Synthesis and Transport Properties of the Family of Zintl Phases Ca₃RESb₃ (RE = La–Nd, Sm, Gd–Tm, Lu): Exploring the Roles of Crystallographic Disorder and Core 4f Electrons for Enhancing Thermoelectric Performance

Abstract

Zintl phases with complex crystal structures have been studied as promising candidate materials for thermoelectric (TE) applications. Here, we report the syntheses of the family of rare-earth metal Zintl phases with the general formula Ca 4−x RE x Sb 3 (x ≈ 1; RE = La−Nd, Sm, Gd−Tm, Lu). The structural elucidation is based on refinements of single-crystal X-ray diffraction data for 12 unique chemical compositions. The cubic structure is confirmed as belonging to the anti-Th 3 P 4 structure type (space group I4̅ 3d, no. 220, Z = 4), where the Ca and RE atoms share the same atomic site with ca. 75 and 25% occupancies, respectively. Such crystallographic disordering of divalent Ca and trivalent RE atoms in the structure provides a pathway to intricate bonding. The latter, together with the presence of heavy elements such as Sb and the lanthanides, are expected to enhance the scattering probability of phonons, thereby leading to low thermal conductivity κ comparable to that of the ordered RE 4 Sb 3 . The drive of the hypothetical parent compound Ca 4 Sb 3 to be stabilized by alloying with rare-earth metals can be understood following the Zintl−Klemm concept, as the resultant formula may be rationalized as (Ca 2+ ) 3 RE 3+ (Sb 3− ) 3 , indicating the realization of closed-shell electronic configurations for all elements. This notion is confirmed by electronic structure calculations, which reveal narrow bandgaps E g = 0.77 and 0.53 eV for Ca 3 LaSb 3 and Ca 3 LuSb 3 , respectively. In addition, the incorporation of RE atoms into the structure drives the phase into a state of a degenerate semiconductor with dominant hole charge carriers.

Materials design, synthesis, and transport properties of disordered rare-earth Zintl bismuthides with the anti-Th3P4 structure type

“…The synthesized and characterized Ca 3 REBi 3 and Ca 3 REBi 1.5 Sb 1.5 phases that are presented here are analogous to the recently reported antimonides Ca 3 RESb 3 , 16 all crystallizing in the anti -Th 3 P 4 structure type with the noncentrosymmetric space group I 4̄3 d , no. 220, Z = 4.…”

“…The typical lanthanide contraction can be observed for the whole series while a slight deviation is observed for the members of the late lanthanides, such as the Tm- and Lu-bearing samples. It is noted that in the reported Ca 3 RESb 3 , 16 the lanthanide contraction was followed by all the phases. The observed deviation here is associated with the refined compositions.…”

“…These observations mirror those previously reported for the Ca 3 RESb 3 phases. 16 Considering these findings, it is plausible to reason that the atomic sizes of the rare-earth metals play a crucial role in crystallizing Ca 3 REBi 3 .…”

“…Due to the inability to achieve single crystal growth via flux synthesis, we turned to the traditional solid-state synthesis approach by direct reaction which was also utilized for the reported Ca 3 RESb 3 , 16 RE 4 Sb 3 15 and Yb 4− x RE x Sb 3 14,15 phases. The procedures are typical of what was reported for Ca 3 RESb 3 and are as follows: stoichiometric amounts of the starting elements were loaded in a Nb tube inside a glove box followed by sealing it under argon gas atmosphere.…”

“…13 Substitution studies aimed to achieve a balanced electron count and with an attendant crystallographic disorder are fueled by the desire to achieve an optimal performing thermoelectric generator. One of such studies carried out is Yb 4−x RE x Sb 3 (RE = La and Sm) 14,15 and the recently reported Ca 4−x RE x Sb 3 (RE = La-Nd, Sm, Gd-Tm, the Lu) phases, 16 denoted hereafter as Ca 3 RESb 3 . The results in these studies clearly demonstrate the role-play of trivalent RE atom substitution on the cubic RE 4 Sb 3 [17][18][19] phases with the anti-Th 3 P 4 structure type in achieving a narrow bandgap semiconducting state due to the closed-shell electronic configuration achieved by all the elements.…”