Single and double-doping effects on the thermoelectric properties of two Zintl compounds: Eu₁₁Bi_8.07(2)Sn_1.93and Eu_10.74(2)K_0.26Bi_9.14(2)Sn_0.86

Abstract

Two Zintl phase thermoelectric compounds of EuKBiSn (x = 0, 0.26(1); y = 0.86(2), 1.93(2)) have been synthesized by a high-temperature solid-state reaction and arc-melting methods. The two isotypic crystal structures are characterized by both single-crystal and powder X-ray diffractions, and adopt a tetragonal HoGe-type structure (space group I4/mmm, Z = 2, Pearson code tI84) containing nine crystallographically independent asymmetric atomic sites in a unit cell. The chemical compositions are confirmed by EDS analysis. The complex crystal structure of the two title compounds can be described as an assembly of three different types of co-facial polyhedra formed by cations and 3-dimensional anionic frameworks surrounding these polyhedra. A quaternary title compound, EuKBiSn, which simultaneously contains both cationic and anionic p-dopants in a single compound, was successfully crystallized for the first time in the AM (A = alkaline-earth metals, rare-earth metals; M = triels, tetrels, pnictogens) series. In particular, two different types of p-dopants K and Sn show particular site-preferences, respectively, where K and Sn prefer to occupy the cationic Wyckoff 4e site and the anionic Wyckoff 8h site. These noticeable site preferences can be elucidated by either a size-factor criterion for the K-doping case or by an electronic-factor criterion for the Sn-doping case. The tight-binding linear muffin-tin orbital calculations show that as the double p-doping is applied to the EuKBiSn system, some extra holes are generated on the electronic structures according to the density of states curves. However, a series of thermoelectric property measurements prove that this extra hole-carrier doping is hardly effective enough to completely suppress a bipolar conduction of holes and electrons due to the rigid metallic band structure of the title system.

On the effect of Ga and In substitutions in the Ca11Bi10and Yb11Bi10bismuthides crystallizing in the tetragonal Ho11Ge10structure type

“…Similarly to other structures with the Ho 11 Ge 10 type (Ge & Corbett, 2012;Choi et al, 2017;Nam et al, 2017), a noticeable elongation of the displacement ellipsoids for the electropositive metal in the 8h position (Ca3) is evident from the anisotropic refinement. This feature was discussed as a result of a peculiar bonding of the atoms in this site to the adjacent nonmetal atoms (Choi et al, 2017). Attempts to refine the occupancy of the Ca3 position in Ca 11 In x Bi 10-x did not yield any noticeable variation from unity.…”

“…If we consider the Bi1-Bi1 i contact as nonbonding, the anionic substructure of Ca 11 In x Bi 10-x can be broken down into (Zhao & Parthé, 1991), and Yb 11 Sn 2 Bi 8 (Ge & Corbett, 2012), though in the latter case, additional Sn may be introduced by replacing Bi in other positions in the crystal structure (Liang et al, 2013). Partial substitution of the 8h site was observed in Eu 11-x K x Sn y Bi 10-y , with Sn and Bi occupying this position as a mixture (Choi et al, 2017). Since Ca 11 In x Bi 10-x was prepared from an In-rich medium, its composition with x ' 1.7 must be close to the highest substitution level achievable in the solid solution, at least under given synthetic conditions.…”

“…Metal pnictides with the composition AE 11 Pn 10 [AE (alkaline earth) = Ca, Sr, Ba, Eu, or Yb; Pn = Sb or Bi], crystallizing in the Ho 11 Ge 10 structure type, were also the focus of several studies on thermoelectric properties due to the low thermal conduction in these phases Choi et al, 2017;Nam et al, 2017). Their complex crystal structure, which can be described as consisting of isolated AE 2+ cations, Bi 3À anions, and polynuclear linear Bi 2 4À and square Bi 4 4À units provide efficient phonon scattering, yielding low values for the lattice part of the thermal conductivity.…”

“…Tuning the band structure of the AE 11 Pn 10 phases by rational doping can be a strategy to enhance their thermoelectric performance. In particular, bipolar conduction can be reduced by Ge substitution for Sb in Ca 11-x Yb x Sb 10 (Nam et al, 2017) and by co-doping in both the cationic and anionic sublattices, as was realized for Eu 11-x K x Sn y Bi 10-y (Choi et al, 2017).…”

Experimental and Theoretical Investigations for the Quaternary Mixed‐Cation Zintl Phase Ca1.82(1)Eu0.18CdSb2

“…Zintl phase is considered as an intrinsically suitable candidate material for the TE application due to its complex crystal structure and semiconducting characteristic . Some of those include the A 14 MSb 11 (A = Ca, Yb; M = Al, Mn), A 11 Pn 10 (A = Ca, Yb, Eu; Pn = Sb, Bi, Sn), A 5 M 2 Pn 6 (A = Ca, Yb; M = Al, In; Pn = Sb, Ge) systems, and AM 2 Pn 2 (A = Ca, Yb, Eu; M = Mg, Zn, Cd; Pn = Sb, Bi) systems . Recently, the Yb 2‐ x Eu x CdSb 2 series having the Eu substitution for Yb in the A 2 CdSb 2 (A = Ca, Yb, Eu, Sr) system was reported for the high Z T value due to its low lattice conductivity originated from the 2‐dimensional (2‐D) crystal structure and the Yb 2+ /Eu 2+ mixed‐cations .…”