Ordered helium trapping and bonding in compressed arsenolite: Synthesis ofAs4O6·2He

Abstract

Compression of arsenolite has been studied from a joint experimental and theoretical point of view. Experiments on this molecular solid at high pressures with different pressure-transmitting media have been interpreted thanks to state-of-the-art ab initio calculations. Our results confirm arsenolite as one of the most compressible minerals and provide evidence for ordered helium trapping above 3 GPa between adamantane-type As 4 O 6 cages. Our calculations indicate that, at relatively small pressures, helium establishes rather localized structural bonds with arsenic forming a compound with stoichiometry As 4 O 6 · 2He. All properties of As 4 O 6 · 2He are different from those of parent As 4 O 6 . In particular, pressure-induced amorphization, which occurs in arsenolite above 15 GPa, is impeded in As 4 O 6 · 2He, thus resulting in a mechanical stability of As 4 O 6 · 2He beyond 30 GPa. Our work paves the way to explore the formation of alternative compounds by pressure-induced trapping and reaction of gases, small atomic or molecular species, in the voids of molecular solids containing active lone electron pairs. DOI: 10.1103/PhysRevB.93.054102Molecular solids are very soft materials with openframework structures composed of molecular units, exhibiting strong covalent interatomic forces, which are linked by weak intermolecular forces, such as van der Waals interactions or hydrogen bonding forces. Pressure is a thermodynamic variable that allows tuning interatomic distances and consequently is a powerful tool to study atomic interactions and the connectivity of different molecular units -hence, the possibility of molecular solids to trap small atoms or molecules. In this sense, molecule trapping has received a lot of interest from the scientific community due to its potential industrial applications covering hydrogen storage [1] and CO 2 segregation from other organic compounds [2], among others. Recently, the small size of helium has triggered the study of pressure-induced helium trapping and how this effect affects the compressibility of host compounds. Those first studies were focused on helium trapping in silica glass [3][4][5] that is an amorphous compound with disordered interstitial voids large enough to host and trap helium. However, more studies need to be made of pressure-induced helium trapping in ordered crystallographic compounds, such as molecular solids.Helium reaction is a challenge since it is one of the most inert elements in nature [6]. Previous studies of helium trapping in glasses have not revealed the pressure-induced reactivity of helium with the glass structure. On the search of the formation of solid structures with helium, the study of * juasant2@upv.es † fjmanjon@upv.es mixtures of helium with other noble gases at high pressures has exhibited successful results. The mix of those fluids under pressure allows the formation of solid van der Waals compounds with exotic stoichiometries [7,8] [He(N 2 ) 11 , Ne(He) 2 ] given by the solubility of helium and dominated by the crystallization proces...

How and Why Does Helium Permeate Nonporous Arsenolite Under High Pressure?

“…It is worth noting that 1) no coexistence of ars and incl was observed when ars powder was used instead of a single crystal, 2) the incl crystal structure proposed by Sans et al. is fully consistent with our crystal structure determination, and 3) pressure‐induced amorphization (PIA) of ars was observed at around 20 GPa when it was subjected to HP without any PTM or with a PTM other than He . The last finding was corroborated by using DFT computations of ars elastic constants, which indicated that it becomes mechanically unstable at 19.7 GPa and violates Born stability criteria.…”

“…We recently discovered that helium can enter nonporous crystals of cubic arsenic(III) oxide, arsenolite, and determined the crystal structure of the as‐formed As 4 O 6 ⋅ 2 He clathrate by using single‐crystal X‐ray diffraction . This was also independently found by Sans and co‐workers for powdered arsenolite samples …”

“…reported the results of their HP synchrotron X‐ray diffraction study on ars powder in various PTMs. An abrupt rise in the lattice parameter, observed at 3 GPa only when helium was used as the PTM, was associated with the formation of the arsenolite–helium inclusion compound and its crystal structure was proposed based on the compression of tiny voids present in ars crystal structure . It is worth noting that 1) no coexistence of ars and incl was observed when ars powder was used instead of a single crystal, 2) the incl crystal structure proposed by Sans et al.…”

β−Bi2O3under compression: Optical and elastic properties and electron density topology analysis

“…In β-Bi 2 O 3 , we note that powder samples were used instead of single crystals because the β phase is a metastable phase that has been stabilized in submicron-sized particles and nanoparticles [71] and in N-and Y-doped Bi 2 O 3 [22,72]. The use of gaseous environments (i.e., He or Ne) as pressure-transmitting media assures a larger hydrostatic regime than liquid media; however, some studies have shown that the use of gas can modify the compressibility of certain materials with porous or open framework structures when gas molecules can enter into the large voids of the crystalline structure with the increase of pressure, as in As 4 O 6 [73], SiO 2 [74], and other porous materials [75]. In this regard, β-Bi 2 O 3 contains large channels in its structure, which are formed by the presence of LEPs from Bi atoms.…”

Vibrational and elastic properties of As4O6 and As4O6·2He at high pressures: Study of dynamical and mechanical stability

“…Additionally, previous works have reported the structural characterization of As 4 O 6 under compression by means of angle dispersive x-ray diffraction (XRD) measurements using different pressure-transmitting media (PTMs) [8,10]. In those works it was shown that arsenolite is one of the most compressible non-hydrated minerals.…”