p-Type to n-Type Conversion through the “Bypass” Phase Transition in the Zintl-Phase Thermoelectric Materials

Six rare-earth (RE) metal-doped n-type Zintl-phase thermoelectric (TE) compounds in the Ca5–x–yYbxREyAl2Sb6 (RE = Pr, Nd, and Sm; 1.26 ≤ x ≤ 3.03; 0.15 ≤ y ≤ 0.45) system have been prepared using arc melting followed by the post-heat treatment, and the isotypic and homotypic crystal structures were carefully determined by the powder and single-crystal analyses. Six title compounds adopted either the Ca5Al2Bi6-type or Ca5Ga2As6-type phase in the orthorhombic Pbam space group (Z = 2, Pearson code oP26) with seven crystallographically independent atomic sites. Interestingly, the Yb-rich compounds originally crystallized in the Ca5Al2Bi6-type phase and maintained their structure type even after the post-heat treatment. On the other hand, the Ca-rich compounds with particular compositions adopted the Ca5Al2Bi6-type phase first and then underwent phase transition to the Ca5Ga2As6-type phase after the post-heat treatment at the high temperature. Moreover, this single-crystal to single-crystal phase transition also brought the p-type to n-type conversion of electrical transport property for the two Ca5Ga2As6-type title compounds—Ca3.46Yb1.35Pr0.19Al2Sb6 and Ca3.30Yb1.50Sm0.20Al2Sb6—according to Seebeck coefficient measurements. As far as we understand, this study is the first example of producing novel n-type Zintl TE compounds by the "bypass" method through the p-type to n-type conversion of identical Zintl compounds in the A5M2Pn6 (A = Ca, Sr, Eu, and Yb; M = Al, Ga, In; Pn = As, Sb, and Bi) system. Theoretical calculations conducted for the three hypothetical models rationalized the specific site preference of RE and the overall electronic structures. Hall effect measurements proved the n-type carrier, and the carrier concentration and carrier mobility of this Ca5Ga2As6-type Ca3.46Yb1.35Pr0.19Al2Sb6 were also evaluated.