The aim of the formation energy calculations was to check the trend in both structure types with increasing rubidium concentration, however claiming the structure stability with respect to the formation energy is not reliable. Therefore, phonon frequencies were used as indicator to investigate the dynamical stability of RbTl. The presence of positive phonon frequencies in a material is an indicator of its dynamic stability and vice versa. In Figure the calculated acoustic branches at gamma (Γ) show that there are no negative frequencies involved.