Inverted-Pyramidal Microcrystals Embedded in an Amorphous Matrix through High-Pressure Reactive Epitaxy

Abstract

High-pressure phases are often unstable at ambient pressure and amorphize during decompression, making their direct recovery as crystalline materials difficult. Here, we demonstrate a route for synthesizing crystalline particles embedded in an amorphous matrix using an unstable high-pressure phase as a transient reaction medium. An amorphous CaSiO3 film deposited on a YAlO3 single-crystal substrate was treated at 15.6 GPa and 1200 °C. After recovery to ambient pressure and temperature, faceted (Ca,Y)(Si,Al)O3 perovskite-related particles were observed at the interface between the film and substrate. Structural and compositional analyses showed that the particles formed through reactive interdiffusion between the film and substrate and grew downward into the substrate while maintaining a crystallographic relationship with the YAlO3 substrate. The particles retained crystallinity at ambient pressure, whereas the surrounding CaSiO3-rich region became amorphous. These results indicate that transient high-pressure phase formation, interfacial reaction, and selective amorphization during decompression can be combined to produce faceted crystalline particles embedded in an amorphous matrix, a structure that is difficult to obtain by conventional crystal-growth methods.