Phosphateion-driven BiPO ₄ :Eu phase transition

Phase transition of crystals is one of the most important topics in condensed matter physics, chemistry, and materials sciences in which the basic physical and chemical properties of the crystals themselves are first determined during atomic rearrangements. In general, achieving the phase transition needs high temperature, high pressure, and heavy doping treatments. Here, we describe a BiPO ₄ :Eu crystal system in which the phase transition from hexagonal to low-Temperature monoclinic structure can be effectively manipulated by a simple method of increasing the anion (PO ₄ ^3- ) concentration in the reaction solution. This is associated with the improvement in the free energy in the crystallographic system in which only the crystallographic phase having the energy lower than the system free energy is finally formed. The observed morphologies of the BiPO ₄ :Eu crystals within the low-Temperature monoclinic structure are divided into four types and each of them stems from the geometrical evolution of the standard monoclinic structure. The low-Temperature monoclinic structure exhibits superior performance in luminescence as compared to hexagonal one due to the extent of significant nonradiative process in the lattices of the latter. The idea provided here could be extended to understanding the effects of reaction anion or cation on the phase transition of other important down/upconversion luminescence materials such as YPO ₄ and Na(Y,Gd)F ₄ . A question on how many crystal systems can achieve phase transition through the method reported here needs and deserves to be explored.