Crystallographic and spectral equivalence of boron-carbide polymorphs

Abstract

The extraordinary properties of boron carbide originate from the collection of polymorphs that comprise fabricated samples. However, traditional nomenclature that differentiates and describes these crystal structures often restricts the ability to fully model variations in material properties and performance. To highlight and transcend these limitations, this study introduces a modified nomenclature and defines new groups of equivalent polymorphs based on quantum-mechanical simulations. Trends in lattice parameters, total energy, and Raman spectra are explained in terms of the unique covalent-bonding environments of boron-carbide polymorphs.

In search of amorphization-resistant boron carbide

“…Numerous factors complicate attempts to explain the amorphization process. While boron carbide is generally thought to have the rhombohedral crystal structure with space group 166 (R3m), the number and arrangement of the carbon and boron atoms in the unit cell may vary [26,27]. Further, because the formation energies of many of these possible polymorphs are within a narrow range, fabricated samples may comprise multiple crystal structures [24,27,28].…”

“…While boron carbide is generally thought to have the rhombohedral crystal structure with space group 166 (R3m), the number and arrangement of the carbon and boron atoms in the unit cell may vary [26,27]. Further, because the formation energies of many of these possible polymorphs are within a narrow range, fabricated samples may comprise multiple crystal structures [24,27,28]. In general, (B 11 C p )CBC is considered the predominant polymorph [24,26,27,[29][30][31][32][33][34], but other polymorphic constituents are frequently debated [27,29,35].…”

“…By supporting the chains, these additives decrease susceptibility for chain bending and hence increase amorphization resistance. However, the simulation domain includes a large number of polymorphs [27] and a large number of admissible additive atoms. Given the varying degrees of participation of different boron-carbide polymorphs in amorphization [24], the most susceptible polymorphs could be targeted.…”

“…Given the varying degrees of participation of different boron-carbide polymorphs in amorphization [24], the most susceptible polymorphs could be targeted. As an example of additive selection, we present relaxed crystal structure of nickel-doped B 4 C (Figure 2(b)) from DFT (simulation parameters similar to [27]). Figure 2(c) shows increased stiffness (curvature of strain energy) of homogeneous compression of this doped structure as compared to pristine B 4 C. Selection of additive atoms requires consideration of atomic size, electronegativity, and thermodynamic stability (see Figure 2(d)).…”

“…Our DFT calculations have identified ten groups of polymorphs catalogued by lo- , cm -1 cal bonding environment in Figure 3(a). They have distinct potential energies (Figure 3(b)), lattice parameters, Raman spectra [27], and susceptibilities to amorphization [24]. However, the ability to identify the presence, abundance, and distribution of these polymorphs is currently not available.…”