Isolated [SbCl₆]^3– Octahedra Are the Only Active Emitters in Rb₇Sb₃Cl₁₆ Nanocrystals

We elucidate here the nature of the emissive states in Rb7Sb3Cl16 nanocrystals (NCs) for which we report a synthesis. These NCs have a crystal structure comprising both isolated [SbCl6]3– octahedra and isolated [Sb2Cl10]4– dimers of octahedra. The optical properties of Rb7Sb3Cl16 NCs are compared with those of Sb-doped Rb3InCl6 NCs, the latter containing only isolated [SbCl6]3– octahedra. The remarkably similar behaviors of the two systems indicate that the optical emission in both materials originates from the isolated octahedra. Density functional theory calculations suggest that the [SbCl6]3– octahedra are optically active in emission because the local arrangement of the Rb+ ions around the octahedra limits the elongation of the Sb–Cl bonds upon excitation, helping to confine the self-trapped exciton in them. Conversely, in the [Sb2Cl10]4– dimers the constraining effect of the surrounding Rb+ ions is less marked and the Sb–Cl bonds fully break upon photoexcitation, opening up an efficient nonradiative channel for the self-trapped excitons.