Persistence of Mixed and Non-intermediate Valence in the High-Pressure Structure of Silver(I,III) Oxide, AgO: A Combined Raman, X-ray Diffraction (XRD), and Density Functional Theory (DFT) Study

The X-ray diffraction data collected up to ca. 56 GPa and the Raman spectra measured up to 74.8 GPa for AgO, or AgIAgIIIO2, which is a prototypical mixed valence (disproportionated) oxide, indicate that two consecutive phase transitions occur: the first-order phase transition occurs between 16.1 GPa and 19.7 GPa, and a second-order phase transition occurs at ca. 40 GPa. All polymorphic forms host the square planar [AgIIIO4] units typical of low-spin AgIII. The disproportionated Imma form persists at least up to 74.8 GPa, as indicated by Raman spectra. Theoretical hybrid density functional theory (DFT) calculations show that the first-order transition is phonon-driven. AgO stubbornly remains disproportionated up to at least 100 GPa—in striking contrast to its copper analogue–and the fundamental band gap of AgO is ∼0.3 eV at this pressure and is weakly pressure-dependent. Metallization of AgO is yet to be achieved.