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 PO43- 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 PO43- concentration through CO32- hydrolyzed procedures to convert hexagonal into tetragonal structure in early stage. And the other is to decrease free RE3+ ion concentration through CO32- - 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.