p-Type Double Doping and the Diamond-like Morphology Shift of the Zintl Phase Thermoelectric Materials: The Ca_11–xA_xSb_10–yGe_z (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

Abstract

Five ternary and quaternary Zintl phases in the solidsolution Ca 11−x A x Sb 10−y Ge z (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 have been successfully synthesized by both of the arc-melting and the molten Pb metal-flux reactions. The crystal structure of these title compounds was characterized by powder and single-crystal X-ray diffractions analyses, and all title compounds crystallized in the Ho 11 Ge 10 -type phase in the tetragonal space group I4/mmm (Z = 4, Pearson code tI84). The complex crystal structure can be described as an assembly of 1) three kinds of cationic polyhedra centered by three different Sb and 2) the cage-shaped anionic frameworks built through the connection of two types of Sb. The newly substituted p-type double dopants of the cationic (Na and Li) and anionic (Ge) elements displayed particular site preferences, which were successfully explained by either the size-factor criterion based on the atomic size or the electronic-factor criterion based on the electronegativity of an element. Quite interestingly, as the reaction conditions were changed, the morphology shift of single crystals in Ca 10.94(3) Na 0.06 Sb 9.58(1) Ge 0.21 occurred from a cubic-shaped to a hummocky-type, to a hopper-type, and eventually to an octahedral-shaped crystal, just like the Yakutian kimberlite diamonds. Moreover, we firmly believe that the inclusion of the p-type Ge dopant for Sb was crucial to trigger this type of morphology shift and complete the octahedral-shaped morphology in the overall crystal-growth mechanism. The theoretical calculations using a DFT method rationalized the observed site preference of Na and the electronic effect of the p-type Ge dopants. The Seebeck coefficient measurements for Ca 10.88(4) Li 0.12 Sb 9.45(1) Ge 0.21 indicated that some portions of electron charge carriers were effectively eliminated by the p-type double dopants using Li and Ge.

Experimental and Theoretical Study on the Nonmagnetic Li/Mg Cosubstitution in the Gd5–x(Li/Mg)xGe4 (x = 1.04, 1.17, 1.53) System

“…The spin-polarized DOS curves are included in Figure S2 in the Supporting Information. The Q value (QVAL) of each atomic site in Gd 5 Ge 4 (Gd 5 Si 4 type) was also evaluated to study the influence of electronic factors on the cationic site preferences. ,− The basis set included Gd 6s, 6p, and 5d orbitals, Li 2s, 2p, and 3d orbitals, Mg 2s, 2p, and 3s orbitals, and Ge 4s, 4p, and 4d orbitals. The 4f wave functions of Gd were considered as core functions.…”

Tuning the Radius Ratio to Enhance Thermoelectric Properties in the Zintl Compounds AM2Sb2 (A = Ba, Sr; M = Zn, Cd)

“…Among the various TE materials, the Zintl phases that have intrinsically complex crystal structures and semiconducting behaviors are considered one of the most promising candidates for this application, and these include the A 14 MPn 11 (A = Ca, Yb; M = Mn, Al; Pn = Sb), , A 11 Pn 10 (A = Ca, Yb; Pn = Ge, Sb, Bi), − A 5 M 2 Pn 6 (A = Ca, Yb; M = Al, In; Pn = Sb, Sn), , and A 2 MPn 2 (A = Ca, Eu, Yb; M = Cd; Pn = Sb) systems. , In particular, worldwide research for the trigonal CaAl 2 Si 2 -type AM 2 Pn 2 (A = Ca, Sr, Ba, Eu, Yb; M = Zn, Cd; Pn = Sb) system has been recently conducted due to its relatively easy dopability and large ZT values. − On the other hand, despite the identical stoichiometry AM 2 Pn 2 , only a few compounds have been reported in the orthorhombic BaCu 2 S 2 -type AM 2 Pn 2 (A = Ba; M = Cu, Zn; Pn = Sb, Te) system for the TE application, such as the Ba 1– x – y Na x Sr y Zn 2 Sb 2 (0 ≤ x ≤ 0.1, 0 ≤ y ≤ 0.1) system, due to its narrow phase-width and limited dopability. − …”

Yb Substitution and Ultralow Thermal Conductivity of the Ca3–xYbxAlSb3 (0 ≤ x ≤ 0.81(1)) System

“…Thermoelectric (TE) materials and devices have been considered one of the sustainable energy solutions for global warming crisis which our modern society confronts since they can directly convert wasted heat generated by different heat sources into electricity. , The efficiency of TE materials is defined by dimensionless figure-of-merit: ZT = σ S 2 T /κ (σ: electrical conductivity, S : Seebeck coefficient, T : absolute temperature, κ: thermal conductivity), and the Zintl phase can be considered as one of the best candidates for TE applications given its high Seebeck coefficients based on the semiconducting character and low thermal conductivities due to the complex crystal structure. , In the last several years, my research team has been investigating various Zintl phase systems for TE material applications, and these include Ca 5– x – y Yb x Eu y Al 2 Sb 6 , Ca 5– x – y Yb x RE y Al 2 Sb 6 (RE = Pr, Nd, Sm), Ba 2– x Sr x Zn 2– y Cd y Sb 2 , and Ca 11– x A x Sb 10– y Ge Z (A = Na, Li) systems. Interestingly, during the recent investigation for the Ca 5– x Yb x Al 2– y In y Sb 6 (Ca 5 Ga 2 As 6 -type) system, the other Zintl compound Ca 3 AlSb 3 adopting the Ca 3 AlAs 3 -type phase was serendipitously obtained as a secondary phase, and due to its structural similarity to the Ca 5 Ga 2 As 6 -type phase and the semiconducting character, we decided to expand our research activity up to the Ca 3 AlAs 3 -type quaternary Ca 3– x Yb x AlSb 3 system.…”