Mysterious SiB(3): Identifying the Relation between α- and β-SiB(3)

[Image: see text] Binary silicon boride SiB(3) has been reported to occur in two forms, as disordered and nonstoichiometric α-SiB(3–x), which relates to the α-rhombohedral phase of boron, and as strictly ordered and stoichiometric β-SiB(3). Similar to other boron-rich icosahedral solids, these SiB(3) phases represent potentially interesting refractory materials. However, their thermal stability, formation conditions, and thermodynamic relation are poorly understood. Here, we map the formation conditions of α-SiB(3–x) and β-SiB(3) and analyze their relative thermodynamic stabilities. α-SiB(3–x) is metastable (with respect to β-SiB(3) and Si), and its formation is kinetically driven. Pure polycrystalline bulk samples may be obtained within hours when heating stoichiometric mixtures of elemental silicon and boron at temperatures 1200–1300 °C. At the same time, α-SiB(3–x) decomposes into SiB(6) and Si, and optimum time-temperature synthesis conditions represent a trade-off between rates of formation and decomposition. The formation of stable β-SiB(3) was observed after prolonged treatment (days to weeks) of elemental mixtures with ratios Si/B = 1:1–1:4 at temperatures 1175–1200 °C. The application of high pressures greatly improves the kinetics of SiB(3) formation and allows decoupling of SiB(3) formation from decomposition. Quantitative formation of β-SiB(3) was seen at 1100 °C for samples pressurized to 5.5–8 GPa. β-SiB(3) decomposes peritectoidally at temperatures between 1250 and 1300 °C. The highly ordered nature of β-SiB(3) is reflected in its Raman spectrum, which features narrow and distinct lines. In contrast, the Raman spectrum of α-SiB(3–x) is characterized by broad bands, which show a clear relation to the vibrational modes of isostructural, ordered B(6)P. The detailed composition and structural properties of disordered α-SiB(3–x) were ascertained by a combination of single-crystal X-ray diffraction and (29)Si magic angle spinning NMR experiments. Notably, the compositions of polycrystalline bulk samples (obtained at T ≤ 1200 °C) and single crystal samples (obtained from Si-rich molten Si–B mixtures at T > 1400 °C) are different, SiB(2.93(7)) and SiB(2.64(2)), respectively. The incorporation of Si in the polar position of B(12) icosahedra results in highly strained cluster units. This disorder feature was accounted for in the refined crystal structure model by splitting the polar position into three sites. The electron-precise composition of α-SiB(3–x) is SiB(2.5) and corresponds to the incorporation of, on average, two Si atoms in each B(12) icosahedron. Accordingly, α-SiB(3–x) constitutes a mixture of B(10)Si(2) and B(11)Si clusters. The structural and phase stability of α-SiB(3–x) were explored using a first-principles cluster expansion. The most stable composition at 0 K is SiB(2.5), which however is unstable with respect to the decomposition β-SiB(3) + Si. Modeling of the configurational and vibrational entropies suggests that α-SiB(3–x) only becomes more stable than β-SiB(3) at temperatures above its decomposition into SiB(6) and Si. Hence, we conclude that α-SiB(3–x) is metastable at all temperatures. Density functional theory electronic structure calculations yield band gaps of similar size for electron-precise α-SiB(2.5) and β-SiB(3), whereas α-SiB(3) represents a p-type conductor.