# "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.

## Key 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.

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Source: https://tomesphere.com/paper/1904.06096