Chromium ions, both trivalent (Cr3þ) and tetravalent (Cr4þ), belong to the family of transition-metal elements with 3d valence electronic levels. More precisely, they belong to the d-block elements, which can lose their valence s electrons as well as the d electrons, thus yielding various oxidation states. When an ion is located in a solid state host such as crystal with ordered lattice atoms or in a short-range ordered structure such as glass or glass-ceramics, it is surrounded by a set of ions (called ligands) in a very specific geometrical configuration. The interaction between the doped ions and the host ligands is an electrostatic Coulombic interaction, whereas the doped ion and the ligands are considered point charges that are arranged in a specific geometry.
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