C16 phase high entropy boride with high magnetic anisotropy

Wiley advanced materials

By transformation of ball-milled powder mix of HfO2, MoO3, Nb2O5, Ta2O5, ZrO2, carbon black, and boron carbide under high-pressure and high-temperature during the formation of high-entropy boride (Hf0.2Mo0.2Nb0.2Ta0.2Zr0.2)B2. The synthesis process started at 1100 °C and a fully dense crystalline material was obtained after annealing at 2000 °C. The synthesized material was powdered and mixed with platinum and loaded in a diamond anvil cell and studied to ultrahigh pressure of 220 GPa. The hexagonal AlB2-type phase of (Hf0.2Mo0.2Nb0.2Ta0.2Zr0.2)B2 was found to be stable up to 30% compression in volume and the variation of axial (c/a) ratio shows a slight anisotropic compression with c-axis being more compressible than the a-axis above 62 GPa. The third order Birch-Murnaghan equation of state fit to the high-pressure data up to 220 GPa results in an ambient pressure volume 𝑉0=28.16 ±0.039 Å3, bulk modulus

𝐾𝑜 = 407 ± 6 GPa, pressure derivative of bulk-modulus 𝐾′0 = 2.73 ± 0.045 GPa which puts HEB on par with the category of incompressible transition metal borides like ReB2 and Os2B3.

Georgetown U

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