Prediction of scandium tetraboride from first-principles calculations: Crystal structures, phase stability, mechanical properties, and hardness* *Project supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant Nos. 11704170 and 61705097) and the Natural Science Foundation of Shandong Province, China (Grant Nos. ZR2016AP02 and ZR2016EMP01).

Abstract

Using the evolutionary methodology for crystal structure prediction, we have predicted the orthorhombic Cmcm and Pnma phases for ScB4. The earlier proposed CrB4-, FeB4-, MnB4-, and ReP4-type structures for ScB4 are excluded. It is first discovered that the Cmcm phase transforms to the Pnma phase at about 18 GPa. Moreover, both phases are dynamically and mechanically stable. The large bulk modulus, shear modulus, and Young’s modulus of the two phases make it an optimistic low compressible material. Moreover, the strong covalent bonding nature of ScB4 is confirmed by the ELF analysis. The strong covalent bonding contributes greatly to its stability.