Pressure-induced transformations ofAgIIF2—towards an‘infinite layer’ d9 material

Abstract

The enthalpy of seven polymorphs ofAgF2 has been scrutinized up to 50 GPa using density functional theory (DFT) calculations. We show thatα-AgIIF2 (Pbca, with its puckered-sheet structure and an elongated octahedral4+2 coordination of Ag) transforms above 15 GPa into a layered polymorphδ (Abma). The Jahn–Teller effect persists and the coordination of Ag(II) is of the4+4 type; the Ag–F–Agbridges are bent. Cubic γ structure of the CaF2 type , and its Pa3 variant (η), rutile(ζ), and tetragonal‘infinite chain’ P4mm structure related to AgFBF4 (ε) arenot preferred in the entire pressure window that was investigated. Electronic structure of high-pressureδ-AgIIF2 form shows features that are characteristic for two-dimensional (2D) materials, a prerequisite forhigh-TC superconductivity.Our calculations also suggest that experimentally observed high-temperatureβ-AgF2 (Imcm, disproportionated, i.e. a charge-density-wave form,AgIAgIIIF4) is indeed metastable; it is slightly endothermic compared toα-AgIIF2.