High-Pressure High-Temperature Synthesis And Thermal Equation Of State Of High-Entropy Transition Metal Boride

A high-entropy transition metal boride (Hf-0.2 Ti-0.2 Zr-0.2 Ta-0.2 Mo-0.2)B-2 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 V-0 = 27.93 +/- 0.03 angstrom (3) 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 K-0 = 344 +/- 39 GPa, dK/dT = -0.108 +/- 0.027 GPa/K, and a temperature dependent volumetric thermal expansion coefficient alpha = alpha (0) + alpha T-1 + alpha T-2(-2). The thermal stability combined with a high bulk-modulus establishes this high-entropy transition metal boride as an ultrahard high-temperature ceramic material.