Fig. 1: Concentration quenching for Mn⁴⁺ in crystals.

At high Mn⁴⁺ doping concentrations the Mn⁴⁺ ions (orange) are in close proximity in the crystal lattice. If the Mn⁴⁺ ions are close together, energy transfer between Mn⁴⁺ ions (dark blue) causes the excitation to migrate through the crystal. Eventually, it may reach a quenching site such as a vacancy or impurity (dashed circle), where the excitation energy is lost as heat. This process competes with radiative emission (red) and reduces the luminescence efficiency