Thermoelectric Properties of (Ba,K)Zn2As2 Crystallized in the ThCr2Si2‑type Structure

The compound Ba1–x K x Zn2As2 has a low-temperature phase (α-phase) crystallized in the α-BaCu2S2-type structure and a high-temperature phase (β-phase) crystallized in the ThCr2Si2-type structure. We successfully obtained the β-phase at room temperature as a metastable state by quenching from above the structural phase transition. This allowed us to determine the thermoelectric properties of the β-phase from room to high temperature in the range of 0.00 ≤ x ≤ 0.10. The lattice thermal conductivity is quite low, with a value less than 1 W/mK at 773 K, independent of x. The effective suppression may be due to lattice instability in the underdoped region and to randomness in the overdoped region. The maximum dimensionless figure-of-merit ZT was 0.30 at 773 K for x = 0.03 with the power factor of 0.61 mW/mK2, which is relatively high for a ThCr2Si2-type structure. The results demonstrate the effectiveness of quenching for obtaining a low lattice thermal conductivity, thus providing a new method for attaining high thermoelectric performance.