We
performed variable-temperature synchrotron powder X-ray diffraction
measurements and impedance spectroscopy under pressure for silver
iodide (AgI) nanoparticles with a diameter of 11 nm. The superionic
conducting α-phase of AgI nanoparticles was successfully stabilized
down to at least 20 °C by applying a pressure of 0.18 GPa, whereas
the transition temperature was 147 °C in bulk AgI at ambient
pressure. To our knowledge, this is the first example of the α-phase
of AgI existing stably at room temperature.