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.

本研究制备了Ca₅₋ₓ₋ᵧYbₓREᵧAl₂Sb₆（RE=镨（Pr）、钕（Nd）、钐（Sm）；1.26 ≤ x ≤ 3.03；0.15 ≤ y ≤ 0.45）体系中的6种稀土（rare-earth, RE）掺杂n型齐尔相（Zintl-phase）热电（thermoelectric, TE）化合物，合成方法为电弧熔炼结合后续热处理。通过粉末衍射与单晶衍射分析，精准确定了该系列化合物的同型与同晶晶体结构。6种目标化合物均以正交晶系Pbam空间群（晶胞Z=2，皮尔逊（Pearson）符号oP26）结晶，存在7个晶体学独立原子位点，其晶型可归为Ca₅Al₂Bi₆型或Ca₅Ga₂As₆型。值得注意的是，富Yb化合物初始结晶为Ca₅Al₂Bi₆型结构，且经后续高温热处理后仍保持该晶型。与之相对，特定组分的富Ca化合物初始以Ca₅Al₂Bi₆型结构存在，经高温热处理后发生相变，转变为Ca₅Ga₂As₆型结构。尤为关键的是，该单晶到单晶的相变还使得两种Ca₅Ga₂As₆型目标化合物——Ca₃.46Yb₁.35Pr₀.19Al₂Sb₆与Ca₃.30Yb₁.50Sm₀.20Al₂Sb₆——的电学输运性质从p型转变为n型，该结论通过塞贝克系数（Seebeck coefficient）测试得以验证。据我们所知，本研究首次通过"旁路"策略，在A₅M₂Pn₆（A=钙（Ca）、锶（Sr）、铕（Eu）、镱（Yb）；M=铝（Al）、镓（Ga）、铟（In）；Pn=砷（As）、锑（Sb）、铋（Bi））体系中，通过相同齐尔相化合物的p型到n型转变，制备出新型n型齐尔相热电化合物。针对3种假设模型开展的理论计算，合理阐释了稀土元素的特定位点偏好与整体电子结构特征。霍尔效应（Hall effect）测试证实了该系列化合物的n型载流子特性，并对Ca₅Ga₂As₆型化合物Ca₃.46Yb₁.35Pr₀.19Al₂Sb₆的载流子浓度与载流子迁移率进行了评估。