Approaching Hypothetical RbTl in Experiment and Theory – X-Ray Structure Determination of Cs1-xRbxTl (x=0.18, 0.42) and a Solid Solution K1-xRbxTl (x≤0.69)

Although the binary alkali metal thallides ATl with A= Li, Na, K, and Cs have been reported in literature, binary RbTl at ambient pressure is still missing. Experiments according to 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 a mixture 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 obtained in experiment and supported by theory. 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 (Cs0.82Rb0.18Tl: C2/c, a=14.4136(4) Å, b=11.1678(3) Å, c=40.8013(11) Å, β=96.353(2) °, V=6527.4(3) ų; Cs0.58Rb0.42Tl: C2/c, a=14.2610(3) Å, b=11.1116(2) Å, c=27.5589(7) Å, β=104.056(2) °, V=4236.30(17) ų). Detailed DFT calculations on both, binary and mixed cation systems were performed and support the experimental results.