On the thermoelectric and magnetic properties, hardness, and crystal structure of the higher boride YbB₆₆

In this work we report the results of our investigation of YbB₆₆ as a potential high-temperature thermoelectric material. A high-quality single crystal has been grown by the optical floating zone method. Thermoelectric transport properties were measured in a temperature range of 373–973 K YbB_66, like REB₆₆ compounds in general, is a p-type semiconductor whose electrical properties can be described by Mott's variable range hopping mechanism. It shows very large Seebeck coefficient ranging from 588 μV K⁻¹ at 373 K to 241 μV K⁻¹ at 973 K and an electrical resistivity ρ that decreases by almost 4 orders of magnitude from 9.4×10⁻¹ Ω m to 2.4 × 10⁻⁴ Ω m. Together with the low thermal conductivity of 2.6 W m⁻¹ K⁻¹ a maximum ZT close to 0.1 around 1000 K was determined with trend of sharp increase toward higher temperatures, which is similar to the previous report of SmB₆₆, and significantly larger than previously reported for Y and Er phases of REB₆₆. However, unlike SmB₆₆, the effective magnetic moment suggests a trivalent state for Yb instead of mixed valence, which was determined by measuring the magnetic susceptibility from 2 to 300 K. The composition of YbB₆₆ was indicated to be metal-rich, which actually may be the origin of the good performance.