Comparison of solid-state crystallization of boron polymorphs at ambient and high pressures

Abstract

Here we report the systematic study of solid-state phase transformations between boron polymorphs: α -B₁₂, β -B₁₀₆, γ -B₂₈, T-B₅₂ and amorphous boron (am-B). It is evident that the Ostwald rule of stages plays an important role during phase transformations not only of amorphous boron, but also of crystalline forms. We have observed the crystallization of tetragonal boron T-B₅₂ from amorphous phase of high purity (99.99%), which, however, cannot be easily distinguished from B₅₀C₂ boron compound. Many factors influence the transformations of amorphous phase, and it is possible to observe not only well-known am-B → α -B₁₂ and am-B → β -B₁₀₆ transformations, but also am-B → T-B₅₂, never reported so far. At ∼14 GPa, the crystallization order becomes β -B₁₀₆→α -B₁₂→γ -B₂₈, while at ∼11 GPa the intermediate crystallization of T-B₅₂ still was observed. This unambiguously indicates that α -B₁₂ is more thermodynamically stable than β -B₁₀₆ at high pressures (HPs) and renders possible to transform, at least partially, common β -phase of high purity into α -B₁₂ at very HPs and moderate temperatures (below 1600 K), i.e. outside the domain of its stability.

Keywords:

• boron
• X-ray diffraction
• phase transformation

Acknowledgements

The authors thank Profs V. L. Solozhenko, A. R. Oganov and L. Dubrovinsky for valuable discussions. The multianvil apparatus of Laboratoire Magmas et Volcans is financially supported by the Centre National de la Recherche Scientifique (Instrument National de l'INSU).