A series of PbmSb2nSem+3n nanocrystals (m = 2, 4, 6 and 8; n = 1) are demonstrated that exist only as a distinct phase on the nanoscale. The nanocrystals aggregates are new compounds adopting the cubic NaCl-type structure. These materials form aggregates comprised of nanocrystallites that are attached at a preferred orientation. Elemental compositions were studied using the complementary techniques of scanning transmission electron microscopy/energy dispersive X-ray spectroscopy and inductively coupled plasma-atomic emission spectroscopy. The new ternary nanocrystal aggregates are moderately monodisperse and exhibit well-defined band gap energies in the mid-IR region. The PbmSb2nSem+3n nanomaterials behave as homogeneous solid solutions with lattice parameter trending as a function of Sb incorporation at room temperature and tend to phase separate into PbSe and Sb2Se3 at 400 degrees C.