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

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 \rightarrow α-B12 and am-B \rightarrow β-B106 transformations, but also am-B \rightarrow T-B52, never reported so far. At ~14 GPa the crystallization order becomes β-B106 \rightarrow α-B12 \rightarrow 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, and renders possible to transform, at least partially, common β-phase of high purity into α-B12 at very high pressures and moderate temperatures (below 1600 K), i.e. outside the domain of its stability.