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 c_ij, 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.