Cation Substitution and Size Effects in Ca2ZnSb2 and Yb2MnSb2: Crystal and Electronic Structures and Thermoelectric Properties

Zintl compounds often feature complex structural fragments and small band gaps, favoring promising thermoelectric properties. In this work, a new phase Ca2ZnSb2 is synthesized and characterized to be a LiGaGe-type structure. It is isotypic to Yb2MnSb2 with half vacancies at transition metal sites and undergoes a phase transition to Ca9Zn4+xSb9 after annealing. Interestingly, Ca2ZnSb2 and Yb2MnSb2 are amenable to diverse doping mechanisms at different sites. Here, by substituting smaller Li on cation sites, two novel layered compounds Ca1.84(1)Li0.16(1)Zn0.84(1)Sb2 and Yb1.82(1)Li0.18(1)Mn0.96(1)Sb2 with the P63/mmc space group are discovered, which can be viewed as derivatives of LiGaGe type. Despite having lower occupancy, the structural stability is improved compared with the prototype compounds owing to the reduced interlayered distances. Besides, the band structure analyses demonstrate that the bands near the Fermi level are mainly governed by the interlayered interaction. Due to the highly disordered structure, Yb1.82Li0.18Mn0.96Sb2 features ultralow thermal conductivity from 0.79 to 0.47 W·m-1·K-1 among the testing range; in addition, a remarkable Seebeck coefficient of 270.77 μV·K-1 at 723 K is observed. The discovery of the Ca2ZnSb2 phase enriches the 2-1-2 map, and the size effect induced by cations provides new ideas for material designing.