Quaternary compounds Tl₂CdSi₃Se₈ and Tl₂CdGe₃Se₈ were found at the Tl₂CdSi(Ge)Se₄–Si(Ge)Se₂ sections of the quasi-ternary systems Tl₂Se–CdSe–Si(Ge)Se₂ by XRD and microstructure analysis methods.

Similar quaternary chalcogenides A^I₂B^IID^IV₃X₈ were reported earlier with alkaline elements (A^I = Cs, Rb, K, Na; В^II = Mg, Mn, Zn, Cd, Hg; D^IV = Ge, Sn; Х = S, Se, Te. Several types of crystal structures were observed in this family of compounds, orthorhombic (S.G. Р2₁2₁2₁), monoclinic (S.G. P2₁/c or P2₁/n), cubic Pa-3. Additionally, similar compositions Cu(Ag)₂CdSn₃S₈ were found in the Cu(Ag)₂S–CdS–SnS₂ systems. The Cu₂CdSn₃S₈ compound is a synthetic analoque of the natural mineral rhodostannite Cu₂FeSn₃S₈ and crystallizes in the tetragonal S.G. I4₁/a. The Ag₂CdSn₃S₈ crystal structure refines well in both tetragonal rhodostannite type (S.G. I4₁/a) and cubic chalcospinel type (S.G. Fd–3m).

The Tl₂CdD^IV₃X₈ compounds (M^IV = Si, Ge; X = Se) are closer to the quaternary phases with alkaline metals with orthorhombic structure. Their structure was determined in the isotropic approximation using the Cs₂CdGe₃Se₈ structure as a model, S.G. Р2₁2₁2₁ with the lattice parameters a=0.7485(1), b=1.2117(3), c=1.7134(3) nm, R_I=0.0953 (Tl₂CdSi₃Se₈) and a=0.7602(3), b=1.2071(2), c=1.7474(2) nm, R_I=0.1204 (Tl₂CdGe₃Se₈).

Each layer 2/_∞[CdD^IV₃Se₈]²⁻ consists of chains 1/_∞[CdD^IVSe₆]^6– that are linked by alternating [CdSe₄] and [D^IVSe₄] tetrahedra by corner sharing along the direction a. Moreover, the adjacent chains are connected into a layer by [D^IV₂Se₆]⁴⁻ dimers by corner sharing along the direction c.