Luminescent properties of Eu-doped magnetic Na3FeF6.

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. This journal is © The Royal Society of Chemistry.

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.

Experimental

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.

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).

Results and discussion

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.

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.

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).

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.