Single and double-doping effects on the thermoelectric properties of two Zintl compounds: Eu11Bi8.07(2)Sn1.93 and Eu10.74(2)K0.26Bi9.14(2)Sn0.86

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

Five ternary and quaternary Zintl phases in the solid-solution 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 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 Ho₁₁Ge₁₀-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_0.06Sb_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_0.12Sb_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.

Early stage of the single-crystal growth and tipping point of the cationic site preference in Gd-doped Zintl phase thermoelectric materials

Three novel Zintl phases in the Ca11−xGdxSb10−y system were synthesized, and the early stage of the single-crystal growth process and the “tipping point” of the cationic site preference for the size-factor criterion were thoroughly investigated.

Two Steps to Improve the Thermoelectric Performance of the Ca5-xYbxAl2-yInySb6 System

A series of quaternary and quinary Zintl phase thermoelectric (TE) compounds, Ca_5-xYb_xAl_2-yIn_ySb₆ (3.07(1) ≤ x ≤ 4.88(2); 0.16(2) ≤ y ≤ 2.00), containing Al/In mixed sites as well as Ca/Yb mixed sites has been successfully synthesized by a direct arc-melting method, and the X-ray diffraction analyses indicated that the products initially adopted an orthorhombic Ba₅Al₂Bi₆-type structure (space group Pbam, Z = 2). However, after a postannealing process at 973 K for 1 month, the particular Yb rich compounds underwent a transformation of the original structure type to a Ca₅Ga₂Sb₆-type phase regardless of the In substitution for Al. The noticeable site preference of cationic Ca and Yb in the three available cationic sites could be understood on the basis of a size match between the central cation and the volume of the anionic polyhedra. The observed phase transition was nicely explained by DFT calculations, proving that the Ca₅Ga₂Sb₆-type phase was energetically more favorable than the Ba₅Al₂Sb₆-type phase for the particular Yb-rich compound. Moreover, this energy difference between the two title phases was originally the result of both the site energy in the Ca site and the bond energies in the [(Al/In)₂Sb₈] anionic building blocks. A series of thermoelectric property data indicated that a two-step process involving a partial/full In substitution for Al and a phase transition from the Ba₅Al₂Sb₆-type to the Ca₅Ga₂Sb₆-type phase successfully enhanced the electrical conductivities and the Seebeck coefficients of the title compounds. This kind of combined effect eventually resulted in a ZT improvement for the quinary compound Ca_1.14(2)Yb_3.86Al_1.68(1)In_0.32Sb₆ by approximately 4 times in comparison to its quaternary predecessor Ca_1.55(1)Yb_3.45Al₂Sb₆.

Anionic Doping and Cationic Site Preference in CaYb4Al2Sb6- xGe x ( x = 0.2, 0.5, 0.7): Origin of the Enhanced Seebeck Coefficient and the Structural Transformation

Three Zintl phase compounds belonging to the CaYb₄Al₂Sb_{6- x}Ge _x ( x = 0.2, 0.5, 0.7; nominal compositions) system with various Ge-doping contents were successfully synthesized by arc-melting and were initially crystallized in the Ba₅Al₂Bi₆-type phase (space group Pbam, Pearson codes oP26). However, after post-heat treatment at an elevated temperature, the originally obtained crystal structure was transformed into the homeotypic Ca₅Ga₂Sb₆-type structure according to powder and single-crystal X-ray diffraction analyses. Two types of crystal structures share some isotypic structural moieties, such as the one-dimensional anionic chains formed by _∞¹[Al₂Sb₈] and the void-filling Ca²⁺/Yb²⁺ mixed cations, but the slightly different spatial arrangements in each unit cell make these two structural types distinguishable. This series of title compounds is originally investigated to examine whether anionic p-type doping using Ge can successfully enhance thermoelectric (TE) properties of the Yb-rich CaYb₄Al₂Sb_{6- x}Ge _x series even after the phase transition from the Ba₅Al₂Bi₆-type to the Ca₅Ga₂Sb₆-type phase. More interestingly, we also reveal that the given structural transformation is triggered by the particularly different site-preference of Ca²⁺ and Yb²⁺ among three available cationic sites in each structure type, which is significantly affected by thermodynamic conditions of this system. Band structure and density of states analyses calculated by density functional theory using the tight-binding linear muffin-tin orbital method also prove that the Ge-doping actually increases band degeneracies and the number of resonant peaks near the Fermi level resulting in the improvement of Seebeck coefficients. Electron localization function analyses for the (0 1 0) sliced-plane and the 3D isosurface nicely illustrates the distortion of the paired-electron densities due to the introduction of Ge. The systematic TE property measurements imply that the attempted anionic p-type doping is indeed effective to improve the TE characteristics of the title CaYb₄Al₂Sb_{6- y}Ge _y system.

On the effect of Ga and In substitutions in the Ca11Bi10 and Yb11Bi10 bismuthides crystallizing in the tetragonal Ho11Ge10 structure type

The Ga- and In-substituted bismuthides Ca₁₁Ga_xBi_10-x, Ca₁₁In_xBi_10-x, Yb₁₁Ga_xBi_10-x, and Yb₁₁In_xBi_10-x (x < 2) can be readily synthesized employing molten Ga or In metals as fluxes. They crystallize in the tetragonal space group I4/mmm and adopt the Ho₁₁Ge₁₀ structure type (Pearson code tI84; Wyckoff sequence n2 m j h2 e2 d). The structural response to the substitution of Bi with smaller and electron-poorer In or Ga has been studied by single-crystal X-ray diffraction methods for the case of Ca₁₁In_xBi_10-x [x = 1.73 (2); octabismuth undecacalcium diindium]. The refinements show that the In atoms substitute Bi only at the 8h site. The refined interatomic distances show an unconventional - for this structure type - bond-length distribution within the anionic sublattice. The latter can be viewed as consisting of isolated Bi^3- anions and [In₄Bi₈^20-] clusters for the idealized Ca₁₁In₂Bi₈ model. Formal electron counting and first-principle calculations show that the peculiar bonding in this compound drives the system toward an electron-precise state, thereby stabilizing the observed bond-length pattern.