| Mutual energy transformation between electricity and heat can be achieved via the thermoelectric effect. The thermoelectric effect has attracted great attention due to its cleanliness and convenience. As energy source, thermoelectric materials can utilize large amounts of industrial waste heat, vehicle emissions, geothermal and solar energy, and can be realized of mechanical refrigeration, which has a very broad application prospects. The thermoelectric performance of a given material is quantified by a non-dimensional thermoelectric figure of merit ZT = S2?T/(?e+?l), where S, ?, T, ?e, and ?l are Seebeck coefficient, electrical conductivity, absolute temperature, electronic and lattice thermal conductivity, respectively. A good thermoelectric material should have large Seebeck coefficient and electrical conductivity and low thermal conductivity. Thus, ideally engineered thermoelectric materials should exhibit “phonon glass, electron crystal” behavior with high mobility and low lattice thermal conductivity, simultaneously. However, it is challenging to obtain a high ZT value since the strong interdependency of each property via the carrier concentration, which hinders the wide application of thermoelectric materials. It is very important to find methods to meet the specified conditions.Zintl phases are prime candidates for applying this concept to obtain high ZT thermoelectric materials. They are usually composed of electropositive cations and polyanions. The cations donate electrons to the anions, which intend to form ionic bonding. For satisfying the charge balance, the anions form covalent bonding, which favors high carrier mobility. In addition, the feature of the combination of ionic and covalent bonding constitutes a complex structure. Such complexity often results in an exceptionally low lattice thermal conductivity due to the increasing scattering phonons. Hence, the study of Zintl phase compounds is a promising way for obtaining good thermoelectric materials. By using the first-principles calculations combined with the semiclassical Boltzmann transport theory, we studied the electronic structures and thermoelectric properties of Sr5Sn2As6 and Ca5Sn2As6, and the reason of the structural change from Ca5Sn2As6 to Ca5Ga2As6 and their thermoelectric properties.After a full optimization for pure Sr5Sn2As6, the lattice structure exhibits high symmetry and large anisotropy, which is composed of one-dimensional infinite chains of corner shared SnAs4 tetrahedron, and theses chains are separated by Sr atoms. The special lattice structure may lead to the unique thermoelectric properties. By studying the transport properties of Sr5Sn2As6 from 300 to 1200 K, the maximum value 248(?V/K) of Seebeck coefficient is slightly larger than the minimum value 202(?V/K), which mainly benefits the appropriate band gap 0.55 eV. As well known, the large Seebeck coefficient over a wide temperature range is conductive to high ZT value over wide temperature. By the simulation of doping, the ZT value of p-type Sr5Sn2As6 is 1.4 times that of n-type one, which is due to the larger effective mass of the valence band. Moreover, both the Seebeck coefficient and the electrical conductivity is large along the z-direction for n-type Sr5Sn2As6, which results in a high ZT value(3.0) along this direction. Meanwhile, the minimum lattice thermal conductivity approaches that of glass. Hence, the good thermoelectric properties of Sr5Sn2As6 may appear along the z-direction.On the other hand, there are two types of the Ca5M2As6(M = Sn, Ga) structure: one is adjacent polyanions were connected by As-As bonding in Ca5Sn2As6 structure type; the other one is that there is no As-As bonding in the structure of Ca5Sn2As6 type. The appearing of As-As bonding or not will strongly affect the thermoelectric properties. Such As-As bonding in Ca5Ga2As6 results in a sharp peak of density of states near the conduction band minimum, which will dramatically increase its n-type Seebeck coefficient. Moreover, the calculated band decomposed charge density shows that the As-As bonding leads to a high charge accumulation along the y-direction for n-type. Combined with the high electrical conductivity along the covalent anion chain direction, a high electrical conductivity may exist in n-type polycrystalline of Ca5Ga2As6. On the other hand, the absence of As-As bonding in A5Sn2As6 results in a sharp peak of density of states near the valence band maximum, which can enhance its p-type Seebeck effect. For Ca5Sn2As6, the small anisotropy of electrical conductivity may induce the high electrical value for its p-type polycrystalline. Meanwhile, the minimum lattice thermal conductivities of Ca5M2As6(M = Sn, Ga) are 0.56 and 0.7 W/mK, respectively. Consequently, n-type Ca5Ga2As6 and p-type Ca5Sn2As6 polycrystalline may have good thermoelectric performance.In addition, the electronic structures and thermoelectric properties of SnSe nanotubes have also been studied. |
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