The energy and force convergence criteria were set at 10 -6 eV and 10 -3 eV/Å, respectively. Additionally, the phonon frequencies of the hypothetical RbTl were calculated using first principles phonon calculations with a finite displacement method implemented in phonopy code interfaced in VASP package. The accuracy of the phonon calculation is sensitive to various technical parameters including supercell size, force convergence symmetry, energy convergence and atomic displacement. In the present calculation, we used default values of 1.0 for force convergence symmetry, energy convergence criteria to 10 -6 eV and default atomic displacement of 0.01 Å. Detail on supercell size can be found in the SI (Table ). In phonon calculations, only the Gamma-point (Γ) was used for k-space sampling.