Hard boron rich boron nitride nanoglasses

Abstract

Boron-rich amorphous boron nitride (BxN1-x, 0.55x0.95) alloys are generated by means of abinitio molecular dynamics simulations and their local structure, mechanical properties and electronic structure are exposed. BN:B phase separations are perceived in all amorphous networks, suggesting that these materials can serve as nanoglass ceramics. The sp(2) hybridization is the main building unit in the BN-rich regions for low boron concentrations, and the models carry locally the signature of the two-dimensional hexagonal BN structure. The amorphous states having both sp(2) and sp(3) hybridizations form for boron contents between 70% and 80%. At higher boron concentrations, sp(3) hybridization with a fraction of similar to 90%-98% is detected as seen in the cubic or wurtize BN crystals. In the boron rich regions, the ideal and defective pentagonal pyramids emerge at 60% boron content, and the first complete B-12 molecule develops at 70% boron concentration. In addition to the B-12 icosahedron, the formation of a cage-like B-16 molecule is, for the first time, discovered in some amorphous alloys. The isolated B-16 molecule is, however, found to be unstable. The Vickers harness calculations reveal that some of these amorphous alloys can serve as hard materials. When their electron properties are considered, all amorphous materials are predicted to be semiconducting.