Effect of Rare-Earth Metals Substitution for Ca on the Crystal Structure and Thermoelectric Properties of the Ca_11–xRE_xSb_10–y System

Abstract

Four novel ternary Zintl phase compounds belonging to the Ca11–x ­RE x ­Sb10–y (RE = La, Ce, Nd, Sm; 0.18(4) ≤ x ≤ 0.43(2), 0.14(1) ≤ y ≤ 0.41(1)) system have been synthesized by arc-melting, and the Ho11Ge10-type crystal structure has been characterized for the isotypic title compounds by both powder and single crystal X-ray diffraction analyses. The intrinsically complex crystal structure is viewed as an assembly of the three distinctively shaped cationic polyhedra built from either seven or nine cations and the anionic frameworks constructed by the “dumbbell-shaped” Sb2 and the “square-shaped” Sb4 moieties. All of the four trivalent rare-earth metals were successfully introduced as n-type dopants to partially substitute divalent Ca atoms in the parental compound Ca11Sb10, which resulted in generating two or three Ca2+/RE3+ mixed-cationic sites. In particular, during these substitutions, we observed a unique site preference of Ca2+ and RE3+ among four available cationic sites, where the rare-earth metals with the higher electronegativities than Ca occupied particular atomic sites having the higher Q values. This type of site preference was conclusively explained by theoretical investigations using the tight-binding linear muffin-tin orbital method. Despite the successful n-type doping, the increased electrical conductivities σ and the decreased Seebeck coefficients S of Ca10.75(3)­Nd0.25­Sb9.82(1) and Ca10.82(4)­Sm0.18­Sb9.86(1) compared to those of Ca11Sb10 still presented the p-type rather than n-type characters. These unexpected behaviors should be attributed to ca. 7–20% of Sb3 deficiencies found at the “square-site” (Wyckoff 8h), which spontaneously occurred to reduce an energetically unfavorable antibonding character of the interatomic interaction between two Sb3 atoms at the square-site. Total and partial density of states of a hypothetical structural model Ca10.5­Nd0.5­Sb10, an SEM image of single crystals of Ca10.57(2)­La0.43­Sb9.59(1), and a TGA result of Ca10.82(4)­Sm0.18­Sb9.86(1) are also provided.

p-Type Double Doping and the Diamond-like Morphology Shift of the Zintl Phase Thermoelectric Materials: The Ca11–xAxSb10–yGez (A = Na, Li; 0.06(3) ≤ x ≤ 0.17(5), 0.19(1) ≤ y ≤ 0.55(1), 0.13(1) ≤ z ≤ 0.22(1)) System

“…More interestingly, the overall crystal structure of these title compounds can be alternatively viewed as an assembly of four distinct polyhedra enclosing four different cationic sites, as illustrated in Figure . First, these four cationic sites can be grouped together based on the coordination numbers: three seven-coordinated sites (Ca1-, Ca2-, and Ca3 sites) and one nine-coordinated site (Ca4/Na mixed site) . These three seven-coordinated sites can be further categorized by the coordination geometries: the Ca1 and Ca3 sites are embedded in the distorted pentagonal bipyramids, whereas the Ca2 site is located at the center of a two-edge-monocapped square pyramid.…”

“…These three seven-coordinated sites can be further categorized by the coordination geometries: the Ca1 and Ca3 sites are embedded in the distorted pentagonal bipyramids, whereas the Ca2 site is located at the center of a two-edge-monocapped square pyramid. On the other hand, the nine-coordinated Ca4/Na mixed site can be described as the two-edge-bicapped square pyramid. , Since the substituting cationic Na and Li showed the specific site preferences, respectively, we tried to understand these phenomena based on the size-factor and the electronic-factor criteria. Further analyses about this site preference will be discussed in the Site Preference section.…”

“…The Cu hearth in the arc melter was carefully cleaned after every usage to eliminate any remaining Sb residue. As a result of this arc-melting synthesis, large amounts of small-sized cubic single crystals were generated on the surface of product ingots, just like our previous works for the Ca 11– x RE x ­Sb 10– y (RE = rare-earth metals; 0.18(4) ≤ x ≤ 0.43(2), 0.14(1) ≤ y ≤ 0.51(2)) systems. , To grow a large single crystal, we applied the metal-flux reaction using the excess amount of Pb metals. Each reactant mixture was loaded in an alumina crucible with the ratio of Ca:A:Sb:Ge:Pb (A = Na, Li) = 10:1:9.5:0.5:30.…”

“…During the last several years, our group has thoroughly investigated the Zintl phase A 11 M 10 (A = alkali-/alkaline-earth/rare-earth metals; M = tetrels, pnictogens) series to exploit its intrinsic characteristics as TE materials based on its exceptionally low thermal conductivity from the complex Ho 11 Ge 10 -type phase and the particular electronic transport property from the poor-metallic/semiconducting behaviors. − Some of our previous results for this series include the RE 11 ­Ge 4 In 6– x M x (RE = La, Ce; M = Li, Ge; x = 1, 1.96), Ce 11 ­Ge 3.73(2) In 6.27 , Ca 11– x Yb x ­Sb 10– y ­Ge z (0 ≤ x ≤ 9; 0 ≤ y ≤ 3; 0 ≤ z ≤ 3), and Eu 11– x K x Bi 10– y Sn y ( x = 0, 0.26; y = 0.86, 1.93) systems. Very recently, we even tried to apply the n -type dopants for this series and successfully synthesized two novel Zintl phase systems: the Ca 11– x RE x ­Sb 10– y (RE = La, Ce, Nd, Sm; 0.18(4) ≤ x ≤ 0.43(2), 0.14(1) ≤ y ≤ 0.41(1)) and Ca 11– x RE x ­Sb 10– y (RE = Tb, Dy, Ho, Er, Tm; 0.35(6) ≤ x ≤ 0.41(4), 0.27(1) ≤ y ≤ 0.51(2)) systems. However, although we successfully added various trivalent n -type dopants, TE properties of these systems remained as p -type given the unneglectable Sb deficiencies found at the Sb3 site.…”

Effect of Cationic and Anionic Doping in the Quinary Zintl Phase Thermoelectric Material Ca5‐xYbxAl2‐yInySb6‐zSnz System

“…Given the fact that thermoelectric (TE) materials can change wasted heat generated by various resources including industrial factories, powerplants, and automobiles into electricity, they are considered as one of the most promising energy recycling technology to decrease global energy consumption 1 . Among the various kinds of TE materials, such as TAGS, 2 Half‐Heusler, 3 PbTe, 4 and skutterudites, 5 Zintl phase is one of the best candidates for high‐temperature TE applications due to its semiconducting character and the low thermal conductivity based on the intrinsically complex structures 6 . Some known examples of these Zintl TE materials include the A 14 MSb 11 (A = Ca, Yb; M = Mn, Al), 7,8 the A 5 Al 2 Sb 6 (A = Ca, Yb), 9 the A 11 Pn 10 (A = Ca, Yb; Pn = Sb, Bi, Sn), 10,11 and the A 2 CdSb 2 (A = Ca, Yb, Eu) 12,13 series.…”