Abstract
Boron suboxide B6O, the hardest known oxide, has an Rm crystal structure (α-B6O) that can be described as an oxygen-stuffed structure of α-boron, or, equivalently, as a cubic close packing of B12 icosahedra with two oxygen atoms occupying all octahedral voids in it. Here we show a new ground state of this compound at ambient conditions, Cmcm-B6O (β-B6O), which in all quantum-mechanical treatments that we tested comes out to be slightly but consistently more stable. Increasing pressure and temperature further stabilizes it with respect to the known α-B6O structure. β-B6O also has a slightly higher hardness and may be synthesized using different experimental protocols. We suggest that β-B6O is present in mixture with α-B6O, and its presence accounts for previously unexplained bands in the experimental Raman spectrum.
Highlights
Boron suboxide B6O, the hardest known oxide, has an R3m crystal structure (α-B6O) that can be described as an oxygen-stuffed structure of α-boron, or, equivalently, as a cubic close packing of B12 icosahedra with two oxygen atoms occupying all octahedral voids in it
Superhard materials are used in many applications, from cutting, grinding and drilling tools to wear-resistant coatings[1,2,3]
The hardness of α-B6O7 was reported to be in the range 30–45 GPa8,9, making it the hardest known oxide[9,10,11]
Summary
Our variable-composition evolutionary searches expectedly find B2O3 and B6O to be the only stable compounds in the B-O system. There are several compounds very close to stability - B2O7 (this is a 2D-form of B2O3 intercalated with oxygen molecules) and oxygen-deficient versions of B6O with B6O-like structures and compositions between B and B6O. Cmcm-B6O (β-B6O, see Table 1 for structural parameters), instead of the well-known R3m-B6O (α-B6O)[7,12,13,14], turned out to be the most stable structure at ambient pressure, as shown in Fig. 1; phonon calculations have confirmed its dynamical stability. Structural parameters and some of the physical properties of β-B6O are shown, in comparison with α-B6O and two related forms of pure boron Structural parameters and some of the physical properties of β-B6O are shown in Table 1, in comparison with α-B6O and two related forms of pure boron
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