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Calculated Mn4+emission energies for various oxides as functions of Mn–O bond length. The experimental values are shown (in red) wherever available. There are three groups of materials: the ones with small O–Mn–O bond angle distortion (black squares), the ones with large O–Mn–O bond angle distortion (blue circles), and phosphates (green triangles). Weak Mn4+-ligand hybridization as a result of long Mn–O bond lengths and/or large O–Mn–O bond angle distortion generally leads to higher emission energies.
Minshu Du (Wed,) studied this question.