The pristine crystalline type-I, type-II, and type-VIII silicon clathrates have been studied using state of the art first-principles calculations based on density functional theory and density functional perturbation theory. We apply quasi-harmonic approximation to study structural stability, the possibility of temperature or pressuredriven phase transitions, along with Gru¨neisen parameters, coefficients of thermal expansion and thermal conductivities to estimate the degree of phonon anharmonicity for selected silicon clathrates. It is shown that a pressure-driven phase transition between type-I and type-II silicon clathrates may occur, and a temperature-driven phase transition between type-I and type-VIII Si clathrates at high temperature is likely. We further show that the relatively high Gru¨neisen parameters (1.5, 1.65, and 1.29, respectively for Si46-I, Si136-II, Si46-VIII), the existence of negative regions in the thermal expansion coefficient curves and very low thermal conductivities all indicate that the phonon anharmonicity in these silicon clathrates is high.
کلید واژگان :First principles, Clathrates, Lattice dynamics
ارزش ریالی : 600000 ریال
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