Lattice dynamics and thermodynamical properties of silicon nitride polymorphs

概要

論文の詳細を見る
The lattice dynamics of three polymorphs of Si3N4, , , and phases, has been calculated by the real space force constant method combined with first-principles calculations. The thermodynamical properties of the three phases are evaluated from the vibrational density of states. The effect of thermal volume expansion is included by quasiharmonic approximation. The volume expansivity, bulk modulus, specific heat, and Helmholtz free energy of the three phases are computed as a function of temperature. All three phases are found to be dynamically stable. -Si3N4 shows lower Helmholtz free energy than -Si3N4 in the temperature range from 0 to 2000 K. In other words, -Si3N4 is not a low-temperature phase but a metastable phase under the ordinary pressure. The presence of planer N-Si3 units and their strong Si-N bonds in two hexagonal polymorphs, - and -Si3N4, is the origin of the high-frequency phonon band at above 25 THz, which is absent in -Si3N4. It plays key roles for anisotropic local vibration of N atoms. The different temperature dependence of specific heat between / and phases can also be ascribed to the structure. The transition pressure from to phase is computed as a function of temperature. It is found to increase with the rise of temperature.