Effect of OH^- on the luminescent efficiency and lifetime of Tb³⁺-doped yttrium orthophosphate synthesized by solution precipitation

In this work, we use the solution precipitation method to synthesize Tb³⁺-doped yttrium orthophosphate, which is a green-emission luminescent material. The evolution of hydrated yttrium orthophosphate (YPO ₄· 2H₂O) to dehydrated yttrium orthophosphate (YPO₄) is observed in the heat-treatment process, simultaneously, accompanying the structural transformation from monoclinic churchite-type to tetragonal xenotime-type structure. Furthermore, the luminescent efficiency of Tb³⁺-doped YPO₄ presents a sharp jump at a critical temperature in this heat-treatment process. Interestingly, this critical temperature is close to the structural transformation temperature. The remarkable change of luminescent efficiency seems to be related to the structural transformation. However, the FTIR and fluorescent decay measurements at 10 and 300 K indicate that the OH group is the origin of luminescent efficiency change. OH^- ions with high vibration frequency provide an efficient means to quench the luminescence. The comparison of the luminescent efficiency, OH^- content, and lifetime of ⁵D₄ of Tb³⁺ between two samples with the same crystal structure proves that the structural transformation has no significant effect on the luminescent efficiency and lifetime. On the basis of these results, it is proposed that correctly preventing OH^- ions inside the host matrix or effectively eliminating them may improve the luminescent efficiency greatly. This idea also may be applied to other optical systems.