Phase Transitions, Structure Evolution, and Thermoelectric Properties Based on A2MnSb2 (A = Ca, Yb)

Layered structures can facilitate “phonon-glass, electron-crystal (PGEC)” properties, which are crucial for the design of the state-of-the-art thermoelectric materials. In this report, two Mn-containing metastable Zintl phases with layered structures, Ca2MnSb2 and Yb2MnSb2, are discovered for the first time. Their structures are isotypic to that of LiZnSb with transition-metal sites partially occupied. Interestingly, these compounds feature very broad homogeneity ranges, which enable flexible tuning on the Mn sites. In addition, close relationships to previously reported Yb9Mn4+xSb9 series have been well established by phase exploration, and structure transition from Yb2MnSb2 to Yb9Mn4+xSb9 was experimentally observed after annealing. The thermal stability of A2MnSb2 (A = Ca, Yb) can be significantly improved through Ag doping on the Mn sites, based on which, systematic investigations on the crystal structures and thermoelectric properties were carried out. Benefited from the complex anionic structures, the thermal conductivity of these phases is extremely low, that is, the Yb-containing material Yb2(Mn0.9Ag0.2)­Sb2 features glasslike thermal conductivity with a minimum value of 0.42 W·m–1·K–1 at 673 K, and meanwhile, a high Seebeck coefficient of 208 μV·K–1 was obtained at 673 K. The discovery of such A2MnSb2 (A = Ca, Yb) phases completed the critical missing members of the A–Mn–Sb ternary family, which is crucial for understanding the structure and properties related to these novel phases, and with the progress of such studies, the design of the state-of-the-art thermoelectric materials based on such Zintl phases is very promising.