Preserving high-pressure solids via freestanding thin-film engineering.

High pressure can significantly alter atomic and electronic structures of materials, resulting in unique properties. However, pressure-induced changes are often reversible, limiting their fundamental research and practical applications under ambient conditions. Here, we introduce a general method to preserve high-pressure solids under ambient conditions. By using freestanding carbon-gold-nanoparticle-carbon sandwiched thin films as precursors, we synthesize nanostructured diamond capsules that encapsulate high-pressure gold via an amorphous carbon-to-diamond transition. The preserved pressure is demonstrated to be tunable, ranging from 15.6 to 26.2 GPa, as the synthesis pressure increases from 32.0 to 56.0 GPa. This study establishes a scalable method to preserve high-pressure solids with controllable particle size and distribution through thin film engineering. Moreover, it enables in situ characterization of high-pressure solids with high spatial resolution at the atomic scale using electron beams, as well as other general diagnostic probes, and provides a viable route for large-scale applications of high-pressure solids.