Lone pair effect, structural distortions, and potential for superconductivity in Tl perovskites

Drawing the analogy to BaBiO₃, we investigate via ab initio electronic structure calculations potential new superconductors of the type ATlX₃ with A = Rb and Cs and X = F, Cl, and Br, with a particular emphasis on RbTlCl₃. On the basis of chemical reasoning, supported by the calculations, we show that Tl-based perovskites have structural and charge instabilities driven by the lone pair effect, similar to the case of BaBiO ₃, effectively becoming A₂Tl⁺Tl ³⁺X₆. We find that upon hole doping of RbTlCl₃, structures without Tl⁺ and Tl³⁺ charge disproportionation become more stable, although the ideal cubic perovskite, often viewed as the best host for superconductivity, should not be the most stable phase in the system. The known superconductor (Sr,K)BiO₃ and hole doped RbTlCl₃, predicted to be most stable in the same tetragonal structure, display highly analogous calculated electronic band structures.