High-pressure chemistry and physics of fluorides

Current diamond anvil cell (DAC) techniques enable performing routine measurements on solids compressed to 100 GPa (million atmospheres). At such conditions the pressure-volume work term (pV) becomes comparable to covalent bond energies therefore considerably affecting the chemistry, structure, and properties of compounds that are otherwise well known and characterized under ambient conditions. [1,2]

Through a combination of Raman spectroscopy and synchrotron powder X-ray diffraction measurements supplemented with density functional theory (DFT) calculations we unraveled the pressure response of two fluoride systems. The first was K₂CuF₄, a prototypical 2D ferromagnetic system. We found that at 10 GPa sliding of [CuF₄]²⁻_∞ layers leads to a transition from the Ruddlesden−Popper phase into a Dion−Jacobson-like structure. This transition results in substantial structural and electronic rearrangement within the planes, resulting in a change from 2D ferromagnetism to 1D antiferromagnetism. [3]

The second system is palladium trifluoride which despite it seemingly simple stoichiometry is a mixed-valent system better formulated as Pd^IIPd^IVF₆. We performed an attempt to verify whether application of high pressure might force this compound to form a genuine Pd^III fluoride (Pd^IIIF₃). Indeed, hybrid density functional calculations predict the thermodynamic preference for single-valent (comproportionated) polymorphs at pressures exceeding 30 GPa. Experimentally we found two phase transitions, with the second one, commencing at ~50 GPa, introducing a monoclinic C2/c phase containing genuine Pd^III centers. Preliminary data suggests that the this phase might host strong one-dimensional antiferromagnetic spin-spin interactions.

References

[1] W. Grochala, R. Hoffmann, J. Feng, N. W. Ashcroft, Angew. Chemie Int. Ed. 46, 3620, 2007.
[2] L. Zhang, Y. Wang, J. Lv, Y. Ma, Nat. Rev. Mater. 2, 17005.
[3] S. B. Pillai, D. Upadhyay, J. Drapała, Z. Mazej, D. Kurzydłowski, J. Phys. Chem. C 128, 17747, 2024.