  Recently CsTlCl3 and CsTlF3 perovskites were theoretically predicted to be
potential superconductors if they are optimally doped. The synthesis of these
two compounds, together with a complete characterization of the samples are
reported. CsTlCl3 is obtained as orange crystals in two different polymorphs: a
tetragonal (I4/m) and a cubic (Fm-3m) phase. CsTlF3 is formed as a light brown
powder, also as a double cubic perovskite (Fm-3m). In all three CsTlX3 phases
Tl1+ and Tl3+ were located in two different crystallographic positions that
accommodate their different bond lengths. In CsTlCl3 some Tl vacancies are
found in the Tl1+ position. The charge ordering between Tl1+ and Tl3+ was
confirmed by x-ray absorption and Raman spectroscopy. The Raman spectroscopy of
CsTlCl3 under high pressure (58 GPa) did not indicate any phase transition to a
possible single Tl2+ state. However, the highly insulating material becomes
less resistive with increasing high pressure, while undergoing a change in the
optical properties, from transparent to deeply opaque red, indicative of a
decrease of the band gap. The theoretical design and experimental validation of
the existence of CsTlF3 and CsTlCl3 cubic perovskites is the necessary first
step in confirming the theoretical prediction of superconductivity in these
materials.
