Phosphate Ion-Driven BiPO₄:Eu Phase Transition

Abstract

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 lowtemperature 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 lowtemperature 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.

Microwave-Driven Hexagonal-to-Monoclinic Transition in BiPO4: An In-Depth Experimental Investigation and First-Principles Study

“…We have associated our experimental morphologies with the B1 and B2 shapes. The A2 and D morphologies generated are similar to that reported by Li et al 21 Furthermore, the polyhedron energy () was calculated and also the energy profiles which allows to connect the ideal morphology with the final experimental morphology were constructed and are depicted in Figure 9 and 10 (bottom) for BiPO 4 hexagonal and monoclinic, respectively. The reaction diagram to obtain the final experimental morphologies of BiPO 4 hexagonal evidences a process barrier less thermodynamically favorable (Figure 9 (bottom)) while the reaction path for BiPO 4 monoclinic presents a minimum of energy via morphology B1 (Figure 10 (bottom)).…”

“…Wang et al also observed the formation of the monoclinic phase using the MAH method, but with the addition of an ionic liquid in the reaction medium 19 . Few studies in the literature also show the conversion of the hexagonal phase to the monoclinic phase by the hydrothermal method 12 through the incorporation of compression 20 or doping processes 21,22 . Chen et al observed complete conversion by employing the time frame of 1 to 3h at a temperature of 200 °C 23 .…”

An Advanced Tunable Multimodal Luminescent La4GeO8: Eu2+, Er3+ Phosphor for Multicolor Anticounterfeiting

“…So far, many photoluminescence (PL) materials such as carbon quantum dots, [ 11,12 ] metal–organic frames, [ 13 ] organic dyes, [ 14 ] perovskite, [ 15 ] phase transition luminescent, [ 16–19 ] and lanthanide‐doped inorganic luminescence materials [ 20,21 ] have been used in anticounterfeiting. Among these materials, lanthanide‐doped inorganic luminescence materials due to color‐tunable, simple recognition, and good photochemical stability are considered the most competitive anticounterfeiting materials.…”

Orthogonally Polarized Luminescence of Single Bismuth Phosphate Microcrystal Doped with Europium

“…Compared with the hexagonal structure, the formation of the monoclinic one needs the crystallographic system in higher free energy, which was confirmed by chemical potential theory as well as the experiments that involve phase transition by varying ion concentration or heating temperature. [32,33] The samples consist of polyhedral microcrystals with lengths ranging from 1 to 5 µm ( Figure S1, Supporting Information). The long axis of the microcrystal is parallel to its crystallographic c-axis.…”

“…The polyhedral morphologies are considered as the geometrical evolutions of the standard mono-clinic structure, in which the most exposed crystallographic faces are low-index (100) and (010) ones due to lower surface energies/activities. [32,34] By monitoring the direct emission wavelength of 593 nm of Eu 3+ ion, the excitation spectrum of the samples ( Figure S2, Supporting Information) shows the 1 S 0 → 3 P 1 transition of Bi 3+ ions and Eu−O charge transfer band as well as a group of shape f−f transitions ranging from 300 to 450 nm within the Eu 3+ 4f 6 electronic configuration. [35,36] The most intense peak is located at 395 nm originating from the 7 F 0 → 5 L 6 transition which is chosen to excite the single BiPO 4 :Eu microcrystals.…”