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 to investigate the dynamical stability of the rubidium-doped KTl and CsTl structure as indicator. The presence of positive phonon frequencies in a material is an indicator of its dynamic stability and vice versa. In Furthermore, the phonon band structure of monoclinic structure (I) (C2/c) in Figure is an indication of stable nature and complements to the experiments (see section 4.2). In addition, calculations on the hypothetical Fddd orthorhombic unit cell setting of Cs0.562Rb0.438Tl also show no negative frequencies, despite not being observed in experiments.