Lattice Dynamics Study of Phonon Instability and Thermal Properties of Type-I Clathrate K(8)Si(46) under High Pressure

For a further understanding of the phase transitions mechanism in type-I silicon clathrates K(8)Si(46), ab initio self-consistent electronic calculations combined with linear-response method have been performed to investigate the vibrational properties of alkali metal K atoms encapsulated type-I silicon-clathrate under pressure within the framework of density functional perturbation theory. Our lattice dynamics simulation results showed that the pressure induced phase transition of K(8)Si(46) was believed to be driven by the phonon instability of the calthrate lattice. Analysis of the evolution of the partial phonon density of state with pressure, a legible dynamic picture for both guest K atoms and host lattice, was given. In addition, based on phonon calculations and combined with quasi-harmonic approximation, the specific heat of K(8)Si(46) was derived, which agreed very well with experimental results. Also, other important thermal properties including the thermal expansion coefficients and Grüneisen parameters of K(8)Si(46) under different temperature and pressure were also predicted.