  Transition metal compounds sometimes exhibit beautiful colors. We report here
on a new oxychloride Ca3ReO5Cl2 which shows unusually distinct pleochroism;
that is, the material exhibits different colors depending on viewing
directions. This ple-ochroism is a consequence of the fact that a complex
crystal field splitting of the 5d orbitals of the Re6+ ion in a
square-pyramidal coordination of low-symmetry occurs accidentally in the energy
range of the visible light spectrum. Since the rele-vant d-d transitions
possess characteristic polarization dependences according to the optical
selection rule, the orbital states are "visible" in Ca3ReO5Cl2.
