Synthesis, physical properties and hardness of a novel YCrB4-type high-entropy boride

Abstract

The YCrB4-type structure is analogous to the AlB2-type one and exists for wide combinations of rare earth and transition metal elements. However, contrary to the large number of AlB2-type high-entropy borides (HEBs), no YCrB4-type HEB has been reported to date. Here, a new 10-cation HEB (Y0.2Dy0.2Ho0.2Tb0.2Lu0.2)(Cr0.2Mo0.2W0.2Re0.2Ru0.2)B4 is synthesized and characterized by x-ray diffraction, electron microscopy, resistivity, magnetic and microhardness measurements. Structural analysis indicates that the HEB is the first member of the orthorhombic YCrB4-type structure family [space group Pbam, a = 5.976(1) Å, b = 11.558(1) Å, c = 3.550(1) Å]. In the cation sublattice, the rare earth (Y, Dy, Ho, Tb, Lu) and transition metal (Cr, Mo, W, Re, Ru) atoms occupy two different cation sites, leading to a remarkably large configurational entropy of 3.22R. This YCrB4-type HEB exhibits paramagnetic and metallic behaviors at high temperature. On cooling to low temperatures, the resistivity shows an upturn below 30 K and a ferromagnetic transition occurs at 6.7 K. In addition, the Vickers hardness of the HEB reaches 23.2 GPa at a load of 0.49 N and 16.4 GPa at a load of 9.8 N. Our work expands both the structural and property diversity of the HEBs.