Abstract
Boron bulk crystals are marked by exceptional structural complexity and unusual related physical phenomena. Recent reports of hydrogenated α-tetragonal and a new δ-orthorhombic boron B52 phase have raised many fundamental questions. Using density functional theory calculations it is shown that hydrogenated α-tetragonal boron has at least two stable stoichiometric compositions, B51H7 and B51H3. Thermodynamic modeling was used to qualitatively reproduce the two-step phase transition reported by (Ekimov et al 2016 J. Mater. Res. 31, 2773) upon annealing, which corresponds to successive transitions from B51H7 to B51H3 to pure B52. The so obtained δ-orthorhombic boron is an ordered, low-temperature phase and α-tetragonal boron is a disordered, high-temperature phase of B52. The two phases are connected by an order-disorder transition, that is associated with the migration of interstitial boron atoms. Atom migration is usually suppressed in strongly bound, covalent crystals. It is shown that the migration of boron atoms is likely to be assisted by the migration of hydrogen atoms upon annealing. These results are in excellent agreement with the above mentioned experiment and they represent an important step forward for the understanding of boron and hydrogenated boron crystals. They further open a new avenue to control or remove the intrinsic defects of covalently bound crystals by utilizing volatile, foreign atoms.
Highlights
Crystalline boron is the last monatomic system for which the phase diagram is not fully determined yet and even the number of phases is not certain
We have studied the structure and thermodynamic stability of hydrogenated α-tetragonal boron, its dehydrogenation process and the transition to δ-orthorhombic boron B52 with density functional theory calculations
Our results show that hydrogenated α-tetragonal boron has at least two stable stoichiometric compositions, B51H7 and B51H3, where the hydrogen atoms are mostly occupying (8j) sites and, to a minor extend, (4g) sites
Summary
Crystalline boron is the last monatomic system for which the phase diagram is not fully determined yet and even the number of phases is not certain. A recent theoretical study by Uemura et al has identified the characteristic features of pure α-T boron B52.19 It was shown that the occupation of interstitial sites as well as non-stoichiometric compositions, i.e. non-integer number of atoms per primitive unit cell, are crucial for the stability of the system. Their results could be used to define a family of α-T boron systems.[11]. The structure of hydrogenated α-T boron, the process of dehydrogenation, and the phase transition to δ-O boron are studied theoretically.
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