Thermoelectric properties of EuZn2Sb2 Zintl compounds: zT enhancement through Yb substitution for Eu

Abstract

AZn2Sb2 (A: Ca, Sr, Eu, Yb) Zintl compounds have shown excellent thermoelectric performance peaking around 700 K. Among these materials, EuZn2Sb2 and YbZn2Sb2 compounds have the highest carrier mobility, making them particularly interesting. We present the thermoelectric properties of EuδZn2Sb2 (δ = 0.9, 1.0, 1.1) and Eu1-xYbxZn2Sb2 compounds (x = 0, 0.1, 0.25, 0.5, 0.75, 1) from 373 to 773 K. Our interest is in studying whether introducing A-site vacancies increases the hole carrier concentration with decreasing Eu content or with increasing Yb substitution, which could lead to optimized power factor at high temperatures. The electrical conductivity showed a non-monotonic change caused by carrier mobility scattering. The thermal conductivity decreased through deviation of the Eu content and through Eu-Yb substitution; in particular, the phonon contribution at 773 K reduced gradually from 1.1 W m−1 K−1 (x = 0) to 0.42 W m−1 K−1 (x = 1.0) with increasing Yb fraction and reached the predicted minimum lattice thermal conductivity. The dimensionless figure of merit increased from 0.58 to 0.68 at 773 K through a 10% Yb substitution, i.e. Eu0.9Yb0.1Zn2Sb2, mainly due to the reduction of phonon thermal conductivity without lowering the power factor.