have the following contradictory features: (a) a high Seebeck coefficient (S) to ensure maximum conversion of waste-heat into useful electricity, (b) a high electrical conductivity () to guarantee an intensive electrical current and (c) a low thermal conductivity () to maintain a high temperature gradient. A high S value requires a high density of states effective masses of the charge-carriers, which in turn requires flat energy bands. A high requires light effective masses of the charge-carriers, which in turn requires very dispersive energy bands [19]. Materials constituted of heavy atoms and crystalised with complex crystalline structure, such as Zintl phases, can ensure a low thermal conductivity [20–22]. The TE conversion efficiency is measured by a dimensionless parameter noted ZT and called figure of merit, which is defined as: , where is the thermal conductivity; is the lattice thermal conductivity and is the electronic thermal conductivity. The above-mentioned thermoelectric parameters, viz., and , are given by the following relationships [2,19]:where e is the electron charge, is Planck's constant, is Boltzmann's constant, is the density of states effective mass, is the absolute temperature, is the charge-carrier concentration, is the charge-carrier electrical mobility, is the charge-carrier relaxation time and is the charge-carrier effective mass.
Structural, elastic, electronic, optical and thermoelectric properties of the Zintl-phase Ae3AlAs3 (Ae = Sr, Ba)
Published in Philosophical Magazine, 2018
A. Benahmed, A. Bouhemadou, B. Alqarni, N. Guechi, Y. Al-Douri, R. Khenata, S. Bin-Omran
Currently, many kinds of thermoelectric materials have been widely studied, such as Zintl-phases, nanostructured compounds, zinc antimonides, oxides, half-Heusler compounds, clathrates and skutterudites [4–12]. Zintl-phases, a broad class of intermetallic compounds characterised by cations that donate their electrons to support the formation of covalency bonded anionic substrates, have emerged as a promising class of materials for thermoelectric applications due to their complex crystal structures, interesting electronic, chemical and physical properties [13,14]. The structural requirements of Zintl-phases are explained by assuming the presence of both anionic networks and electropositive cations [13]. The anionic networks are covalent and the cationic part is ionic in nature. The resulting mix of ionic and covalent bonds frequently leads to complex crystal structures with large unit cells; such a complex crystal can enable them to have low thermal conductivity [15–17]. Additionally, the Zintl-phase chemistry suggests that the fundamental transport parameters can be modified by doping to achieve a good balance between S and σ, consequently thus can lead to a high power factor [18,19]. Therefore, Zintl-phases provide desired characteristics for high ZT and improved thermoelectric performance since their thermal conductivities are intrinsically low. This has been demonstrated in several previous studies, including Ca3AlSb3 [20], Ca5Al2Sb6 [21] and Ca5Ga2As6 [22]. This further actuates us to search for other possible new Zintl-phase materials for suitable TE candidates. In the current study, we are interested in investigating the Ae3AlAs3 (Ae = Sr, Ba) compounds that were recently synthesised [23]. These compounds crystalise in the Ba3AlSb3 structure type with the space group Cmce. The structural properties of the title compounds, including the lattice parameters and atomic position coordinates, have been investigated using single-crystal X-ray diffraction. The band structure and density of states of Ba3AlAs3 have been carried out using the tight-binding linear muffin-tin orbital (TB-LMTO) method [23]. To the best of our knowledge, some basic physical properties, such as elastic, optical and thermoelectric properties of these newly synthesised compounds are not studied. Therefore, the main object of the present study is the investigation of the structural, elastic, electronic, optical and thermoelectric properties of the Ae3AlAs3 (Ae = Sr, Ba) compounds. The structure of the present paper is as follows: Section 2 describes briefly the computation setting. Section 3 reports and discusses the obtained results, with subsections dedicated to structural, elastic, electronic, optical and thermoelectric properties. The paper is finished with a general conclusion.