Tuning of Emission by Eu³⁺ Concentration in a Pyrophosphate: The Effect of Local Symmetry

The local symmetry of a Eu³⁺ ion has a crucial effect on its luminescence properties. In this work, we show that the red/orange ratio in the emission of Eu³⁺-doped MgIn₂P₄O₁₄ phosphors is tunable by adjusting the Eu³⁺ concentrations, due to the change in the local symmetry of metal ions. The substitution of Eu³⁺ for In³⁺ lowers the distortion in the lattice of monoclinic MgIn₂P₄O₁₄, and an increase in Eu³⁺ doping concentration causes the metal ion sites to shift closer to an inverse center, leading to a reversal of the relative emission intensity of a magnetic dipole transition to an electric dipole transition. Meanwhile, the higher asymmetry of metal ion sites occupied by Eu³⁺ in MgIn₂P₄O₁₄ makes the luminescence less thermally stable than that in Mg₃In₄P₆O24.