These new thallates crystallized in the Cmcm space group, with a = 0.33255, b = 1.1022, and c = 1.0479nm for Tl2CaO4 and a = 0.33431, b = 1.1159, and c = 1.3499nm for Ca2Tl2O5. An  ab initio  determination of their structures from powder X-ray data showed that they were closely related, and represented the first 2 members of a series with the general formula: Tl2CanOn+3. The first member, Tl2CaO4, was isotypical with Ti2CaO4 and could be described as being a chemical twin of [Ti2O4] rock salt layers. The second member, Ca2Tl2O5, was derived from the first by increasing the thickness of the rock-salt layers. That is, by replacing double ribbons of edge-sharing octahedra with triple ribbons. In both structures, chemical twinning was induced by the ability of Ca to adopt a trigonal prismatic coordination in prismatic tunnels. The direction of the rock-salt layer was invariable, and was parallel to the (113) plane of the cubic structure.

F.Goutenoire, V.Caignaert, M.Hervieu, C.Michel, B.Raveau: Journal of Solid State Chemistry, 1995, 114[2], 428-34