Thermoelectric properties of metallic hexaborides $R$B$_6$ ($R$ = La, Pr, Nd, Gd)

Seebeck coefficient has been studied on the single crystals of metallic hexaborides $R$B$_6$ ($R$ = La, Pr, Nd, Gd) at temperatures of 2–300 K. The experiment has shown that the signal is limited by the values $|S|\le$ 1.5 $\mu$V/K for all tested materials. The data obtained for the systems LaB$_6$ and GdB$_6$ were approximated by phonon drag contribution caused by quasi-local (Einstein) mode with characteristic temperatures $\Theta_E$(LaB$_6$) $\approx$ 240 K and $\Theta_E$(GdB$_6$) $\approx$ 180 K. On the contrary, the crystal-line electric field effect induces the inversion between negative and positive types of thermoelectricity, which complicates the simulation of phonon drag in the case of PrB$_6$ and NdB$_6$.