Synthesis and Properties of Charge-Ordered Thallium Halide Perovskites, CsTl⁺0.5Tl³⁺0.5X₃ (X = F or Cl): Theoretical Precursors for Superconductivity?

Recently, CsTlCl 3 and CsTlF 3 perovskites were theoretically predicted to be potential superconductors if they were optimally doped. The syntheses of these two compounds together with a complete characterization of the samples are reported. CsTlCl 3 was obtained as orange crystals in two different polymorphs: a tetragonal phase ( I 4/ m ) and a cubic phase ( Fm 3̅ m ). CsTlF 3 was formed as a light brown powder, and also as a double cubic perovskite ( Fm 3̅ m ). In all three CsTlX 3 phases, Tl + and Tl 3+ were located in two different crystallographic positions that accommodate their different bond lengths. In CsTlCl 3, some Tl vacancies were found in the Tl + position. The charge ordering between Tl + and Tl 3+ was confirmed by X-ray absorption and Raman spectroscopy. The Raman spectroscopy of CsTlCl 3 at high pressure (58 GPa) did not indicate any phase transition to a possible single Tl 2+ state. However, the highly insulating material became less resistive with an increasing high pressure, while it underwent a change in its optical properties, from transparent to deeply opaque red, indicative of a decrease in the magnitude of the band gap. The theoretical design and experimental validation of the existence of CsTlF 3 and CsTlCl 3 cubic perovskites are the necessary first steps in confirming the theoretical prediction of superconductivity in these materials.