Novel sp3-hybridized framework structure of group 14 elements

Using genetic algorithm atomic structure prediction method and first-principles calculations, we discovered a novel low-energy metastable structure of group 14 elements in P42/mnm symmetry. The P42/mnm structure is a cage-like distorted sp3-hybridized framework structure with the cage's volume $\sim$ 4\% larger than the average cage's volume of the clathrate type-I structure, indicating P42/mnm structure a good gases or metal atoms encapsulation structure. The band structure calculations show that P42/mnm Si and Ge are semiconducting with energy band gaps close to the optimal values for optoelectronic or photovoltaic applications. The metal atom encapsulation P42/mnm structure of group 14 elements could also be a candidate for rattling-mediated superconducting or ``a phonon glass and an electrical crystal'' thermoelectric materials.