| Summary: | A high-entropy transition metal boride (Hf0.2 Ti0.2 Zr0.2 Ta0.2 Mo0.2)B2 sample was synthesized under high-pressure and high-temperature starting from ball-milled oxide precursors (HfO2, TiO2, ZrO2, Ta2O5, and MoO3) mixed with graphite and boron-carbide. Experiments were conducted in a large-volume Paris–Edinburgh press combined with in situ energy dispersive x-ray diffraction. The hexagonal AlB2 phase with an ambient pressure volume V0 = 27.93 ± 0.03 Å3 was synthesized at a pressure of 0.9 GPa and temperatures above 1373 K. High-pressure high-temperature studies on the synthesized high-entropy transition metal boride sample were performed up to 7.6 GPa and 1873 K. The thermal equation of state fitted to the experimental data resulted in an ambient pressure bulk-modulus K0 = 344 ± 39 GPa, dK/dT = −0.108 ± 0.027 GPa/K, and a temperature dependent volumetric thermal expansion coefficient α = α0 + α1T + α2 T−2. The thermal stability combined with a high bulk-modulus establishes this high-entropy transition metal boride as an ultrahard high-temperature ceramic material.
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