title: Discovery of elusive structures of multifunctional transition-metal borides.
abstract: A definitive determination of crystal structures is an important prerequisite for designing and exploiting new functional materials. Even though tungsten and molybdenum borides (TMBx) are the prototype for transition-metal light-element compounds with multiple functionalities, their elusive crystal structures have puzzled scientists for decades. Here, we discover that the long-assumed TMB2 phases with the simple hP3 structure (hP3-TMB2) are in fact a family of complex TMB3 polytypes with a nanoscale ordering along the axial direction. Compared with the energetically unfavorable and dynamically unstable hP3-TMB2 phase, the energetically more favorable and dynamically stable TMB3 polytypes explain the experimental structural parameters, mechanical properties, and X-ray diffraction (XRD) patterns better. We demonstrate that such a structural and compositional modification from the hP3-TMB2 phases to the TMB3 polytypes originates from the relief of the strong antibonding interaction between d electrons by removing one third of metal atoms systematically. These results resolve the longstanding structural mystery of this class of metal borides and uncover a hidden family of polytypic structures. Moreover, these polytypic structures provide an additional hardening mechanism by forming nanoscale interlocks that may strongly hinder the interlayer sliding movements, which promises to open a new avenue towards designing novel superhard nanocomposite materials by exploiting the coexistence of various polytypes.
authors.affiliation: College of Engineering Science and Technology, Shanghai Ocean University, Shanghai 201306, China. ycliang@shou.edu.cn.
authors.forename: Yongcheng
authors.initials: Y
authors.lastname: Liang
authors.affiliation: College of Engineering Science and Technology, Shanghai Ocean University, Shanghai 201306, China. ycliang@shou.edu.cn.
authors.forename: Zhaobing
authors.initials: Z
authors.lastname: Wu
authors.affiliation: College of Engineering Science and Technology, Shanghai Ocean University, Shanghai 201306, China. ycliang@shou.edu.cn and State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. wqzhang@mail.sic.ac.cn.
authors.forename: Xun
authors.initials: X
authors.lastname: Yuan
authors.affiliation: State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. wqzhang@mail.sic.ac.cn and Materials Genome Institute and Department of Physics, Shanghai University, Shanghai 200444, China.
authors.forename: Wenqing
authors.initials: W
authors.lastname: Zhang
authors.affiliation: Materials Genome Institute and Department of Physics, Shanghai University, Shanghai 200444, China and Department of Physics, University at Buffalo, State University of New York, Buffalo, New York 14260, USA. pzhang3@buffalo.edu.
authors.forename: Peihong
authors.initials: P
authors.lastname: Zhang