![]() ![]() Ir and Os are neighboring 5d transition metals in the periodic table and both have similar electronegativity, so Ir–B compounds were also considered as potential high hardness materials after the discovery of superhard OsB 2. 16,24 This finding better resolves the previous controversy over the structure of WB 4. 26 Recent theoretical studies using USPEX, a global crystal structure search technique, have systematically studied the W–B system and have predicted a new stable compound, superhard tungsten pentaboride WB 5, suggesting that the long-debated “WB 4” and the newly predicted WB 5 are actually the same material. Subsequent high-temperature experiments show that orthogonal phase FeB 4 is a potential superconductor and superhard material, and its hardness is 43–70 GPa. have systematically studied the Fe–B system 20 and found that the orthogonal structure oP10-FeB 4 is metastable above the convex hull curve, which indicates that this compound may be stable at finite temperature. 21 In addition, the systematic study of the compounds can also guide subsequent experiments and solve the uncertainty of boride structure caused by the difficulty in determining the position of lighter boron atoms in experiments. Besides, they also found that the previously high-temperature synthesized YB 4 is in metastable phase in the Y–B system. used CALYPSO to systematically study Y–B system and found a new stable phase R m-YB 6 with the hardness of about 37 GPa. 20–25 Although previous researchers have done a lot of research on the Y–B binary system for a long time, Ding et al. 19 Therefore, previous researchers have systematically studied many binary transition metal borides by using structure search to find some borides with high hardness and fully understand the thermodynamic properties of the system. compounds with Vickers hardness > 40 GPa) because it can search for a series of stable and metastable phases only with given components, which provides a powerful tool for the systematic exploration of binary transition metal borides. 14–18Ĭrystal structure search has been widely applied to predict binary hard and superhard materials ( i.e. Notably, the combination of structure searching algorithms with first principles calculations, play a key role in understanding the origin of high hardness and in accelerating the discovery of some hard materials. 13) (37.4 GPa) have high hardness and can be synthesized under normal or low relative pressure through arc melting, making their production cost lower and easier to expand. 1–8 Previous experimental studies have shown that many transition metal borides such as ReB 2 (ref. 1 Introduction With increasing demands for hard materials in industrial applications and the strict requirements in complex environments, transition metal borides have become the most important industrial materials because of their excellent wear resistance, ultra-incompressibility, high hardness, and high melting point. We further find that the four Ir–B phases of P2 1/ m-Ir 2B, C2/ m-Ir 3B 2, P2 1/ m-IrB, and Fmm2-Ir 4B 3 possess dominant Ir–B covalent bonding character, while strong B–B and Ir–B covalent bonds are present in Cm-Ir 4B 5 and Pnma-Ir 3B 4, which are responsible for their excellent mechanical properties. Remarkably, these iridium borides are all ductile. The anisotropic indexes and the three-dimensional surface constructions of Young's modulus indicate that Ir–B compounds are anisotropic with the sequence of the elastic anisotropy of Ir 2B > IrB > Ir 4B 5 > Ir 3B 4 > Ir 4B 3 > Ir 3B 2. C2/ m-Ir 3B 2 is predicted to possess the highest Vickers hardnesses, with a Vickers hardness of 13.1 GPa and 19.4 GPa based on Chen's model and Mazhnik-Oganov's model respectively, and a high fracture toughness of 5.17 MPa m 0.5. ![]() The high bulk modulus of 301 GPa, highest shear modulus of 148 GPa, and smallest Poisson's ratio of 0.29 for C2/ m-Ir 3B 2 make it a promising low compressible material. As a result, besides three stable phases of C2/ m-Ir 3B 2, Fmm2-Ir 4B 3, and Cm-Ir 4B 5, three promising metastable phases, namely, P2 1/ m-Ir 2B, P2 1/ m-IrB, and Pnma-Ir 3B 4, whose energies are within 20 meV per atom above the convex hull curve, are also identified at ambient pressure. We present results of an unbiased structure search for the lowest energy crystalline structures of various stoichiometric iridium borides, using first-principles calculations combined with particle swarm optimization algorithms. ![]()
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