Synthesis of millimeter-scale high-hardness polycrystalline stishovite with nano-sized grains via high-pressure transformation of microcrystalline α-SiO2 precursor

Existing polycrystalline stishovite bulk synthesized from traditional strategies generally suffer from low purity, grain coarsening and minuscule sample size. Here, we successfully synthesized millimeter-scale high-performance polycrystalline stishovite with nano-sized grains from microcrystalline α-SiO 2 powder in a large-volume press. An anomalous variation in grain size from micrometer to nanometer is realized by high-pressure transformation of microcrystalline precursors. Remarkably, the sample with an average grain size of 91.6 nm synthesized at 15.6 GPa and 800 °C possesses the highest Vickers and Knoop hardness among reported pure stishovite materials, meanwhile higher indentation crack resistance ( K IC = 3.1 MPa m 1/2 ), about double that of stishovite single crystals. The outstanding mechanical properties stem from the synergistic effects of pure stishovite phase, dense microstructure, fine grain size, and high-concentration dislocations. This work provides valuable approach for synthesizing advanced structural ceramics and next-generation superhard materials by high-pressure transformation of microcrystalline precursors.