Contradicting Influence of Zn Alloying on Electronic and Thermal Properties of a YbCd₂Sb₂-Based Zintl Phase at 700 K

Zintl compounds are promising thermoelectric materials for power generation as their electronic and thermal transport properties can be simultaneously engineered with anion/cation alloying. Recently, a peak thermoelectric figure-of-merit, zT, of 1.4 was achieved in a (Yb_0.9Mg_0.1)Cd_1.2Mg_0.4Zn_0.4Sb₂ Zintl phase at 700 K. Although the effects of alloying Zn in lattice thermal conductivity had been studied thoroughly, how the Zn alloying affects its electronic transport properties has not yet been fully investigated. This study evaluates how the Zn alloying at Cd sites alters the band parameters of (Yb_0.9Mg_0.1)Cd_1.6−xMg_0.4Zn_xSb₂ (x=0-0.6) using the Single Parabolic Band model at 700 K. The Zn alloying increased the density-of-states effective mass (m_d^*) from 0.87 to 0.97 m₀. Among Zn-alloyed samples, the m_d^* of the x=0.4 sample was the lowest (0.93 m₀). The Zn alloying decreased the non-degenerate mobility (μ₀) from 71 to 57 cm² s⁻¹ V⁻¹. Regardless of Zn alloying content, the μ₀ of the Zn-alloyed samples were similar (∼57 cm² s⁻¹ V⁻¹). Consequently, the x=0.4 with the highest zT exhibited the lowest weighted mobility (μ_W). The lowest μ_W represents the lowest theoretical electronic transport properties among other x. The highest zT at x=0.4 despite the lowest μ_W was explained with a significant lattice thermal conductivity reduction achieved with Zn alloying with x=0.4, which outweighed the deteriorated electronic transport properties also due to the alloying.