Hole Doping and Structural Transformation in CsTl_1–xHg_xCl₃

CsTlCl₃ and CsTlF₃ perovskites have been theoretically predicted to be superconductors when properly hole-doped. Both compounds have been previously prepared as pure compounds: CsTlCl₃ in a tetragonal (I4/m) and a cubic (Fm3̅m) perovskite polymorph and CsTlF₃ as a cubic perovskite (Fm3̅m). In this work, substitution of Tl in CsTlCl₃ with Hg is reported, in an attempt to hole-dope the system and induce superconductivity. The whole series CsTl_1–xHg_xCl₃ (x = 0.0, 0.1, 0.2, 0.4, 0.6, and 0.8) was prepared. CsTl_0.9Hg_0.1Cl₃ is tetragonal as the more stable phase of CsTlCl₃. However, CsTl_0.8Hg_0.2Cl₃ is already cubic with the space group Fm3̅m and with two different positions for Tl⁺ and Tl³⁺. For x = 0.4 and 0.5, solid solutions could not be formed. For x ≥ 0.6, the samples are primitive cubic perovskites with one crystallographic position for Tl⁺, Tl³⁺, and Hg²⁺. All of the samples formed are insulating, and there is no signature of superconductivity. X-ray absorption spectroscopy indicates that all of the samples have a mixed-valence state of Tl⁺ and Tl³⁺. Raman spectroscopy shows the presence of the active Tl–Cl–Tl stretching mode over the whole series and the intensity of the Tl–Cl–Hg mode increases with increasing Hg content. First-principle calculations confirmed that the phases are insulators in their ground state and that Hg is not a good dopant in the search for superconductivity in this system.