Metallic Borides, La₂Re₃B₇ and La₃Re₂B₅, Featuring Extensive Boron–Boron Bonding

Abstract

La2Re3B7 and La3Re2B5 have been synthesized in single-crystalline form from a molten La/Ni eutectic at 1000 °C in the first example of the flux crystal growth of ternary rare-earth rhenium borides. Both compounds crystallize in their own orthorhombic structure types, with La2Re3B7 (space group Pcca) having lattice parameters a = 7.657(2) Å, b = 6.755(1) Å, and c = 11.617(2) Å, and La3Re2B5 (space group Pmma) having lattice parameters a = 10.809(2) Å, b = 5.287(1) Å, and c = 5.747(1) Å. The compounds possess three-dimensional framework structures that are built up from rhenium boride polyhedra and boron-boron bonding. La3Re2B5 features fairly common B2 dumbbells, whereas La2Re3B7 has unique one-dimensional subunits composed of alternating triangular B3 and trans-B4 zigzag chain fragments. Also observed in La3Re2B5 is an unusual coordination of B by an octahedron of La atoms. Electronic band structure calculations predict that La2Re3B7 is a semimetal, which is observed in the electrical resistivity data as measured on single crystals, with behavior obeying the Bloch-Grüneisen model and a room-temperature resistivity ρ300 K of ∼375 μΩ cm. The electronic band structure calculations also suggest that La3Re2B5 is a regular metal.

General Fabrication of Boride, Carbide, and Nitride Nanocrystals via a Metal-Hydrolysis-Assisted Process

“…Borides, carbides, and nitrides exhibit an unusual combination of physical and chemical properties including high thermal, mechanical, and chemical stabilities, high chemical inertness, wide band gap, high electrical conductivity owing to their features of strong chemical bonds, close-packed lattice, homogeneous and dense structure, etc. − They comprise a class of applications, such as thermionic electron emission sources (LaB 6 , CeB 6 ), electroanalysis (TiB 2 , ZrB 2 , TiN), catalysts (W 2 C, Mo 2 C, VC, NbN, MoN), − interconnects in integrated circuits (TiC, NdC), electronic devices (SiC), lubricants and optical coatings (BN), high-temperature refractory ceramics (MgSiN 2 ), and Li ion battery materials (SiB 3 , VN, Ti 3 C 2 T x ). − Devising synthesis routes for these borides, carbides, and nitrides and studying their properties and structural polymorphism are active and important areas of current research. − …”