Effect of Rare-Earth Metals Substitution for Ca on the Crystal Structure and Thermoelectric Properties of the Ca11–xRExSb10–y System

Abstract

Four novel ternary Zintl phase compounds belonging to the Ca11–xRExSb10–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.