Abstract
Single crystals of the mixed-valent compounds Hg~7~Se~3~O~13~H~2~ (I) and Hg~8~Se~4~O~17~H~2~ (II) were grown under hydrothermal conditions. The crystal structures were solved and refined from single crystal X-ray data collected at room temperature (I: R__3, Z = 3, a = 5.9239(3), c = 37.096(3) Å, 1465 structure factors, 60 parameters, R[F
^2^ > 2__σ(F
^2^)] = 0.0383, wR(F
^2^ all) = 0.0970, Goof = 1.047; II: P__31__c, Z = 2, a = 5.8908(6), c = 31.048(3) Å, 1889 structure factors, 69 parameters, R[F
^2^ > 2__σ__(F
^2^)] = 0.0662, wR(F
^2^ all) = 0.1626, Goof = 1.088). Both compounds show a multifarious crystal chemistry and contain triangular Hg~3~
^4+^ cluster cations, Hg~2~
^2+^ dumbbells, Hg^2+^ cations, HgO~2~
^2–^ mercurate groups, and trigonal-pyramidal SeO~3~
^2–^ as well as tetrahedral SeO~4~
^2–^ anions and water molecules as simple building units. The description in terms of crystal chemistry leads to the more detailed formulae (Hg^IV/3^
~3~) (Hg^I^
~2~)(Hg^II^)(Hg^II^O~2~)(Se^IV^O~3~)~2~(Se^VI^O~4~)(H~2~O) for I and (Hg^IV/3^
~3~)(Hg^I^
~2~)(Hg^II^)~2~(Hg^II^O~2~)(Se^IV^O~3~)~2~(Se^VI^O~4~)~2~(H~2~O) for II, respectively. Characteristic features of the closely related structures are building blocks formed by parallel layers of Hg~3~
^4+^ triangles, layers of disordered Hg~2~
^2+^ dumbbells, layers of SeO~4~
^2–^ and HgO~2~
^2–^ groups, and layers of SeO~3~
^2–^ groups which are all stacked along [001]. The layers are linked into a complex three-dimensional framework via common O atoms and H~2~O molecules. In comparison with Hg~7~Se~3~O~13~H~2~, the structure of Hg~8~Se~4~O~17~H~2~ contains an additional and disordered HgSeO~4~ layer which leads to a significant distortion of parts of the resulting structure.