Influence of Thermally Activated Solid-State Crystal-to-Crystal Structural Transformation on the Thermoelectric Properties of the Ca_5–xYb_xAl₂Sb₆ (1.0 ≤ x ≤ 5.0) System

Abstract

The solid-solution Zintl compounds with the mixed cations of Ca 2+ and Yb 2+ in the Ca 5−x Yb x Al 2 Sb 6 (1.0 ≤ x ≤ 5.0) system have been synthesized by high-temperature solid-state reactions. Two slightly different crystal structures of the Ba 5 Al 2 Bi 6 -type and Ca 5 Ga 2 Sb 6 -type phases have been characterized for seven compounds with 2.5 ≤ x ≤ 5.0 and three compounds with 1.0 ≤ x ≤ 2.0, respectively, by both powder and single-crystal X-ray diffraction analyses. The two title phases adopt the orthorhombic space group Pbam (Z = 2, oP26) with seven independent asymmetric atomic sites and share certain structural similarities, including infinite one-dimensional [Al 2 Sb 8 ] double chains and isolated space-filling Ca 2+ /Yb 2+ cations. Interestingly, we reveal the crystal-to-crystal solid-state structural transformation of the Yb-rich compound Ca 1.5 Yb 3.5 Al 2 Sb 6 from the Ba 5 Al 2 Bi 6 -type to the Ca 5 Ga 2 Sb 6 -type phase through the postannealing process, which can be rationalized as the phase transition from the kinetically more stable structure to the thermodynamically more stable crystal structure on the basis of theoretical calculations. Discrepancies of the local coordination geometries of the anionic [Al 2 Sb 8 ] units and the geometrical arrangements of structural building moieties in the two distinct phases provoke the different electrical properties of metallic and semiconducting conduction, respectively, for the Ba 5 Al 2 Bi 6 -type and Ca 5 Ga 2 Sb 6 -type phases. Density of states and crystal orbital Hamilton population analyses based on tight-binding linear muffin-tin orbital calculations prove that the band-gap opening in the Ca 5 Ga 2 Sb 6 -type phase should mainly be attributed to an extended bond distance of the bridging Sb−Sb in the [Al 2 Sb 8 ] unit. A series of thermoelectric (TE) property measurements indicates that the phase transition via the postannealing process eventually results in an enhancement of the TE performance of Yb-rich Ca 1.5 Yb 3.5 Al 2 Sb 6 .

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

“…This kind of the solid‐state single‐crystal to single‐crystal phase transition triggered by the post‐heat‐treatment observed in Ca 1.02 Yb 3.98 Al 1.48 In 0.52 Sb 6 is known to occur only for the Yb‐rich compounds in the Ca/Yb‐mixture containing A 5 M 2 Pn 6 (A = alkali metal, alkaline‐earth metals, rare‐earth metals; M = Al, Ga, In; Pn = pnictogens) system 9,16,17 . Some of these examples include the Ca 5‐ x Yb x Al 2 Sb 6 (1 ≤ x ≤ 5), 9 the CaYb 4 Al 2 Sb 6‐ x Ge x ( x = 0.2, 0.5, 0.7), 18 and Ca 5‐ x Yb x Al 2‐ y In y Sb 6 (3.07 (1) ≤ x ≤ 4.88 (2); 0.16 (2) ≤ y ≤ 2.00) 19 systems.…”

“…The isotypic crystal structure of two title compounds is illustrated in Figure 3, and the overall structure can be described as a combination of (1) the infinite one‐dimensional (1D) anionic chains of propagating along the c ‐axis direction and (2) the three different types of cationic sites filling the space between these 1D chains 9 …”

“…Moreover, three cationic sites are surrounded by either six or seven Sb or Sb/Sn mixtures. The Ca1/Yb1‐mixed site (Figure 3(c)) and the Ca3/Yb3‐mixed site (Figure 3(e)) are located at the center of the slightly distorted octahedra, respectively, whereas the Ca2/Yb2‐mixed site (Figure 3(d)) is located at the center of the two‐edges‐capped square‐pyramid 9 . Interestingly, between the two 6‐coordinated cationic sites, the Ca3/Yb3‐mixed site has a more symmetric octahedral geometry than the Ca1/Yb1‐mixed site based on the interatomic distances between the central atomic site and the surrounding Sb or (Sb/Sn) sites.…”

“…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. Recently, Prof. Snyder reported that the carrier mobility of the Ca 5 Al 2‐ x In x Sb 6 system increased due to the In‐substitution for Al 14 .…”

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 .…”