Six rare-earth (RE) metal-doped n-type Zintl-phase thermoelectric (TE) compounds in the Ca 5−x−y Yb x RE y Al 2 Sb 6 (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 Ca 5 Al 2 Bi 6 -type or Ca 5 Ga 2 As 6 -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 Ca 5 Al 2 Bi 6 -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 Ca 5 Al 2 Bi 6 -type phase first and then underwent phase transition to the Ca 5 Ga 2 As 6 -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 Ca 5 Ga 2 As 6 -type title compoundsCa 3.46 Yb 1.35 Pr 0.19 Al 2 Sb 6 and Ca 3.30 Yb 1.50 Sm 0.20 Al 2 Sb 6 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 A 5 M 2 Pn 6 (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 Ca 5 Ga 2 As 6 -type Ca 3.46 Yb 1.35 Pr 0.19 Al 2 Sb 6 were also evaluated.

Bi 3 (SeO 3 ) 3 (Se 2 O 5 )F, consisting of extremely distortive lone pair cations as well as a very electronegative fluoride anion has been synthesized in high yield via a unique hydrothermal condition using the starting oxides and a small amount of a highly concentrated aqueous HF solution. Bi 3 (SeO 3 ) 3 (Se 2 O 5 )F with the polar monoclinic space group, P2 1 , exhibits a three-dimensional structure composed of BiO 7 , BiO 6 F, SeO 3 , and Se 2 O 5 polyhedra. The infrared (IR) spectral data of Bi 3 (SeO 3 ) 3 (Se 2 O 5 )F do not just confirm the existence of all the constituting bonds but also indicate a wide transparent IR region over 1000 cm −1 for the compound. The reported selenite fluoride also reveals a large bandgap of ca. 3.8 eV attributed to the distortions arising from the constituting asymmetric units as well as the highly electronegative F − anion. Electron localized function (ELF) calculations clearly visualize unsymmetrical polyhedra of Bi 3+ and Se 4+ by presenting the stereoactive lone pairs on each cation. Bi 3 (SeO 3 ) 3 (Se 2 O 5 )F exhibits a very large second-harmonic generation (SHG) response of 8 times that of KH 2 PO 4 (KDP) and type-I phase-matching behavior. A closer structural analysis as well as dipole moment calculations consistently suggest that the origin of the very large SHG response of Bi 3 (SeO 3 ) 3 (Se 2 O 5 )F is a net moment toward the [010] direction arising from the polyhedra of highly distortive lone pair cations.

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Two dopants with quinary Zintl phases Ca1.02Yb3.98Al1.48In0.52Sb6 and Ca5Al1.73In0.27Sb5.44Sn0.56 were synthesized by arc melting followed by annealing to evaluate the possibility of exploiting them as p‐type thermoelectric materials. The phase purity and crystal structures were carefully determined by powder X‐ray diffraction and single‐crystal X‐ray diffraction analyses, and the refined isotypic Ca5Ga2Sb6‐type structures were described as an assembly of the one‐dimensional anionic frameworks and the space‐filling cationic sites. The newly added cationic Yb and anionic Sn showed specific site‐preferences, which was elucidated by either the size‐factor or the electronic‐factor criterion. Moreover, these dopings were closely related to the phase transition of Ca1.02Yb3.98Al1.48In0.52Sb6 and the possible increase in electrical transport property of Ca5Al1.73In0.27Sb5.44Sn0.56. In particular, the thorough study for band structure of the Sn‐doped compound proves that the increased band extrema and degeneracies in the valence band can increase Seebeck coefficients, and the introduced atomic mixtures can lower thermal conductivities by increasing phonon scattering of title compounds.

A series of Eu-doped Zintl compounds belonging to the Ca5–x–y Yb x Eu y Al2Sb6 (x = 0, 1.12; 0 ≤ y ≤ 0.63(2)) system have been successfully synthesized by both the arc-melting and the molten Pb-flux methods. All of the five title compounds initially crystallized in the Ca5Ga2As6-type phase (space group Pbam, Z = 2, Pearson code oP26) and maintained their original structure even after the post-heat treatment, unlike the recently reported n-type Zintl analogues in the Ca5–x–y Yb x RE y Al2Sb6 (RE = Pr, Nd, Sm) systems, which underwent a phase transition from the Ca5Ga2As6-type to the Ca5Al2Bi6-type phase after annealing. This research aimed to understand the origin of the structural preference of the title Ca5–x–y Yb x Eu y Al2Sb6 system, whether it was affected by the valence electron count or the cationic size. Electrical transport property measurements showed an increase in electrical conductivities and a decrease of Seebeck coefficients for Ca4.89(1)Eu0.11Al2Sb6, Ca4.82(1)Eu0.18Al2Sb6, and Ca4.62(1)Eu0.38Al2Sb6, compared to the parental compound Ca5Al2Sb6. Hole effect measurements proved that these changes should be attributed to the reduced carrier concentration and enhanced carrier mobility. The comprehensive density functional theory calculations including electron density map analysis for the hypothetical model Ca4.5Eu0.5Al2Sb6 revealed that the polarity between Al and Sb forming the anionic frameworks decreased as the Eu-dopants were introduced, which eventually affected the carrier mobility in the anionic frameworks. Thermal conductivity measurements proved that the Eu-doping successfully lowered the lattice thermal conductivity because of the enhanced atomic disordering. The magnetization measurements for Ca4.37(2)Eu0.63Al2Sb6 showed a typical Curie–Weiss behavior with weak antiferromagnetic nearest-neighbor interactions with θp = −5.07 K.