Synthesis and Ultrahigh Pressure Compression of High-Entropy Boride (Hf 0.2 Mo 0.2 Nb 0.2 Ta 0.2 Zr 0.2 )B 2 to 220 GPa.

The high-entropy boride (Hf 0.2 Mo 0.2 Nb 0.2 Ta 0.2 Zr 0.2 )B 2 material was synthesized under high-pressures and high-temperatures in a large-volume Paris-Edinburgh (PE) press from a ball-milled powder mix of HfO 2 , MoO 3 , Nb 2 O 5, Ta 2 O 5 , ZrO 2 , 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 AlB 2 -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 AlB 2 -type phase of (Hf 0.2 Mo 0.2 Nb 0.2 Ta 0.2 Zr 0.2 )B 2 was found to be stable up to the highest pressure of 220 GPa reached in this study (volume compression V/V 0 = 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 (Hf 0.2 Mo 0.2 Nb 0.2 Ta 0.2 Zr 0.2 )B 2 material is stable to ultrahigh pressures and temperatures and exhibit high bulk modulus similar to other incompressible transition metal borides like ReB 2 and Os 2 B 3 .