title: Polytypism in superhard transition-metal triborides.
abstract: The quest of novel compounds with special structures and unusual functionalities continues to be a central challenge to modern materials science. Even though their exact structures have puzzled scientists for decades, superhard transition-metal borides (TMBs) have long been believed to exist only in simple crystal structures. Here, we report on a polytypic phenomenon in superhard WB3 and MoB3 with a series of energetically degenerate structures due to the random stacking of metal layers amongst the interlocking boron layers. Such polytypism can create a multiphase solid-solution compound with a large number of interfaces amongst different polytypes, and these interfaces will strongly hinder the interlayer sliding movement within each polytype, thereby further increase the hardness of this particular material. Furthermore, in contrast to the conventional knowledge that intrinsically strong chemical bonds in superhard materials should lead to high lattice thermal conductivity, the polytypic TMB3 manifest anomalously low lattice thermal conductivity due to structural disorders and phonon folding. These findings promise to open a new avenue to searching for novel superhard materials with additional functionalities.
authors.affiliation: 1] College of Engineering Science and Technology, Shanghai Ocean University, Shanghai 201306, China [2].
authors.forename: Yongcheng
authors.initials: Y
authors.lastname: Liang
authors.affiliation: 1] State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China [2] Materials Science and Engineering Department, University of Washington, Seattle, WA 98195-2120, USA [3].
authors.forename: Jiong
authors.initials: J
authors.lastname: Yang
authors.affiliation: State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China.
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.
authors.forename: Wujie
authors.initials: W
authors.lastname: Qiu
authors.affiliation: School of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai 200092, China.
authors.forename: Zheng
authors.initials: Z
authors.lastname: Zhong
authors.affiliation: Materials Science and Engineering Department, University of Washington, Seattle, WA 98195-2120, USA.
authors.forename: Jihui
authors.initials: J
authors.lastname: Yang
authors.affiliation: State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China.
authors.forename: Wenqing
authors.initials: W
authors.lastname: Zhang