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

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₊Sb₉ 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 Ca_1.84(1)Li_0.16(1)Zn_0.84(1)Sb₂ and Yb_1.82(1)Li_0.18(1)Mn_0.96(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, Yb_1.82Li_0.18Mn_0.96Sb₂ 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.