Thermoelectric properties of p- and n-type Eu${}_5$Sn${}_2$As${}_6$

Eu${}_5$Sn${}_2$As${}_6$ is a Zintl phase crystalizing in the orthorhombic space group Pbam with one-dimensional chains of corner-shared SnAs${}_4$ tetrahedra running in the c-direction. Eu${}_5$Sn${}_2$As${}_6$ has an impressive room temperature Seebeck of >100 µV/K and < - 100 µV/K at 600 K crossing from p- to n-type at 650 K. The maximum thermoelectric figure of merit, zT, for Eu${}_5$Sn${}_2$As${}_6$ is small (0.075), comparable to that of the Zintl phase Ca${}_5$Al${}_2$Sb${}_6$ whose thermoelectric performance was improved by doping Na onto the Ca sites. In this study, we show that the thermoelectric properties of Eu${}_5$Sn${}_2$As${}_6$ can be improved by substituting with K or La. The series Eu${}_{5-x}$K${}_x$Sn${}_2$As${}_6$ provides an increase in maximum zT of 0.22 for x=0.15 due to a decrease resistivity while the onset of bipolar conduction systematically increases in temperature. Upon La substitution, Eu${}_{5-x}$La${}_x$Sn${}_2$As${}_6$ results in a new n-type Zintl phase across the temperature range of 300-800 K. (© 2022 Wiley-VCH GmbH)