Europium valence control in the hydrothermal synthesis of apatites and borosilicates
Graphical abstract
Introduction
Europium is somewhat unique as a lanthanide ion in that it has two oxidation states Eu2+ (f7) and Eu3+ (f6) of reasonable stability. Although the Eu2+ state is susceptible to oxidation under some conditions it has good stability in the solid state, and the two oxidation states can be accessed and controlled relatively easily [1]. The Eu2+ state is of particular interest for its strong blue emission as well as its scintillation properties [2], [3], [4], [5], [6], [7]. The Eu3+ state also has unique optical properties with a relatively strong emission in the orange-red region of the visible spectrum [8], [9], [10], [11]. Interestingly, its visible red emission spectrum is highly sensitive to its coordination environment. Thus in addition to its possible use as a phosphor [12], [13], it is also a good diagnostic ion for coordination environments [14], [15].
One major area of interest is regarding the physical properties of europium, in particular its properties as a scintillator ion, primarily in its divalent state [16], [17], [18], but also in its trivalent oxidation state [19]. The hydrothermal synthetic method offers control over oxidation states and is especially suitable for lower oxidation state metal ions [20]. Thus it seems like an excellent technique for the exploration of various europium-containing solids. In particular, the technique is useful for amphoteric metal oxides such as silicates and borates. Since borates and silicates containing rare earth or alkaline earth metal sites are often suitable hosts for lanthanide dopant ions, and in particular are excellent solid hosts for scintillators and luminescent materials, we began the exploration of both Eu2+ and Eu3+ doped into borosilicates. We previously found that borosilicate glasses are an attractive starting material for the synthesis of lanthanide-containing borates, silicates, and borosilicates [21]. In addition we found that these materials have a significant probability of crystallizing in noncentrosymmetric space groups, raising the possibility of forming self-frequency doubling laser crystals in the presence of appropriate lanthanide dopants.
Given the ability to control the oxidation state of the europium in crystalline solids [22], [23], we began to investigate the chemistry of various europium oxidation states via hydrothermal crystal growth. In this paper we present our results related to the synthesis and spectroscopy of Eu in silicates and borosilicates, with particular emphasis on the ability to control the valence of the Eu ion in the hydrothermal synthesis, where Eu is present as a dopant or a fundamental building ion in the resulting crystals. We describe the formation of novel Eu containing silicate and borosilicate single crystals using a high temperature hydrothermal synthetic method starting with various premade borosilicate glasses. We also explore Eu doping in barium borosilicate systems, where Ba2+ serves as both a good structural building block and a host site in the lattice for Eu2+ ions. In this case europium ions can be introduced either by impregnation in the original glass feedstock or by addition of suitable Eu reagents to the hydrothermal reaction mixture. The study reports a simple method to control the oxidation state of the europium ion by the addition of reducing agents to the reaction mixture to isolate only Eu2+ containing products. Given the extensive array of borosilicate glass feedstocks available we anticipate that huge range of new crystalline solids can be synthesized using this approach.
Section snippets
Preparation of borosilicate glass feedstock
Three types of barium borosilicate glasses were prepared for use as feedstock for subsequent hydrothermal synthesis. The first, “BaSiBO”, was prepared by the solid state reaction of 2.5 g (36 mmol) B2O3 (Alfa Aesar, 98%), 0.72 g (12 mmol) SiO2 (Alfa Aesar 99.9%), and 1.84 g (12 mmol) BaO (Alfa Aesar 99.5%) in a platinum crucible in air at 1000 °C for 18 h. Additionally, two different europium-doped barium borosilicate glasses were similarly prepared by addition of Eu2O3 or EuCl2 to the
Synthesis and crystal chemistry of europium apatites
The initial motivation for the current study was the synthesis of europium doped borosilicates. As an ongoing study in the synthesis of novel borosilicates, we have observed a variety of structures and rich chemistry by hydrothermal treatment of an amorphous starting material to produce crystalline phases. This synthetic methodology was extended to the attempts to prepared europium doped borosilicate as described herein. In doing so, crystalline products were observed where europium was present
Conclusions
We began an investigation to explore the scope and limitations of using high temperature hydrothermal synthesis to introduce europium ions into silicate and borosilicate lattices as a possible emitter for visible light or use as a scintillator. Europium is unique among the lanthanides in that is has a range of useful emission properties for two stable oxidation states, Eu2+ and Eu3+. In a previous report we introduced the concept of using high temperature hydrothermal reactions to employ Eu3+
Acknowledgements
We would like to thank the National Science Foundation (DMR-0305377, DMR-0907395 and DMR-1410727) for funding and support.
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