Stability of mixed carbon-silicon clathrates

We examine the thermodynamic and dynamic stability (i.e., phonon dispersion) of mixed silicon-carbon clathrate frameworks using first-principles calculations as a function of pressure and composition. Silicon atoms were substituted on special framework Wyckoff positions in the Type-I and Type-II empty carbon clathrate structures over a broad compositional range, and the enthalpies of the mixed clathrates were compared to pure silicon and carbon clathrates, as well as the thermodynamic ground states. While all mixed clathrates examined were found to be metastable with respect to elemental formation components and/or silicon carbide, certain empty binary host lattices are found to be lower-energy phases than the pure-component clathrate endmembers at high pressure, in particular Type-I C22Si24 and Type-II C32Si104. This enhanced energetic stability is rationalized by a decrease of energy upon doping specific crystallographic positions. When occupied by small guest ions like Li+ and Na+, these mixed C-Si clathrate structures exhibit minima in their formation enthalpies under high-pressure conditions, providing insights into potential synthetic pathways.