YbCd2Sb2-based Zintl phases have been identified as promising materials for thermoelectric applications due to their high Seebeck coefficient and electrical conductivity. However, their high thermal conductivity limits their overall thermoelectric performance. To address this, Mg has recently been introduced as an alloying element at Cd atomic sites to reduce the lattice thermal conductivity of YbCd2Sb2. Zhang et al. have reported a high zT (a figure-of-merit for the thermoelectric performance) of 1.4 at 700 K in Yb(Cd0.8Mg0.2)2Sb2. They have demonstrated that the high zT is due to significantly suppressed phonon transport, in other words, low lattice thermal conductivity. They attributed the significantly low lattice thermal conductivity to severely distorted lattices that could not be described even with the Debye-Callaway model. Here, the Debye-Callaway model and Callaway-von Baeyer model have been utilized to evaluate the effect of Mg alloying on the lattice thermal conductivity of Yb(Cd1-xMgx)2Sb2 (x = 0, 0.1, 0.2) by estimating their theoretical lattice thermal conductivities. We found that appropriately fitting the parameter included in the phonon relaxation rate (of the Debye-Callaway model), which represents a fractional change of bulk modulus to that of local bond length, could describe the significantly suppressed lattice thermal conductivities of Yb(Cd1-xMgx)2Sb2 (x = 0, 0.1, 0.2).
