Zhao Bo, Wang Lu, Tao Qiang, Zhu Pinwen
State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People's Republic of China.
J Phys Condens Matter. 2023 Feb 24;35(17). doi: 10.1088/1361-648X/acbad6.
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 fromhybridization 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 () 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/is the most possible structure to meet the superhard standard. The shear modulus () generates a positive effect for hardness of HB-TMBs, but the effect from bulk modulus () is complex. Importantly, materials with a value of/less than 1.1 are more possible to achieve the superhard standard. As for the electronic properties, almost all TMBand TMBstructures 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 ZrBcomes from the channels between B-B-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.
高硼含量的过渡金属硼化物（HB-TMBs）最近被认为是超硬多功能材料的有前途的候选者。高硬度源于高含量硼（B）原子形成的共价键骨架以抵抗变形。过渡金属的高价电子密度以及硼和过渡金属杂化产生的特殊电子结构是多功能性的来源。然而，不同HB-TMBs硬度变化的原因仍然是个谜，因为硬度是一种主要与结构、化学键和机械各向异性相关的复杂性质。硼原子丰富的杂化类型在HB-TMBs中产生了丰富的结构。研究结构与硬度或多功能性之间的内在相互作用对于寻找新型功能性超硬材料具有重要意义。在这篇综述中，总结了HB-TMBs的稳定结构、硬度和多功能性。得出的结论是，HB-TMBs的结构主要由硼层和过渡金属层的夹心堆叠组成。HB-TMBs的硬度随原子半径减小呈增加趋势。强硼骨架中的多面体为HB-TMBs提供硬度支撑，其中C2/是最有可能满足超硬标准的结构。剪切模量（）对HB-TMBs的硬度产生积极影响，但体积模量（）的影响较为复杂。重要的是，/值小于1.1的材料更有可能达到超硬标准。至于电子性质，几乎所有的TMB和TMB结构都表现出金属性质，它们在费米能级附近的态密度来自过渡金属的d电子。具有较高硼比例的HB-TMBs（如ZrB）的优异电学性质来自于B-B键和过渡金属d轨道之间的通道。一些HB-TMBs还因特殊结构而表现出超导性，其中大多数在费米能级附近过渡金属原子的d电子比硼原子的p电子具有更强的杂化。这项工作对于进一步理解和发现HB-TMBs中的新型功能性超硬材料具有重要意义。
