It has been shown previously that the thermoelectric properties of the Zintl phase compound YbCd2Sb2 can be finely tuned via Zn substitution at the Cd-site in the anionic (Cd2Sb2)2− framework. Here we report the results of the investigation of isoelectronic substitution of Yb by Ca. The p-type Yb1−xCaxCd2Sb2 (0.2≤x≤0.8) samples have been synthesized via a solid-state reaction followed by suitable cooling, annealing, grinding, and spark plasma sintering densification processes. In samples with x=0.2, 0.4, 0.5, 0.6, 0.8, the electrical conductivity, Seebeck coefficient, and thermal conductivity measurements have been carried out in the temperature range from 300 to 650 K. It is found that the Ca substitution effectively lowers the thermal conductivity for all samples at high temperature, while it significantly increases the Seebeck coefficient. As a result, the dimensionless figure of merit ZT of 0.96 has been attained at 650 K for samples with x=0.4, while the value is 0.78 for the unsubstituted YbCd2Sb2

Samples of Zintl phase YbCd 2-x Mn x Sb 2 (CaAl 2 Si 2 -type, space group P3m1) were prepared by a solid-state reaction followed by a suitable annealing and spark plasma sintering (SPS) for densification. Investigations were carried out on chemical substitution of Cd by Mn in order to optimize the thermoelectric figure of merit ZT in the solid solution system YbCd 2-x Mn x Sb 2 (x = 0, 0.05, 0.1, 0.15, 0.2, 0.4, 0.6, 0.8, 1.0, 1.5 and 2.0). Seebeck coefficients, electrical and thermal conductivities of all the samples were measured in the temperature range 300-650 K. The presence of Mn (x Յ 0.2) substan-