Role of Eu-Doping in the Electron Transport Behavior in the Zintl Thermoelectric Ca5-x-yYbxEuyAl2Sb6 System

초록

A series of Eu-doped Zintl compounds belonging to the Ca5-x-yYbxEuyAl2Sb6 (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-yYbxREyAl2Sb6 (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-yYbxEuyAl2Sb6 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 theta p = -5.07 K.

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