Abstract Investigating crystal phase transition helps in understanding new crystal structures and obtaining novel performance. The conventional approaches to realizing phase transition usually require extreme conditions such as high temperature, high pressure, or heavy ion doping. In this work, we achieve phase transition from hexagonal to tetragonal YPO4:Eu crystals just by increasing the concentration ratio of (NH4)2HPO4 to NH4H2PO4 in the precursor. When increasing the ratio, the underlying transition mechanism modifies the free energy of the crystalline system and in turn selects the tetragonal phase with higher formation energy. The difference in preferred growth direction endows tetragonal and hexagonal microcrystals with sphere- and prism-like morphology, respectively. The luminescence intensity is stronger in the tetragonal than the hexagonal phase, due to stronger non-radiative energy transfer induced by high-frequency vibrations in the hexagonal lattice. The efficient method provided here allows convenient production of tetragonal samples with high luminescence intensity on a large scale, and could be well applied to other phosphate systems for phase selection.

中文翻译：

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(NH4)2HPO4 和 NH4H2PO4 在 YPO4:Eu 相变和发光增强中的作用

摘要 研究晶相转变有助于理解新的晶体结构和获得新的性能。实现相变的传统方法通常需要极端条件，例如高温、高压或重离子掺杂。在这项工作中，我们仅通过增加前体中 (NH4)2HPO4 与 NH4H2PO4 的浓度比来实现从六方到四方 YPO4:Eu 晶体的相变。当增加比例时，潜在的转变机制改变了晶系的自由能，进而选择了具有更高形成能的四方相。优选生长方向的差异分别赋予四方和六方微晶以球状和棱柱状形态。由于六方晶格中的高频振动引起的非辐射能量转移更强，四方相中的发光强度比六方相更强。这里提供的有效方法可以方便地大规模生产具有高发光强度的四方样品，并且可以很好地应用于其他磷酸盐系统的相选择。