Freezing in Resonance Structures for Better Packing: XeF₂ Becomes (XeF⁺)(F⁻) at Large Compression

Abstract

Recent high-pressure experiments conducted on xenon difluoride (XeF(2)) suggested that this compound undergoes several phase transitions up to 100 GPa, becoming metallic above 70 GPa. In this theoretical study, in contrast to experiment, we find that the ambient pressure molecular structure of xenon difluoride, of I4/mmm symmetry, remains the most stable one up to 105 GPa. In our computations, the structures suggested from experiment have either much higher enthalpies than the I4/mmm structure or converge to that structure upon geometry optimization. We discuss these discrepancies between experiment and calculation and point to an alternative interpretation of the measured cell vectors of XeF(2) at high pressure. At pressures exceeding those studied experimentally, above 105 GPa, the I4/mmm structure transforms to one of Pnma symmetry. The Pnma phase contains bent FXeF molecules, with unequal Xe-F distances, and begins to bring other fluorines into the coordination sphere of the Xe. Further compression of this structure up to 200 GPa essentially results in self-dissociation of XeF(2) into an ionic solid (i.e., [XeF](+)F(-)), similar to what is observed for nitrous oxide (N(2)O) at high pressure.

Predicting crystal structures and properties of matter under extreme conditions via quantum mechanics: the pressure is on

“…Even compounds as exotic as XeF 2 , with its hypervalent F-Xe-F bonding, have been calculated to undergo transformation to the ionic (XeF) + F À polymorph at sufficiently high pressures. 168 This process is analogous to what is observed in acidic environments (for example, addition of SbF 5 Lewis acid to XeF 2 leads to the formation of (XeF) + salt), but predicted here to take place by the application of pressure alone. 168 …”

“…168 This process is analogous to what is observed in acidic environments (for example, addition of SbF 5 Lewis acid to XeF 2 leads to the formation of (XeF) + salt), but predicted here to take place by the application of pressure alone. 168 …”

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

“…We find that F-Kr-F molecules in KrF 2 undergo slight bending in response to compression, in analogy to ArF 2 , but we do not encounter any signs of auto-ionization, as predicted for XeF 2 [42]. Interestingly, our structure searches do not identify any phases containing KrF + and Kr 2 F 3 + cations, which are known to form upon reaction of KrF 2 with fluoride acceptors [7].…”

“…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.…”

“…We utilized the generalized gradient approximation (GGA) in the form of the Perdew-Burke-Ernzerhof (PBE) functional [61], which was successfully applied in previous high-pressure studies on xenon fluorides [42,44], as well as a recent study on the ambient-pressure structures of KrF 2 [62]. The projector augmented-wave (PAW) method was used [63], as implemented in the VASP 5.2 code.…”