A theoretical prediction of a new silicon allotrope: tP36-Si

[Display omitted] •The new silicon phase, tP36-Si, has a narrow direct band gap of 0.58 eV.•Its spectral absorption can range from the infrared, visible to ultraviolet light.•The optical properties of tP36-Si are superior to the known diamond Si-I. We predict a new silicon allotrope (called tP36-Si) in theory by first-principles calculations. It consists of 36Si atoms in a unit tetragonal cell with a P42/nmc space group, and exhibits a small density of 2.10 g/cm3. The phonon dispersion and the elastic constants indicate that this structure has dynamic stability and mechanical stability. TP36-Si is a direct semiconductor with a narrow band gap of 0.58 eV, which was calculated by the HSE06 functional. Moreover, the optical properties of tP36-Si are superior to Si-I, since its spectral absorption can range from infrared, visible to ultraviolet light. The calculated X-ray diffraction (XRD) patterns and Raman spectra will provide a theoretical basis for future experimental characterization. Thus, tP36-Si may possess potential applications in the fields of electronic, optical and photovoltaic devices due to its narrow direct band gap and good optical absorption.