supplyment

Information on the lattice parameters of the crystal structure is given in the Supplementary Material, which also includes polyhedral structure diagrams for the Pnma (Y) phase, the Pnnm phase and the P21/m phase. In addition, the following are included: volume versus pressure curves of the three phases, phonon spectra at different pressures, shortest bond lengths of the three phases and bond angle variations of the transition states from Pnma (Y) to Pnnm, charge transfers of the three phases at different pressures, and -pCOHP and -ICOHP calculated for Sn-I, Sn-Sn, and I-I, respectively, at different pressures for the three phases.