Boron oxides under pressure: Prediction of the hardest oxides

Abstract

We search for stable compounds of boron and oxygen at pressures from 0 to 500 GPa using the ab initio evolutionary algorithm USPEX. Only two stable stoichiometries of boron oxides, namely, B 6 O and B 2 O 3 , are found to be stable, in good agreement with experiment. A hitherto unknown phase of B 6 O at ambient pressure, Cmcm-B 6 O, has recently been predicted by us and observed experimentally. For B 2 O 3 , we predict three previously unknown stable high-pressure phases-two of these (Cmc2 1 and P 2 1 2 1 2 1) are dynamically and mechanically stable at ambient pressure, and should be quenchable to ambient conditions. Their predicted hardnesses, reaching 33-35 GPa, make them harder than SiO 2-stishovite. These are the hardest known oxides (if one disregards B 6 O, which is essentially a boron-based insertion compound). Under pressure, the coordination number of boron atoms changes from 3 to 4 to 6, skipping fivefold coordination.

Computational discovery and characterization of new B2O phases

“…Although recent calculations show that the B2O is slightly above the convex hull, 6 it is noted that our predicted structures are energetically favorable compared with the previously experimental structure. 19,21 We have further examined the phase stability using first-principles molecular dynamic (MD) simulations for the Cmmm and P-3 phases in the NVT ensemble near the room temperature (300 K).…”

“…Such fascinating icosahedral clusters are also usually found in boron-rich compounds, e.g., B4C, B6O, B12As2, and B12P2. [2][3][4][5][6][7][8] Although common morphological features based on the B12 icosahedron include a twelve-atom cluster with each boron atom located on the vertex of the icosahedron, the bonding networks and electronic states are very sensitive to changes in chemical composition. The covalent bonding distribution in the framework of B12 in α-B involves 13 internal bonding orbitals and 12 external bonding connected by direct bonding or through additional atoms.…”

Phase relations, and mechanical and electronic properties of nickel borides, carbides, and nitrides from ab initio calculations

“…Crystal structure prediction calculations of pure Ni performed earlier by similar procedure showed its stability in Fm 3̄ m ( fcc ) structure up to 400 GPa. 36,37 For pure boron, carbon, and nitrogen the following data have been used: B- R 3̄ m , - Pnnm , - Cmca , and - P 6 3 / mcm ; 38 C- P 6 3 mc (graphite) and - Fd 3̄ m (diamond); N- Pa 3̄, - I 2 1 3, 39 , - Pba 2, 40 and - I 4̄3 m . 41 The pressures of phase transition between listed polymorphic modifications are shown in Fig.…”

Thermodynamic stability of borophene, B2O3 and other B1−xOx sheets

“…Boron is also known to have a high chemical affinity to oxygen and therefore boron-rich (nano) structures can only be prepared under inert conditions or vacuum; so is borophene. For bulk phases the boron-oxygen system is well studied and several works on the phase diagram exist [13][14][15]. Thermodynamically stable are the pure 'icosahedral' boron phases in their various forms [16], boron suboxide B 6 O [17] and boron trioxide B O 2 3 that is a vitreous phase under ambient conditions [12] but can be crystalline when synthesized under pressure [15,18,19].…”

“…For bulk phases the boron-oxygen system is well studied and several works on the phase diagram exist [13][14][15]. Thermodynamically stable are the pure 'icosahedral' boron phases in their various forms [16], boron suboxide B 6 O [17] and boron trioxide B O 2 3 that is a vitreous phase under ambient conditions [12] but can be crystalline when synthesized under pressure [15,18,19]. Sometimes boron monoxide B O 2 [20,21] is also considered but its stability and existence is strongly debated [13,14,22,23].…”