Hydrogenation, dehydrogenation of α-tetragonal boron and its transition to δ-orthorhombic boron

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 B 52 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, B 51 H 7 and B 51 H 3 . 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 B 51 H 7 to B 51 H 3 to pure B 52 . The so obtained δ-orthorhombic boron is an ordered, low-temperature phase and α-tetragonal boron is a disordered, high-temperature phase of B 52 . 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.

A thermodynamic description of the glass state and its application to glass transition

“…However, deviations from these values are often observed owing to its kinetics; now, diamond can be synthesized even at low-pressure processes [57,58]. Our experience shows that the kinetics of reaction agents drastically the phase boundary of boron crystals [59].…”

Boranes: The Boron Subhydride B104.67H3 with a Distorted β-Boron Crystal Structure

“…In conclusion, we have found a boron subhydride compound of composition B 104.67 (4) H 3 with the lowest amount of hydrogen to date. 44,46 The three hydrogen atoms per primitive unit cell form ideal B−H−B bridging bonds. 34 B 104.67(4) H 3 is the first compound with a noncentrosymmetrically distorted βboron framework.…”

“…Theory resulted in stable boron hydrides of compositions B 51 H 7 and B 51 H 3 , respectively. 44,46 Here, we report the crystal structure of ordered B 104.67(4) H 3 with a distorted β-boron structure. While this composition was produced by accident, the most prominent difference to βboron is the distortion of the framework from R3̅ m toward noncentrosymmetric R3m.…”

“…Annealing led to the release of hydrogen in two steps, resulting in B 51.5 H 4.7 and finally true α-t-boron (annealing at 770 K) or an orthorhombically distorted variant (δ-o-boron; annealing at 1020 K). Theory resulted in stable boron hydrides of compositions B 51 H 7 and B 51 H 3 , respectively. , …”