"Visible" 5d orbital states in a pleochroic oxychloride

TL;DR

This paper reports the discovery of a new oxychloride compound exhibiting distinct pleochroism caused by complex crystal field splitting of Re6+ 5d orbitals, making orbital states visible through polarization-dependent optical properties.

Contribution

It introduces a novel oxychloride material with visible 5d orbital states due to low-symmetry crystal field effects, highlighting the link between crystal structure and optical properties.

Findings

Ca3ReO5Cl2 exhibits strong pleochroism.

5d orbital states are directly observable via optical polarization.

Crystal field splitting causes visible d-d transitions.

Abstract

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.