Luminescent properties of Eu-doped magnetic Na₃FeF₆

Abstract

Sodium iron fluoride (Na₃FeF₆) is a colorless ferromagnetic fluoride with a monoclinic crystal structure (space group P21/c), and it is expected to be an ideal platform for exploring magneto-optical interactions. In the present work, Eu³⁺ doped Na₃FeF₆ micro-powders were synthesized by a hydrothermal method, and the structures were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The optical properties were examined using UV-Vis spectra and fluorescence spectra, and the results show that the emission spectra can be finely tuned by the hydrothermal reaction temperature and doping concentration of Eu ions. We found that Na₃FeF₆ doped with 5% Eu³⁺ synthesized at 196 °C exhibited the optimal red emission under excitation at 395 nm. The magnetization of Na₃FeF₆:5% Eu³⁺ decreased rapidly from about 7.85 emu g⁻¹ at 5 K to 0.4 emu g⁻¹ at 60 K, then slowly decreased with temperature increase from 60 K to 300 K. This Eu³⁺ doped Na₃FeF₆ powder is expected to find potential applications in the field of magneto-optical modulation and relevant devices.

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1. Introduction

Due to the applications in biomedical imaging, cancer therapy and sensing, magneto-optical bi-functional nanomaterials have attracted growing attention in the past decade.^1–3 Magnetic nanomaterials such as superparamagnetic nanocrystals can be used as drug carriers and magnetic resonance imaging materials in the bio-medicine field. The introduction of optical functionality to these magnetic nanocrystals could greatly extend their application in diverse fields. For example, the magneto-optical hybrid nanoparticles formed by the combination of upconversion nanoparticles and magnetic Fe₃O₄ nanoparticles can be used both as drug carriers and as nanoscale magneto-optical nanoprobes.^4–10

In recent years, the research of magneto-optical bi-functional nanomaterials mostly focuses on the combination of magnetic and optical properties in a single particle, such as rare-earth (RE) doped materials.^11–15 The matrix of these magneto-optical materials is mostly not magnetic or only paramagnetic. Therefore, the study of the magneto-optical effect, especially the interaction between magnetism and photoluminescence, requires the use of strong magnetic field. The introduction of strong (ferromagnetic) magnetic materials can only be realized through the fabrication of core–shell structure, such as the Fe₃O₄@Y₂O₃:Eu.¹⁶ However, most ferromagnetic materials including Fe₃O₄ strongly absorbs visible light and quenches the emission of the RE ions.^17–23 Therefore, the search of a colorless magnetic host could be of great interest for the investigation of the magneto-optical effect.

Sodium iron fluoride (Na₃FeF₆) is a colorless, ferromagnetic fluoride, and it is an ideal platform for the exploration of magneto-optical interactions.^24–26 In this work, Na₃FeF₆ doped with Eu³⁺ ions were prepared by hydrothermal method. The Na₃FeF₆:Eu powders were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM). Under excitation by UV light, visible emission can be observed from the Na₃FeF₆:Eu, and luminescence intensity from Na₃FeF₆:Eu were optimized by adjusting different reaction temperature and doping concentration of Eu ions. This Na₃FeF₆:Eu phosphors might be explored as a magneto-optical dual-functional material.

2. Experimental

2.1 Sample synthesis

The powders of Fe(NO₃)₃, NaF, and NH₄HF₂ were dissolved in deionized water respectively to get a concentration of 0.1mol l⁻¹ Fe(NO₃)₃, 0.5 mol l⁻¹ NaF and 0.5mol l⁻¹ NH₄HF₂ for each solutions. These solutions were mixed with a volume ratio of v[Fe(NO₃)₃] : v[NaF] : v[NH₄HF₂] = 1 : 1 : 3, and then 3 ml HF were added into the mixed solutions (75 ml). Finally, 0.1 mol l⁻¹ Eu(NO₃)₃ was added to the above solution with the volume ratio of v[Eu] : v[Fe] = 3%, 5%, 7%, 10%, 15%, 18%. The mixed solution was transferred into an autoclave, and then heated at 196 °C for 16 hours. After cooling to room temperature, the products were collected from the resultant solutions by centrifugation at 8000 rpm for 20 min. The obtained phosphor powders were washed for three times and then dried at 60 °C.

2.2 Characterizations

The crystal structures of all the samples were studied by X-ray diffraction (XRD) with a RIGAKU D/MAX 2550/PC system operated at a step size of 0.02° at a scanning speed of 5° min⁻¹ using Cu Kα radiation (λ = 1.5406 Å). Scanning electron microscope (SEM) images were taken using a Hitachi S-4800 scanning electron microscope. The UV-Vis spectra diffuse reflectance spectra of the films were measured at room temperature on a UV-3600 Plus spectrophotometer. The luminescence spectra of the samples were investigated using a Japan F-4600 fluorescence spectrophotometer. Magnetic properties were performed on a Quantum Design SQUID MPMS XL-7 (SQUID).

3. Results and discussion

3.1 Structure of the Na₃FeF₆

Fig. 1a shows the crystal structure of Na₃FeF₆ projected along the a-axis. The structure of Na₃FeF₆ is isomorphic with cryolyte-like structures (K₂NaScF₆, Na₃CrF₆ and Na₃AlF₆) with the monoclinic symmetry that belongs to the space group P21/c.^27,28 There are three sodium sites, namely the Na1 site that is located at the distorted octahedral site of NaF₆, the Na2 site at the bi-pyramidal site of (NaF₅), and finally the Na3 site at the distorted tetrahedral site of NaF₄ (Fig. 1(b)). All the Fe atoms are located at the distorted FeF₆ octahedral sites. Furthermore, it can be observed that Na1 octahedral sites and Na3 tetrahedral sites actually share corner sites. Na1 octahedral sites share edges with Na2 bipyramid sites. Fe-containing octahedrons share corners with Na1 and Na3 sites share edges with Na2 sites. The Fe³⁺ ions site can be replaced by Eu³⁺ ions ions when Eu ions doping in the Na₃FeF₆ structure (Fig. 1a).

Fig. 1 (a) Crystal structure of Na₃FeF₆ projected along the a axis. (b) A detailed view of three different sodium sites in the Na₃FeF₆ crystal structure.

3.2 Characterization of the Na₃FeF₆

Fig. 2 shows the XRD patterns of sodium iron fluoride doped with Eu³⁺ at concentrations of 3%, 5%, 7%, 10%, and 15%. As concentration of the Eu ions increase from 3% to 7%, the diffraction peak gradually shifts to the left, diffraction angle θ decreases. According to the Bragg equation (2d sin θ = nλ), d increases with the decreasing of θ. The reason for this may be that as the concentration of Eu goes up, more Eu ions are incorporated into the Na₃FeF₆ lattice. Compared with Fe ions, Eu ions has a larger ionic radius. Therefore, the lattice constant would increase with the increase in the concentration of Eu ions in the lattice. The diffraction peaks of the Na₃FeF₆ with 5% Eu³⁺ doping are the highest, indicating the best crystallinity. The increase of Eu³⁺ concentration above 5% leads to growth of lattice strain that prevents the further enhancement of crystallization. As can be seen from the XRD pattern, the solubility limit for Eu³⁺ ions in the lattice of Na₃FeF₆ is lower than 15%, at which considerable amount secondary phase appears due to the collapse of the lattice.⁶ From the thermal analysis, the fluoride host Na₃FeF₆ is stable at temperatures up to around 255 °C. At higher temperatures, a strong endothermic peak appears accompanied with high weight loss in multiple stages, indicating the decomposition of the material (Fig. 3a). The detailed reactions involved in the decomposition remain to the unraveled. TG–TDA curver of Na₃FeF₆:5% Eu³⁺ is similarity with the pristine Na₃FeF₆ powder, indicating that the structure of the Na₃FeF₆ is stable when the 5% Eu³⁺ doping in Na₃FeF₆.

Fig. 2 X-ray diffraction patterns for samples of the Na₃FeF₆:Eu³⁺ with doping different concentrations of Eu³⁺.

Fig. 3 TG and DTA curves: (a) Na₃FeF₆ and (b) Na₃FeF₆:5% Eu³⁺.

Fig. 4 and S2a† presents the typical SEM images of powders of Na₃FeF₆ doped with 5% Eu³⁺ and pristine Na₃FeF₆ powder. Octahedral particles are clearly observed, which is in accordance with the crystal structure of Na₃FeF₆ (Fig. 1), The average radius of these particles is around 4 μm and the surfaces of the particles are not smooth. The SEM image and the corresponding energy-dispersive spectrum EDS of the Na₃FeF₆:5% Eu (Fig. S1†) and pristine Na₃FeF₆ powder (Fig. S3†) shows elemental distribution of Na, Fe and F is revealed by elemental energy spectrum analysis and elemental Eu distribution in the Na₃FeF₆:5% Eu structure. Fig. S2b† shows the images of powders of Na₃FeF₆ doped with 15% Eu³⁺ including both octahedral particles and secondary phase with nanorod. The results in accordance with the XRD pattern from Na₃FeF₆:15% Eu. Fig. 4c–f show the SEM image and the corresponding EDS mapping images of Fe, Na, F and Eu elements. The result shows that all of the elements were detected and uniformly distributed.

Fig. 4 SEM images image of the Na₃FeF₆:5% Eu³⁺ powders and the corresponding EDS mapping images of F, Fe, Na and Eu elements.

3.3 Photoluminescence properties

Fig. 5 shows the absorption spectra of Na₃FeF₆ doped with different concentrations of Eu³⁺ (3%, 5%, 7%, 10%, 15% and 18%) in the spectral range of 200–750 nm. It can be clearly seen that the Eu-doped Na₃FeF₆ powders exhibit obvious ultraviolet absorption at wavelength short than 300 nm. A small peak located at 395 nm can be attributed to the f–f transition of Eu³⁺, while other transitions are not observed due to their weak transition probability. Obviously, the intensity of absorbance at 395 nm increases from 3% Eu to 5% Eu doped samples. At doping concentrations higher than 10%, the absorbance does not increase further, implying the doping limit of RE ions in the host of Na₃FeF₆.

Fig. 5 Absorption spectra of the Na₃FeF₆ powder doped with different concentrations of Eu³⁺.

As shown in Fig. 6a, the excitation spectra of 5% Eu³⁺ doped Na₃FeF₆ are measured at the wavelength of 615 nm. It can be observed from the excitation spectrum that there is a strong excitation band between 380–400 nm with a peak at around 395 nm, which can be well ascribed to the different f–f transitions of Eu³⁺. Fig. 6b presents the emission spectra of Na₃FeF₆ doped with different concentrations of Eu³⁺ ions. The strongest emission peak is at 615 nm, which is attributed to the ⁵D₀–⁷F₂ transitions of Eu³⁺.¹² Increase of the Eu³⁺ doping level leads to the growth of emission intensity and the strongest intensity is observed for 5% Eu³⁺ doping. At doping levels higher than 5%, the emission intensity no longer increases due to concentration quenching. It is generally explained by the competition of two parallel processes: the emission process and the quenching process by self-absorption as well as cross-relaxation between Eu³⁺ ions.⁷ At low Eu³⁺ doping concentrations, ion–ion interaction is negligibly small.⁸ Above a certain Eu³⁺ concentration, the interactions between nearby Eu³⁺ ions becomes strong, which provides new energy dissipation pathways and reduces the rate of emission.⁹

Fig. 6 (a) Excitation spectrum of Na₃FeF₆:5% Eu³⁺. (b) Emission spectra of Na₃FeF₆ doped with different concentration of Eu³⁺. (c) Emission spectra of the Na₃FeF₆:5% Eu³⁺ synthesized at different temperature. (d) Decay curves of the red (615 nm) emission under pulsed 395 nm excitation for the powder of Na₃FeF₆:5% Eu³⁺.

Fig. 6c shows the emission spectra of the Na₃FeF₆:5% Eu³⁺ powders (the 5% Eu³⁺ doping Na₃FeF₆ was optimized from the Fig. 6b) synthesized at different temperatures. Under the excitation at 395 nm, the emission spectra show two peaks at 596 nm and 615 nm, respectively. The stronger peak at 615 nm is due to the ⁵D₀–⁷F₂ transition of the Eu ions and the other peak at 596 nm is owing to ⁵D₀–⁷F₁ transition. The emission intensity is the highest for the sample obtained at 196 °C, and a higher synthesis temperature is not favorable for improving optical property. The decay curves of red (615 nm) emission was measured under pulsed 395 nm excitation (Fig. 6d). By fitting the decay curve with a bi-exponential decay function, we obtain a lifetime of 19.9 ms, which fall into the typical value for the red emission of Eu³⁺.

The magnetization properties of Na₃FeF₆:5% Eu³⁺ particle was detected during warming up from 5 K to 300 K under field of 1000 Oe was applied. It was observed that the magnetization decrease rapidly from about 7.85 emu g⁻¹ at 5 K to 0.4 emu g⁻¹ at 60 K, then slowly decreasing with temperature increase from 60 K to 300 K (Fig. 7). The magnetization versus magnetic field (M–H) curves of Na₃FeF₆:5% Eu³⁺ particle by SQUID under the maximum magnetic field of 50 000 Oe at 300 K. It can be seen that the magnetization increase linearly from −4.15 emu g⁻¹ to 4.14 emu g⁻¹ under magnetic field from −50 000 Oe to 5000 Oe. The results considered that Na₃FeF₆:5% Eu³⁺ is paramagnetic material. Fig. S5† shows the magnetization curves of Na₃FeF₆ doped with different concentration of Eu³⁺ ions at 300 K. Increase of the Eu³⁺ doping level leads to the decrease of paramagnetic intensity, it maybe due to the higher paramagnetic intensity from Fe³⁺ ions compare with Eu³⁺ ions and the positions of Fe³⁺ ions in Na₃FeF₆ structure were gradually replaced by the Eu³⁺ ions.

Fig. 7 The magnetization curve of Na₃FeF₆:5% Eu³⁺ particle at different temperature (a) and 300 K (b).

4. Conclusions

The phosphor powder of Na₃FeF₆:Eu is prepared by hydrothermal method. The structures of the Na₃FeF₆:Eu powders were characterized by X-ray diffraction (XRD) and SEM. Octahedral particles with homogeneous surfaces are obtained. Optical measurement indicates that Na₃FeF₆ doped with 5% Eu³⁺ and synthesized at the temperature of 196 °C shows the strongest emission intensity. The magnetization of Na₃FeF₆:5% Eu³⁺ decrease rapidly from about 7.85 emu g⁻¹ at 5 K to 0.4 emu g⁻¹ at 60 K, then slowly decreasing with temperature increase from 60 K to 300 K. The results pave the way for the study of the magneto-optical effect in this material.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (Grant No. 2018YFB1107200), the National Natural Science Foundation of China (Grant No. 61404071, 61575091, 61675094, 51132004, 51072054, 51102209, 51472091, 51772270 and 51471082), Key projects of Henan Provincial Education Department (No. 16A140029 and No. 14B40010), the National Basic Research Program of China (2011CB808100), the Natural Science Foundation of Henan Province of China (162300410201), the Henan Provincial Department of Science and Technology Research Project (162102210303), the Fundamental and Cutting-edge Technology Research Program of Henan Province (152300410218), the Key Scientific Research Projects of Henan Province (Grants 16A140014 and 16A140016), the Program for Young Teachers of Higher School in Henan Province (2013GGJS-153), the Program for Innovation Talents (in Science and Technology) in University of Henan Province (16HASTIT044), Youth Backbone Teacher Training Program in Henan province (2017GGJS137) Open funds of State Key Laboratory of Precision Spectroscopy, East China Normal University and State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra07137c

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