Tuning of Emission by Eu Concentration in a Pyrophosphate: the Effect of Local Symmetry.

The local symmetry of a Eu ion has a crucial effect on its luminescence properties. In this work, we show that the red/orange ratio in the emission of Eu-doped MgInPO phosphors is tunable by adjusting the Eu concentrations, due to the change in the local symmetry of metal ions. The substitution of Eu for In lowers the distortion in the lattice of monoclinic MgInPO, and an increase in Eu 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 Eu in MgInPO makes the luminescence less thermally stable than that in MgInPO.