Transition-metal borides with the tantalum boride (Ta3B4) crystal structure: their electronic and bonding properties

Chemical vapor deposition of metal borides: 6. The formation of neodymium boride thin film materials from polyhedral boron clusters and metal halides by chemical vapor deposition

“…[2a] The structure of neodymium hexaboride (NdB 6 ), shown in Fig. 1, is an example of this latter complex three-dimensional structural type, in which B 6 octahedra are arranged in a body-centered cubic lattice with the octahedra linked to the apices of other octahedra in all six directions, giving a rigid yet relatively open structure. The strong multicenter, covalent bonding of these boron polyhedra is believed to impart the observed high stability, hardness, and high melting points to most of the boride materials.…”

“…The strong multicenter, covalent bonding of these boron polyhedra is believed to impart the observed high stability, hardness, and high melting points to most of the boride materials. [3] While it is not possible to account for the boride structures in simple bonding terms, it is generally believed that the metal center donates electrons to the boron units in the boron-rich compounds, such as NdB 6 . In the case of NdB 6 , the closo-boron octahedra require 14 valence electrons (2n + 2), of which 12 are provided by the boron atoms.…”

“…[3] While it is not possible to account for the boride structures in simple bonding terms, it is generally believed that the metal center donates electrons to the boron units in the boron-rich compounds, such as NdB 6 . In the case of NdB 6 , the closo-boron octahedra require 14 valence electrons (2n + 2), of which 12 are provided by the boron atoms. [4] If the neodymium then provides two electrons to the cage, one 'free' valence electron should remain per metal center, making the material an excellent conductor.…”

“…The best electronic description of these materials, however, comes from a more complex molecular orbital treatment such as that recently presented by Hoffmann for Ta 3 B 4 . [6] Several metal boride epitaxial thin films, such as HfB 2 , have recently been studied carefully by XPS and related techniques in efforts to probe the structures and properties of these materials. [7] The unique structural and electronic diversity of the metal borides continues to inspire both theoretical and synthetic investigations of these materials.…”

“…[9] One additional important characteristic property of metal borides is that they possess electrical conductivities of a metallic order, and several borides, such as LaB 6 and TiB 2 , have electrical resistances very much lower than the corresponding pure metals (e.g. over five times lower for TiB 2 ).…”