Defect control in Ca1−δCeδAg1−δSb (δ ≈ 0.15) through Nb doping

Abstract

Zintl phases with the nominal compositions Ca_1−δRE_δAg_1−δSb (RE = La, Ce, Pr, Nd, Sm; δ ≈ 0.15) are interesting due to their unique crystal structures and potential as high temperature thermoelectrics. Their structures generally feature the LiGaGe type structure with substantial vacancies on the Ag sites. The formation of such defects can be explained by the electronic effects, with which 18 electrons are required to stabilize the CaAgSb Zintl system. Since the substitution of Ca²⁺ with RE³⁺ will lead to electron excess, the formation of Ag defects will be an intrinsic character of such compounds to maintain their electron precise nature. In this work, the material Ca_0.85Ce_0.15Ag_0.85Sb was selected for a detailed study on defect chemistry. In order to better understand the mechanism related to the defect formation and control in this system, we conducted a series of experiments aimed at controlling the point defects in Ca_0.85Ce_0.15Ag_0.85Sb. This strategy was realized by intentionally doping Nb, which resulted in the discovery of a series of low defect density materials Ca_0.725+xNb_0.1−xCe_0.15AgSb (0 ≤ x ≤ 0.05). In this work, an interesting defect controlling strategy on Zintl phases was demonstrated, which suggested the high flexibility of Ca_1−δRE_δAg_1−δSb in the optimization of thermoelectric properties.