Size effect on heat capacity and phase transitions in Cu6PS5I superionic conductors

Cu6PS5I single crystals were grown using chemical vapor transport and Cu6PS5I micro- and nanocrystalline powders were prepared. Structural studies were performed using X-ray diffraction analysis and scanning electron microscopy. The influence of grain size on the phase transitions (PTs) and heat capacity in Cu6PS5I single crystals and micro- and nanocrystalline powders is studied in the temperature range 70-300 K. With average grain size reduction, the ferroelastic and superionic first-order PTs and the structural second-order PT shift towards higher temperatures. Besides, the anomaly at the second-order PT is fully smeared with average grain size reduction, while at high temperatures the new feature appears which is assigned to a “surface” PT.