First-principles prediction of crystal structure and physical properties of ScB3

The structure and multifunctionality of high-boron transition metal borides. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2023;35:173001. [PMID: 36758243 DOI: 10.1088/1361-648x/acbad6] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

High boron content transition metal (TM) borides (HB-TMBs) have recently been regarded as the promising candidate for superhard multifunctional materials. High hardness stems from the covalent bond skeleton formed by high content of boron (B) atoms to resist deformation. High valence electron density of TM and special electronic structure fromp-dhybridization of B and TM are the sources of multifunction. However, the reason of hardness variation in different HB-TMBs is still a puzzle because hardness is a complex property mainly associated with structures, chemical bonds, and mechanical anisotropy. Rich types of hybridization in B atoms (sp, sp², sp³) generate abundant structures in HB-TMBs. Studying the intrinsic interaction of structures and hardness or multifunction is significant to search new functional superhard materials. In this review, the stable structure, hardness, and multifunctionality of HB-TMBs are summarized. It is concluded that the structures of HB-TMBs are mainly composed by sandwiched stacking of B and TM layers. The hardness of HB-TMBs shows a increasing tendency with the decreasing atom radius. The polyhedron in strong B skeleton provides hardness support for HB-TMBs, among which C2/mis the most possible structure to meet the superhard standard. The shear modulus (G₀) generates a positive effect for hardness of HB-TMBs, but the effect from bulk modulus (G₀) is complex. Importantly, materials with a value ofB₀/G₀less than 1.1 are more possible to achieve the superhard standard. As for the electronic properties, almost all TMB₃and TMB₄structures exhibit metallic properties, and their density of states near the Fermi level are derived from the d electrons of TM. The excellent electrical property of HB-TMBs with higher B ratio such as ZrB₁₂comes from the channels between B-Bπ-bond and TM-d orbitals. Some HB-TMBs also indicate superconductivity from special structures, most of them have stronger hybridization of d electrons from TM atoms than p electrons from B atoms near the Fermi level. This work is meaningful to further understand and uncover new functional superhard materials in HB-TMBs.

Pressure-induced evolution of structures and promising superconductivity of ScB6

Unique multicenter bonding in boron-rich materials leads to the formation of complicated structures and intriguing properties. ScB₆, as a sister compound, possibly possesses high hardness and superconducting critical temperature in this family under ambient pressure. Here, phase transitions, chemical bonding states and electronic properties of ScB₆ at high pressure are uncovered using particle swarm optimization (PSO) combined with first-principles calculations. The phase sequence of P2₁/m → C2/m → Cmcm for ScB₆ has been identified under high pressure. Interestingly, the evolution of a boron framework is from a graphene-like layer to a planar B₄ ring, B₆ and B₇ cycle, and non-planar B₈ cycle, which interconnect a graphene-like network. These phases of ScB₆ are expected to be hard materials due to the excellent mechanical behaviors by the mechanical property calculations. Although the metallic features of the three phases reduce their hardness, the further electron-phonon coupling calculations indicate that the three phases of ScB₆ are superconducting phases under high pressures.