"Er(SePh)(2.5)I-0.5" reacts with elemental S to give (THF)(10)Er6S6I6, a double cubane cluster with one face of the Er4S4 cube capped by an additional Er2S2. Reactions with a mixture of elemental S/Se results in the formation of (THF)(14)Er10S6(Se-2)(6)I-6, a cluster composed of an Er6S6 double cubane core, with two "Er-2(Se-2)(3)" units condensed onto opposing rectangular sides of the Er6S6 fragment. This deposition of Er2Se6 totally encapsulates the two central Er with chalcogen atoms (4 S, 4 Se) and excludes neutral THIF donors or iodides from the two primary coordination spheres. The Er-10 compound is the first lanthanide cluster to contain internal, chalcogen encapsulated Ln. This cluster shows strong fluorescence at 1544 nm with a measured decay time of 3 ms and an estimated quantum efficiency of 78%, which is comparable to Er doped solid-state materials. The unusual fluorescence spectral properties of (THF)(14)Er10S6Se12I6 are unprecedented for a molecular Er complex and are attributed to the low phonon energy host environment provided by the I-, S2-, and Se-2(2-) ligands.