Mysterious SiB₃: Identifying the Relation between α- and β-SiB₃

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Binary silicon borides (SiB₃) are refractory materials with potential applications.
• Two forms, α-SiB_3- and β-SiB₃, are known but their thermal stability and formation conditions are poorly understood.

Purpose of the Study:

• To map the formation conditions of α-SiB_3- and β-SiB₃.
• To analyze the relative thermodynamic stabilities of these SiB₃ phases.
• To investigate the structural and electronic properties of SiB₃.

Main Methods:

• Synthesis of SiB₃ phases under varying temperatures, pressures, and compositions.
• Characterization using X-ray diffraction and Raman spectroscopy.
• First-principles cluster expansion and density functional theory calculations.

Main Results:

• α-SiB_3- is a kinetically driven, metastable phase formed at lower temperatures and shorter times.
• Stable β-SiB₃ requires prolonged synthesis times or high pressures.
• β-SiB₃ decomposes above 1250 °C, while α-SiB_3- decomposes into SiB₆ and Si.
• Disordered α-SiB_3- exhibits strained Si incorporation in B₁₂ icosahedra, with electron-precise composition SiB_2.5.
• DFT calculations show similar band gaps for α-SiB_2.5 and β-SiB₃.

Conclusions:

• α-SiB_3- is metastable under all conditions due to kinetic formation pathways.
• Optimized synthesis conditions balance formation and decomposition rates.
• High pressure significantly enhances β-SiB₃ formation kinetics.
• Structural disorder in α-SiB_3- influences its properties and stability.