Low-Energy Floppy Modes in High-Temperature Ceramics

Enhancement of the phonon density of states due to low-energy floppy modes in crystalline and glassy states of the high-temperature ceramic S${\mathrm{i}}_3$ ${\mathrm{N}}_4$ is investigated using the molecular-dynamics method. Floppy modes appear continuously in the glass as the connectivity of the system is reduced. In the crystal, they appear suddenly at 20% volume expansion. The density of states due to the floppy modes varies linearly with energy, and the specific heat is significantly enhanced by the floppy modes.