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 phase solid solutions in the Ca 11− x Gd x Sb 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 I 4/ mmm ( Z = 4, Pearson code tI 84) space group and crystallized in the Ho 11 Ge 10 -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− x RE x Sb 10− y (RE = rare-earth metals) series. Theoretical investigations indicated that the energetically unfavorable antibonding character of the Sb-Sb interaction on the Sb 4 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− x Gd x Sb 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 0.37 Sb 9.74(1) compared to those of two previously reported Ca 10.75(3) Nd 0.26 Sb 9.82(1) and Ca 10.82(4) Sm 0.18 Sb 9.86(1) , which contained the relatively "lighter" Nd and Sm dopants. Three novel Zintl phases in the Ca 11− x Gd x Sb 10− 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.