Structural, Electronic, Elastic Properties, and Phase Transitions of Type-I and Type-VIII Sr8Al16Sn30 Clathrates from First-principles Calculations

Abstract

In this work, the phase transitions, the elastic properties, and the electronic properties of Sr8Al16Sn30 under different pressures are investigated by first-principle calculations based on density functional theory. The E–V curves show that the type-VIII is the stable ground state phase, while the type-I is considered to be metastable at the lowest energy. Ignoring the temperature effect, type-VIII clathrate is always more stable than type-I, and the indication of phase transitions from type-I to type-VIII structure is not observed under increasing hydrostatic pressure. The obtained elastic constant values indicating that two structures under 0 and 3 GPa are stable, and not stable under 7 GPa. The dependences of the elastic constants cij, the elastic modulus, the Poisson ratio, the brittle and ductile behavior, and the elastic anisotropy on pressure in two structures are further analyzed. The electronic structures of type-I and type-VIII clathrates significantly change under increased pressure. The type-I clathrate with a narrow gap turns into the metal, and for type-VIII the band gap decreases in 3 GPa and the band gap increases in 7 GPa. This show that pressure tuning plays an important role in improving material properties.