Correlation between Tc and Crystal Structure in S-Doped FeSe Superconductors under Pressure: Studied by X-ray Diffraction of FeSe0.8S0.2 at Low Temperatures (Correlation between T-c and Crystal Structure in S-Doped FeSe Superconductors under Pressure: Studied by X-ray Diffraction of FeSe0.8S0.2 at Low Temperatures)