The contribution of the coordinated water to ⁵D₄ population in YPO₄ hydrates doped with low concentration of Tb³⁺

This work reports an interesting result on the photoluminescence of YPO₄ · 2H₂O doped with low concentration of Tb³⁺, which was synthesized at low temperature by aqueous route. In the as-synthesized Y_0.098PO₄:Tb_0.002 · 2H₂O, in which the hydration water is coordinated to the rare earth ions, the emissions of ⁵D₄ of Tb³⁺ are much more intense than the emissions of ⁵D₃. After dehydration by annealing, an opposite result is achieved, i.e., the emissions of ⁵D₄ of Tb³⁺ are much lower than the emissions of ⁵D₃. The Fourier-transform infrared (FTIR) and fluorescent decay spectra demonstrate that the coordinated water molecule in the hydrated sample contributes to the relaxation of ⁵D₃ to ⁵D₄, thus increasing the population of the excited state ⁵D₄ of Tb³⁺ in the as-synthesized sample.