Computer modeling of allo-Si and allo-Ge polymorphs

Computer models of possible structures of the so-called alto-Si and allo-Ge phases (polymorphs of the respective elements), reported to be formed upon topotactic demetalation of the layered compounds NaLi3Si6 and Li7Ge12, are built and energy-relaxed using atomistic and periodic density functional theory (DFT) methods. The resulting clathrate-like frameworks contain straight intracrystalline tunnel-like voids with cross sections defined by seven-, eight-, or 10-atom rings. In alto-Si structures these tunnels are one-dimensional: the most stable and symmetric structure (as predicted by DFT), containing the widest tunnels, coincides with zeolite type TON. In most of the allo-Ge structures lateral seven-, eight-, or 10-member ring openings connect mutually the tunnels, leading to two-dimensional- or three-dimensional-connected intracrystalline voids which may allow easy guest atom diffusion. The possibility is discussed that these structures, potentially accessible in experimental preparations, could incorporate flexibly occluded metal atoms leading to low-dimensional metal-in-semiconductor characteristics and interesting physical or chemical properties. Comparison of the results from DFT and atomistic simulations indicate that these latter, using Stillinger-Webber potentials, may be less adequate to predict the energies of the most distorted structures.