Equilibrium and metastable phase transitions in silicon nitride at high pressure: A first-principles and experimental study

Article

关键词: POST-SPINEL PHASES;    EQUATION-OF-STATE;    SYNCHROTRON X-RAY;    AB-INITIO;    HIGH-TEMPERATURE;    THERMODYNAMIC PROPERTIES;    THERMAL-EXPANSION;    CRYSTAL-STRUCTURE;    CUBIC SI3N4;    BETA-SI3N4;   

DOI  :  10.1103/PhysRevB.84.014113

来源: SCIE

【 摘 要 】

We have combined first-principles calculations and high-pressure experiments to study pressure-induced phase transitions in silicon nitride (Si3N4). Within the quasi-harmonic approximation, we predict that the alpha phase is always metastable relative to the beta phase over a wide pressure-temperature range. Our lattice vibration calculations indicate that there are two significant and competing phonon-softening mechanisms in the beta-Si3N4, while phonon softening in the alpha-Si3N4 is rather moderate. When the previously observed equilibrium high-pressure and high-temperature beta -> gamma transition is bypassed at room temperature (RT) due to kinetic reasons, the beta phase is predicted to undergo a first-order structural transformation to a denser P (6) over bar phase above 39 GPa. The estimated enthalpy barrier height is less than 70 meV/atom, which suggests that the transition is kinetically possible around RT. This predicted new high-pressure metastable phase should be classified as a postphenacite phase. Our high-pressure x-ray diffraction experiment confirms this predicted RT phase transition around 34 GPa. No similar RT phase transition is predicted for alpha-Si3N4. Furthermore, we discuss the differences in the pressure dependencies of phonon modes among the alpha, beta, and gamma phases and the consequences on their thermal properties. We attribute the phonon modes with negative Gruneisen ratios in the alpha and beta phases as the cause of the predicted negative thermal expansion coefficients (TECs) at low temperatures in these two phases, and predict no negative TECs in the gamma phase.

【 授权许可】

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