Unexpected Trend in Stability of Xe–F Compounds under Pressure Driven by Xe–Xe Covalent Bonds

Abstract

Xenon difluoride is the first and the most stable of hundreds of noble-gas (Ng) compounds. These compounds reveal the rich chemistry of Ng's. No stable compound that contains a Ng-Ng bond has been reported previously. Recent experiments have shown intriguing behaviors of this exemplar compound under high pressure, including increased coordination numbers and an insulator-to-metal transition. None of the behaviors can be explained by electronic-structure calculations with fixed stoichiometry. We therefore conducted a structure search of xenon-fluorine compounds with various stoichiometries and studied their stabilities under pressure using first-principles calculations. Our results revealed, unexpectedly, that pressure stabilizes xenon-fluorine compounds selectively, including xenon tetrafluoride, xenon hexafluoride, and the xenon-rich compound XeF. Xenon difluoride becomes unstable above 81 GPa and yields metallic products. These compounds contain xenon-xenon covalent bonds and may form intercalated graphitic xenon lattices, which stabilize xenon-rich compounds and promote the decomposition of xenon difluoride.

High-Pressure Reactivity of Kr and F2—Stabilization of Krypton in the +4 Oxidation State

“…For Kr 2 F, our structure investigations identified a phase with I4/mcm symmetry, one that remains the lowest-enthalpy structure in the whole pressure range studied. This polymorph is isostructural with a previously proposed high-pressure phase of Xe 2 F [44]. Similarly to that structure, it contains an interpenetrating network of graphitic Kr layers (Figure 4a) with a Kr-Kr bond of about 2.5 Å at 150 GPa, which is ca 8% shorter than the Kr-Kr separation in solid krypton at the same pressure ( Table 1).…”

“…In case of Xe2F, Peng et al indicated formation of covalent Xe-Xe bonds [44]. For Kr2F we find, however, that ELF values between nearest-neighbor Kr atoms do not exceed 0.15 (Figure 4c), which hints at lack of Kr-Kr covalent bonding within the graphitic layers.…”

“…The second-nearest Kr-Kr contacts in Kr 2 F are much longer (2.83-2.84 Å, Figure 4b), and the length of Kr-F contacts hints at no covalent bonding between krypton and fluorine (Table 1). In case of Xe 2 F, Peng et al indicated formation of covalent Xe-Xe bonds [44]. For Kr 2 F we find, however, that ELF values between nearest-neighbor Kr atoms do not exceed 0.15 (Figure 4c), which hints at lack of Kr-Kr covalent bonding within the graphitic layers.…”

“…Even at 200 GPa, the Kr-F bond within these molecules (1.77 Å) is more than 20% shorter than the closest Kr-F intermolecular contact (2.26 Å). In fact, the crystal structure of KrF 4 is analogous to the predicted high-pressure phase of XeF 4 , which is also molecular even at large compression [44].…”

“…Values close to 1 are generally interpreted as indicating paired electrons (i.e., lone pairs, bonds, and core electrons). Typical covalent bonds are characterized by ELF values of exceeding 0.7; however, previous studies indicated that the maximum ELF value for the Xe-F bond in XeF 2 is only 0.5 [42,44]. This can be rationalized by taking into account that this bond arises from a three-center two-electron interaction, and is characterized by a bond order of 1/2.…”