Synthesis and Ultrahigh Pressure Compression of High-Entropy Boride (Hf_0.2Mo_0.2Nb_0.2Ta_0.2Zr_0.2)B₂ to 220 GPa

The high-entropy boride (Hf0.2Mo0.2Nb0.2Ta0.2Zr0.2)B2 material was synthesized under high-pressures and high-temperatures in a large-volume Paris-Edinburgh (PE) press from a ball-milled powder mix of HfO2, MoO3, Nb2O5, Ta2O5, ZrO2, carbon black, and boron carbide. The transformation process was monitored in situ by energy-dispersive x-ray diffraction with conversion starting at 1100 °C and completed by 2000 °C with the formation of a single hexagonal AlB2-type phase. The synthesized sample was recovered, powdered, and mixed with platinum pressure marker and studied under high pressure by angle-dispersive x-ray diffraction in a diamond anvil cell. The hexagonal AlB2-type phase of (Hf0.2Mo0.2Nb0.2Ta0.2Zr0.2)B2 was found to be stable up to the highest pressure of 220 GPa reached in this study (volume compression V/V0 = 0.70). The third order Birch-Murnaghan equation of state fit to the high-pressure data up to 220 GPa results in an ambient pressure unit cell volume V0=28.16±0.04 Å3, bulk modulusKo = 407 ± 6 GPa, pressure derivative of bulk-modulus K0′ = 2.73 ± 0.045 GPa. Our study indicates that this high-entropy boride (Hf0.2Mo0.2Nb0.2Ta0.2Zr0.2)B2 material is stable to ultrahigh pressures and temperatures and exhibit high bulk modulus similar to other incompressible transition metal borides like ReB2 and Os2B3.