Multi-site occupancies and dependent photoluminescence of Ca9Mg1.5(PO4)7:Eu2+ phosphors: A bifunctional platform for optical thermometer and plant growth lighting

Herein, we demonstrate an optical thermometer based on single Eu2+ doped Ca9Mg1.5(PO4)7 phosphors, which were prepared by traditional solid-state reaction technique under a reduction atmosphere. Considerations on the bond length obtained by the crystal structure refinement and the dependent photoluminescence performances allow to assign the two distinct emission bands to Eu2+ ions occupied Ca1–Ca3 and Mg2 sites. Moreover, the blue and red emitting bands perfectly match with the photosynthetic action spectrum, which can enhance the indoor plant photosynthesis. The optimal doping content of Eu2+ ions in this Ca9Mg1.5(PO4)7 system is 3 mol%. The corresponding concentration quenching effect is verified as dipole–dipole interaction with the critical distance of 3.315 nm. Furthermore, by exploiting the fluorescence intensity technique, the optical thermal resistance properties of Ca9Mg1.5(PO4)7:Eu2+ are identified based on the temperature dependent emission spectra in a range of 303–523 K. In detail, the maximum absolute and relative sensitivity Sa and Sr of Ca9Mg1.5(PO4)7:Eu2+ thermometer are as high as 0.637%/K and 0.3155 K–1, respectively. Consequently, the Eu2+ doped Ca9Mg1.5(PO4)7 phosphors establish a bifunctional platform for both optical thermometer and plant growth lighting via multi-site occupancies.