Approaching Hypothetical RbTl in Experiments and Theory – X-ray Structure Determination of Cs _{1– <i>x</i>} Rb <i> _x </i> Tl ( <i>x</i> = 0.18, 0.42) and a Solid Solution K _{1– <i>x</i>} Rb <i> _x </i> Tl (

Although the binary alkali metal thallides ATl with A = Li, Na, K, and Cs have been reported in the literature, binary RbTl at ambient pressure is still missing. Experiments with a 1:1 ratio of Rb:Tl, either according to Zintl's procedure in low-temperature experiments in liquid ammonia or classical solid-state synthesis at high temperature, did not result in the desired product. Therefore, several ternary compositions with mixtures of K/Rb and Cs/Rb have been prepared. For K/Rb mixtures, a solid solution in the KTl structure type, up to a proportion of 69% rubidium, could be obtained. Site occupancy preferences for rubidium on the alkali metal sites in the KTl type are observed in experiments and supported by theoretical calculations. In contrast to Rb/K mixtures being realizable in the KTl structure type, Rb/Cs mixtures did not allow for the isolation of materials according to the CsTl structure type. Instead, two new monoclinic compounds could be isolated (Cs_0.82Rb_0.18Tl: <i>C</i>2/<i>c</i>, <i>a</i> = 14.4136(4) Å, <i>b</i> = 11.1678(3) Å, <i>c</i> = 40.8013(11) Å, β = 96.353(2)°, <i>V</i> = 6527.4(3) ų; Cs_0.58Rb_0.42Tl: <i>C</i>2/<i>c</i>, <i>a</i> = 14.2610(3) Å, <i>b</i> = 11.1116(2) Å, <i>c</i> = 27.5589(7) Å, β = 104.056(2)°, <i>V</i> = 4236.30(17) ų). Detailed DFT calculations on both binary and mixed cation systems were performed and support the experimental results.