“Visible” 5d Orbital States in a Pleochroic Oxychloride

Transition metal compounds sometimes exhibit attractive colors. Here, we report a new oxychloride, Ca₃ReO₅Cl₂, that shows unusually distinct pleochroism; that is, the material exhibits different colors depending on the viewing direction. This pleochroism is a consequence of the coincidental complex crystal field splitting of the 5d orbitals of the Re⁶⁺ ion in a square-pyramidal coordination of low symmetry in the energy range of the visible spectrum. Since the relevant d–d transitions show characteristic polarization dependence according to the optical selection rule, the orbital states are “visible” in Ca₃ReO₅Cl₂.