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

Abstract

Three novel Zintl phase solid solutions in the Ca_11−xGd_xSb_10−y (0.36(2) ≤ x ≤ 0.58(4), 0.26(1) ≤ y ≤ 0.37(1)) system have been successfully synthesized by arc melting, and their crystal structures were carefully characterized by powder and single-crystal X-ray diffraction analyses. During the synthesis, as we changed the reaction time, we were able to observe the early stages of the crystal growth process for a cubic-shaped single crystal with hummocky-type patterns on it. All three title compounds adopted the tetragonal I4/mmm (Z = 4, Pearson code tI84) space group and crystallized in the Ho₁₁Ge₁₀-type phase. The overall complex crystal structure contained three Ca/Gd mixed sites with particular site preferences and one Sb site with some occupational deficiencies. A series of DFT calculations and detailed structure analyses proved that the observed cationic site preference should be attributed to the size-factor criterion rather than the electronic-factor criterion, and the Gd-containing title compounds became a tipping point for the size factor criterion in the overall Ca_11−xRE_xSb_10−y (RE = rare-earth metals) series. Theoretical investigations indicated that the energetically unfavorable antibonding character of the Sb–Sb interaction on the Sb₄ tetramers caused ca. 2.6–3.7% (fractional percentage) of Sb deficiencies and eventually resulted in keeping the p-type character of the Ca_11−xGd_xSb_10−y system despite the successful n-type Gd doping. According to the temperature-dependent thermoelectric property measurements, the relatively “heavier” Gd dopants successfully improved the electrical conductivity, Seebeck coefficient, and thermal conductivity of Ca_10.63(4)Gd_0.37Sb_9.74(1) compared to those of two previously reported Ca_10.75(3)Nd_0.26Sb_9.82(1) and Ca_10.82(4)Sm_0.18Sb_9.86(1), which contained the relatively “lighter” Nd and Sm dopants.