Observation of valence band crossing: The thermoelectric properties of CaZn₂Sb₂-CaMg₂Sb₂ solid solution

CaAl₂Si₂ type Zintl phases have long been known to be promising thermoelectric materials. Here we report for the first time on the thermoelectric properties of CaMg₂Sb₂ along with the transport properties of CaZn₂Sb₂-CaMg₂Sb₂ solid solution. The charge carrier tuning in this system was carried out by substituting divalent Ca²⁺ with monovalent Na⁺. To check a possible band convergence, we applied an effective mass analysis to our samples and found an abrupt doubling of the samples' effective masses as the composition switches from Zn-rich to Mg-rich. We further analyzed the effect that alloy scattering plays in the lattice thermal conductivity of our samples with a Modified Klemens model. We showed that the reduction seen in the lattice thermal conductivity of the alloyed samples can be well explained based on the mass difference of Mg and Zn in the poly-anionic metal site. Our best p-doped sample with a composition of Ca_.99Na_.01MgZnSb₂ displays a relatively high peak zT of 0.87 at 850 K.