The phase transitions of alpha-Si3N4 and beta-Si3N4 have been investigated by shock compression through the recovery technique and Hugoniot measurements. alpha- and beta-Si3N4 are transformed into a cubic spinel structure (c-Si3N4). The yield of c-Si3N4 increases with increasing shock pressure and reaches 100% at 63 GPa. The shock-synthesized c-Si3N4 powders are nanocrystals and display a high-temperature metastability up to about 1620 K. c-Si3N4 is one of the hard materials based on the measured equation of state. c-Si3N4 powders have been characterized by electron microscopy and Si-29 magic angle spinning NMR spectroscopy. The purification and separation method has been developed to obtain pure c-Si3N4 powders.