Hugoniot curve (shock velocity vs particle velocity and pressure vs density) of $\beta -{\mathrm{Si}}_3{\mathrm{N}}_4$ has been measured up to 150 GPa by a two-stage light gas gun and an inclined-mirror method. The Hugoniot elastic limit is detected to be about 16 GPa. Above ∼36 GPa, a phase transition from $\beta -{\mathrm{Si}}_3{\mathrm{N}}_4$ to $c-{\mathrm{Si}}_3{\mathrm{N}}_4$ is discovered. The kinetics is very sluggish. This phase transition does not seem to complete at the highest pressure achieved in this work, but an extrapolation gives a pressure of ∼180 GPa for the completion. Isentrope of $c-{\mathrm{Si}}_3{\mathrm{N}}_4$ has been determined from the Hugoniot data by fitting the Birch-Murnaghan equation of state. The zero-pressure bulk modulus and its first pressure derivative are found to be $300\pm 10\mathrm{GPa}$ and $3.0\pm 0.1,$ respectively. These values are in agreement with the first-principles calculations, and therefore strongly support that $c-{\mathrm{Si}}_3{\mathrm{N}}_4$ is a low compressibility phase.

• Received 14 June 2000

DOI:https://doi.org/10.1103/PhysRevB.62.11412