Na₂CO₃-inducing YPO₄:Eu phase transformation and related luminescence

Herein we describe a Eu³⁺ (5 at%)-doped YPO₄ nano-/microcrystal system in which crystal phase transformation from hexagonal to tetragonal structure takes place by increasing the PO₄ ^3- concentration under reaction solution. Also, we find an interesting phenomenon that introduction of Na₂CO₃ and improvement of its concentration under reaction solution will induce a phase transformation from hexagonal to tetragonal, and then back to hexagonal one. To the authors’ knowledge, this, for the first time, is observed in YPO₄ system. The Na₂CO₃ plays two important roles in the phase transformation process. One is to increase PO₄ ^3- concentration through CO₃ ^2- hydrolyzed procedures to convert hexagonal into tetragonal structure in early stage. And the other is to decrease free RE³⁺ ion concentration through CO₃ ^2- - RE³⁺ complexes to reverse tetragonal into hexagonal structure in latter stage. Moreover, we confirm that luminescence changes induced by the phase transformation result from the change of Eu³⁺ non-radiative transition rate.