Thermoelectric and magnetic properties of spark plasma sintered REB₆₆ (RE = Y, Sm, Ho, Tm, Yb)

A synthesis route was successfully established to reliably synthesize the high temperature thermoelectric material REB₆₆ (RE = Y, Sm, Ho, Tm, Yb) much more readily than the typical single crystal growth. Thermoelectric properties of HoB₆₆ and TmB₆₆ are also investigated for the first time. The REB₆₆ samples synthesized by spark plasma sintering showed thermoelectric properties equivalent to those measured on single crystals. A figure of merit ZT around 0.1 at 973 K was obtained with a sharply increasing trend toward higher temperatures. Strikingly, the thermoelectric properties of these compounds appear to be almost independent of the microstructure. The nature of the rare earth does not seem to strongly affect the thermoelectric properties, differences originating mainly from different compositions or the presence of secondary phases. Relatively large negative Curie-Weiss temperatures θ were observed, and an increasing coupling indicated from HoB₆₆ to YbB₆₆. This could be indicative of an unusual coupling mechanism.