Three-dimensional buckled honeycomb boron lattice with vacancies as an intermediate phase on the transition pathway from α-B to γ-B

In the phase diagram of elemental boron, an unknown high-pressure form was identified as γ-orthorhombic boron (γ-B28), provoking studies of the extraordinary properties of its main building blocks, B12 icosahedra and B2 dumbbells. Although two low-pressure phases, α- and β-rhombohedral boron (α-B12 and β-B106), are also composed of icosahedra, the detailed kinetics and mechanisms of the structural transition from α-B12 or β-B106 to γ-B28 remain poorly understood. We report on new metastable boron phases formed during the transition in high-pressure high-temperature conditions that were discovered using the crystal structure search method. The metastable phases are understood to be a three-dimensional buckled defective honeycomb lattice in which boron vacancies lead to a dynamically and mechanically stable structure with triangular motifs. We suggest that the metastable phases act as intermediate states on the transition pathway from α-B12 to γ-B28 owing to their structural flexibility and low enthalpies, in the framework of Ostwald’s step rule. The route taken by boron atoms rearranging themselves at high pressure and temperature has been uncovered by scientists in Korea and the USA. The crystal structures of boron include those with an orthorhombic arrangement and others with a rhombohedral shape. When subjected to high pressures and high temperatures, boron can switch between these arrangements, but the pathway for this change was not well understood. Now, Kee Joo Chang at Korea Advanced Institute of Science and Technology and co-workers have discovered a new atomic arrangement that exists as an intermediate phase while boron atoms move from one stable form to another. By investigating this phase transition using an ab initio evolutionary crystal-structure search method, they discovered a metastable state that exists temporarily during the transition that has a three-dimensional buckled honeycomb arrangement. We report the discovery of novel metastable boron allotropes, called hex-Bν, using the crystal structure search method. First-principles calculations reveal that hex-Bν shows not only dynamical, mechanical stability but also structural flexibility under high pressures and high temperatures. We suggest a new pressure-induced transition pathway from α-B12 to γ-B28 under extreme conditions, in which our discovered allotropes can act as intermediate phases. This work provides a possible solution to the long-standing fundamental question on the phase transition mechanism between boron allotropes.