Na2CO3-inducing YPO4:Eu phase transformation and related luminescence

Abstract Herein we describe a Eu 3+ (5 at%)-doped YPO 4 nano-/microcrystal system in which crystal phase transformation from hexagonal to tetragonal structure takes place by increasing the PO 4 3- concentration under reaction solution. Also, we find an interesting phenomenon that introduction of Na 2 CO 3 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 YPO 4 system. The Na 2 CO 3 plays two important roles in the phase transformation process. One is to increase PO 4 3- concentration through CO 3 2- hydrolyzed procedures to convert hexagonal into tetragonal structure in early stage. And the other is to decrease free RE 3+ ion concentration through CO 3 2- - RE 3+ 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 Eu 3+ non-radiative transition rate.