2. Experimental details
These synthesis experiments were conducted using “Diacell HeliosDAC” diamond anvil cell (DAC) (Almax EasyLab Inc.), equipped with an internal resistive heater that allowed the sample heating up to 1000 °C. A K-type thermocouple located between the diamonds and the gasket was used for the temperature control. Diamond anvils with 820 μm flat culets were used in the cell and samples were loaded into a 250 μm hole of Inconel gasket. For pressure calibration a routine method based on the R1 luminescence line shift of a ruby crystal, was utilized [19]. During the heating a flow of mixture of argon and hydrogen gas was used inside the cell to prevent cell oxidation. Each synthesis experiment was conducted for 2 h and the specific conditions at which the successful synthesis of AgGaO2 was achieved included pressures between 1.5 and 10 GPa, and temperatures from 480 °C to 620 °C. A total of about 20 samples were prepared including several control samples. Most of the precursor samples consisted of Ag2O and Ga2O3 powders mixed at different weight ratios. A few control samples consisting of pure Ag2O or Ga2O3 powders were also prepared. Some of the control experiments, conducted to monitor the effect of high pressure, involved heating of portions of the Ag2O–Ga2O3 precursors under normal atmospheric pressure (i.e., 1 atm.). Details including samples and experimental conditions used for all synthesis experiments conducted for this study are summarized in Table S1 and Fig. S1. Products of the synthesis experiments, including all controlled samples were thoroughly characterized. Raman measurements were conducted using red laser excitation of 633 nm and Renishaw inVia Raman/PL Microscope. Scanning electron microscopy (SEM) was done using FEI Nova 600 field emission gun microscope. For various types of transmission electron microscopy (TEM)-based measurements, including high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX) we used FEI Tecnai F20 TEM. TEM samples were prepared by dispersing powders on dedicated 3.05 mm-diameter, 300 mesh copper grid-supported amorphous lacey carbon films.