title: Crystal Field Splitting is Limiting the Stability and Strength of Ultra-incompressible Orthorhombic Transition Metal Tetraborides.
abstract: The lattice stability and mechanical strengths of the supposedly superhard transition metal tetraborides (TmB4, Tm = Cr, Mn and Fe) evoked recently much attention from the scientific community due to the potential applications of these materials, as well as because of general scientific interests. In the present study, we show that the surprising stabilization of these compounds from a high symmetry to a low symmetry structure is accomplished by an in-plane rotation of the boron network, which maximizes the in-plane hybridization by crystal field splitting between d orbitals of Tm and p orbitals of B. Studies of mechanical and electronic properties of TmB4 suggest that these tetraborides cannot be intrinsically superhard. The mechanical instability is facilitated by a unique in-plane or out-of-plane weakening of the three-dimensional covalent bond network of boron along different shear deformation paths. These results shed a novel view on the origin of the stability and strength of orthorhombic TmB4, highlighting the importance of combinational analysis of a variety of parameters related to plastic deformation of the crystalline materials when attempting to design new ultra-incompressible, and potentially strong and hard solids.
authors.affiliation: School of Materials Science and Engineering, and International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191, P. R. China.|Theoretical division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
authors.forename: R F
authors.initials: RF
authors.lastname: Zhang
authors.affiliation: Theoretical division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.|State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi 030001, P. R. China.
authors.forename: X D
authors.initials: XD
authors.lastname: Wen
authors.affiliation: IT4Innovations Center, VSB-Technical University of Ostrava, CZ-708 33 Ostrava, Czech Republic.
authors.forename: D
authors.initials: D
authors.lastname: Legut
authors.affiliation: School of Materials Science and Engineering, and International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191, P. R. China.
authors.forename: Z H
authors.initials: ZH
authors.lastname: Fu
authors.affiliation: Department of Chemistry, Technical University Munich, Lichtenbergstr. 4, D-85747 Garching, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Veprek
authors.affiliation: Department of Chemistry, State University of New York at Buffalo, Buffalo, NY 14260-3000, USA.
authors.forename: E
authors.initials: E
authors.lastname: Zurek
authors.affiliation: Geophysical Laboratory, Carnegie Institution of Washington, NW Washington, DC 20015, USA.|Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, P.R. China.
authors.forename: H K
authors.initials: HK
authors.lastname: Mao