Tailoring Photoluminescence and X-ray Storage Properties in Mechanochemically Prepared Nanocrystalline CaF₂:Sm3+

1. Introduction

CaF₂ belongs to the alkaline earth metal fluoride (MF₂) compounds which crystallize in the cubic structure with the Fm$\overline{3}$m space group.[1,2] The Ca²⁺ ions lie at the nodes in the face-centered lattice, while the F^- lie at the center of the octants.[3,4] There has been growing interest in studying the optical properties of lanthanide (Ln) doped CaF₂ due to the high transmittance from the far UV to the mid IR range, high chemical resistance, and low refractive index of this host.[5]

Nanocrystalline CaF₂:Ln has been prepared by a wide variety of methods such as co-precipitation,[6,7] sol-gel process,[8] hydrothermal synthesis,[9,10] and thermal decomposition of precursors.[11] In recent years, high-energy ball milling has increasingly been applied to synthesize stoichiometric and non-stoichiometric solid solutions with minimal or solvent free routes.[12,13,14,15] In this process, the mechanical energy caused by the high speed collision of balls in the ball milling jar forces the reagents to react and turn into fine powders that can be on the nanoscale.[16] This method has advantages in increasing the material reactivity, uniform spatial distribution of elements, and reducing the possibility of multi-phase formation.[17,18] Heise et al. [19] successfully synthesized Eu³⁺ doped MF₂ (M = Ca, Sr, and Ba) powders by ball milling M(OAc)₂, Eu(OAc)₃, and NH₄F, and crystallite sizes in the range of 12 to 18 nm were obtained. Molaiyan and Witter also reported the preparation of the CaF₂:Sm³⁺ electrolyte by ball milling anhydrous CaF₂ and SmF₃ in stoichiometric compositions of Sm_1-yCa_yF_3-y (0 $\le$ y $\le$ 0.15) using a Tanchen planetary ball mill.[14] Although ball milling is a facile method in preparing nanocrystalline powders, this method has still not been widely applied to the preparation of MF₂:Ln materials for optical applications.

We have previously reported that nanocrystalline CaF₂:Sm³⁺ prepared by a co-precipitation method can serve as a relatively efficient photoluminescent X-ray storage phosphor, with the storage mechanism based on the reduction of Sm³⁺ to Sm²⁺ upon exposure to X-irradiation.[20] In the present study, we report the mechanochemical synthesis of nanocrystalline CaF₂:Sm³⁺ by ball milling Ca(OAc)₂, Sm(OAc)₃, and NH₄F at room temperature. The synthesized powders were characterized by XRD, electron microscopy, and luminescence spectroscopy. The effects of the ball milling time, Sm concentration, and post-annealing on the generation of Sm²⁺ by X-ray was investigated in detail using photoluminescence measurements.

2. Experimental Methods

Nanocrystalline CaF₂:ySm³⁺ (y = mol%) was prepared by ball milling Ca(OAc)₂·H₂O (May & Baker Ltd), Sm(OAc)₃.xH₂O (Sigma Aldrich), and NH₄F (Sigma Aldrich) according to the following solid-state reaction:

$$\begin{gathered} {\left(1\text{–}y\right)\mathrm{Ca}\left(\mathrm{OAc}\right)}_2{\text{·}\mathrm{H}}_2\mathrm{O}+{\left(y\right)\mathrm{Sm}\left(\mathrm{OAc}\right)}_3{\text{·}x\mathrm{H}}_2\mathrm{O}+\left(2+y\right){\mathrm{NH}}_4\mathrm{F}\text{→} \\ {\mathrm{Ca}}_{1-y}{\mathrm{Sm}}_{\mathrm{y}}{\mathrm{F}}_{2+y}+\left(2+y\right){\mathrm{NH}}_3+(2+y)\mathrm{HOAc}+{(1\text{–}y+\mathit{xy})\mathrm{H}}_2\mathrm{O} \end{gathered}$$

(1)

Reagents (with y = 0.1%) were premixed and ground using a mortar and pestle before being transferred into a 12 ml zirconia ball mill jar with six 5 mm diameter zirconia balls. The mixtures were then ball milled for 1, 3, 5 or 8 h to investigate the dependence of physical properties on ball milling time. The ball milling was performed using a Fritsch Planetary Mill (Pulverisette 7) at 10 Hz. The obtained mixture was dried overnight in an oven (Labec, Model H323) at 60 °C. The final product was then ground using a mortar and pestle to yield a homogenous nanocrystalline powder. Nanocrystalline CaF₂:ySm³⁺ powders with different Sm concentrations (y = 0, 0.05, 0.1, 0.3, 0.5, 1, 3, and 5 %) were also prepared with a ball milling time of 8 h. Post-annealing by using a muffle furnace (Labec, CEMLS-SD) was conducted at temperatures of 200, 300, and 400 °C in air.

The phase purity of samples was characterized by powder X-ray diffraction (XRD) on a Rigaku MiniFlex-600 benchtop diffractometer with Cu-Kα radiation (λ = 0.154 nm, 40 kV and 15 mA) with a scanning step and speed of 0.01° and 0.5°/min, respectively. Data was collected in the 2θ range of 10 to 100°. TEM imaging was undertaken by a FEI Tecnai G2 Spirit transmission electron microscope.

Photoluminescence (PL) spectra of Sm³⁺ were measured by using a Horiba Jobin-Yvon Spex FluoroMax-3 fluorometer at room temperature with 405 nm excitation. Sm²⁺ luminescence spectra were recorded on a Spex 500 M monochromator (150 grooves/mm grating), equipped with an Andor iDus camera (DV401A-BV Si CCD). A closed-cycle cryostat (CTI-Cryogenics Cryodyne model 22) was used to cool the sample to 27 K. In this case, the samples were excited by a focused 635 nm laser diode. The powders were manually pressed into a counterbore of 5 mm diameter and 0.5 mm depth on an aluminium holder.

The X-ray based reduction of Sm³⁺ to Sm²⁺ was undertaken on the Rigaku Miniflex-600 benchtop powder XRD diffractometer at a 2θ angle of 30° (dose rate ~15 mGy s^-1). The X-ray dose was cross-calibrated against a Sirona HELIODENT Plus dental X-ray source.

3. Results and Discussions

The XRD patterns of nanocrystalline CaF₂:0.1%Sm³⁺ prepared by ball milling with periods of 1, 3, 5 and 8 h are shown in Figure1a. In Figure 1b the XRD patterns of CaF₂:ySm³⁺ ball milled for 8 h with different concentrations of Sm³⁺ (0 ≤ y ≤ 5 %) are illustrated. Finally, in Figure 1c the XRD patterns of CaF₂:0.1%Sm³⁺ (8 h ball milling period) annealed at temperatures of 200, 300, and 400 °C are shown. The patterns were compared with the standard CaF₂ data (PDF-1000043) taken from the Crystallography Open Database.[21] Results from Rietveld refinements obtained by the MAUD[22] software package are summarized in Table 1. The goodness of fit G = R_wp/R_exp is < 1.5 for all refinements, i.e. implying good fits.[23] As follows from the figures, all the prominent peaks could be indexed to the cubic CaF₂ structure with the Fm$\overline{3}$m space group.[1,2]

As seen in Figure 1a, impurity peaks are still visible after 1 h of milling. A more complete phase formation of nanocrystalline CaF₂ can be observed after 3 h. Importantly, prolonged ball milling broadened the diffractions peaks, and this was caused by the decrease of the average crystallite size of CaF₂:0.1%Sm³⁺ from 12 ± 1 to 8 ± 1 nm for ball milling times of 1 to 8 h (Table 1a). A 0.14% expansion of the lattice parameter was also observed with this decrease in the crystallite size. It is noted here that the use of hydrated salts in ball milling may accelerate the formation of CaF₂:ySm³⁺ due to the higher mobility of ions and this was also observed in the preparation of nanocrystalline BaFCl.[24]

Interestingly, a reduction of the average crystallite size of CaF₂:ySm³⁺ from 12 ± 1 to 6 ± 1 nm (Table 1b) was observed when the Sm³⁺ concentration was increased from 0 to 5 %. The lattice parameter also increased by 0.17 % in this case. The latter is most likely caused by the mechanism of charge compensation as Sm³⁺ substitutes Ca²⁺. The excess positive charge must be compensated by defects such as O^2- impurity ions, substituting F^- in the lattice, and/or interstitial F^-. Also, the electronic repulsion of the ions may increase the lattice parameter.[25,26] Importantly, Sm³⁺ can easily substitute Ca²⁺ in the O_h symmetry with eightfold (bcc) coordination, due to their similar ionic radii (Sm³⁺ = 1.08 Å, compared to Ca²⁺ = 1.12 Å) [27] and, importantly, phase purity is retained for Sm³⁺ concentrations up to 5%.

As follows from Figure 1c, the annealing of CaF₂:0.1% Sm³⁺ at 200, 300, and 400 °C significantly narrowed the diffraction peaks. From the Rietveld refinements average crystallite sizes of 12, 22, and 46 ± 1 nm, respectively, were obtained for these annealing temperatures (Table 1c). The crystallite size appeared to grow by ~T^3.4 upon annealing up to 400 °C.

Typical TEM micrographs of CaF₂:0.1%Sm³⁺ prepared by ball milling are displayed in Figure 2. The observed crystallite/particle size distribution was in good agreement with the average crystallite sizes obtained from the Rietveld refinements. In particular, annealing the sample to 400 °C significantly increased the particle size. A micrograph of CaF₂:0.5%Sm³⁺ prepared by co-precipitation[20] with an average crystallite size of 46 ± 1 nm is shown in Figure 2e for comparison.

Photoluminescence spectra of nanocrystalline CaF₂:0.1%Sm³⁺ prepared by ball milling for 8 h before and after 360 Gy X-irradiation (Cu-Kα) are shown in Figure 3. Sm³⁺ emission lines centered at 566, 604, 645 and 704 nm (Figure 3a) correspond to ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2, and 11/2) f-f transitions, respectively.[28,29,30] Sm^{3+ 4}G_5/2 → ⁶H_5/2 and ⁶H_7/2 transitions contain magnetic and electric dipole contributions that obey the selection rules $∆J$ = 0, $\pm$ 1, while the other two transitions ⁴G_5/2 → ⁶H_9/2 and ⁶H_11/2 are purely electric dipole transitions ($∆J$ ≤ 6).[31] The symmetry of the local environment of the trivalent 4f ions can be identified by the relative intensity ratio of electric dipole to magnetic dipole transitions (I_R = ⁴G_5/2 → ⁶H_9/2 / ⁴G_5/2 → ⁶H_5/2).[32] The present work indicated that most of the Sm³⁺ ions occupied the inversion symmetry sites of the CaF₂ host lattice since the IR is < 1.[32,33,34] Note, however, that charge compensation will in principle lower the site symmetry.

Upon 360 Gy X-irradiation, the luminescence of Sm³⁺ decreased as is seen in Figure 3a accompanied by the rise of the electric dipole allowed Sm²⁺ 4f⁵5d (T_1u) → 4f^{6 7}F₁ (T_1g) transition at 708.2 nm with vibronic side bands (transverse optical phonon mode of CaF₂ due to the O_h⁵ group symmetry) (Figure 3b).[35,36,37] We stress here that no Sm²⁺ luminescence was observed before X-irradiation indicating that the Sm ions entered the CaF₂ host lattice in their +3 oxidation state. In contrast, Liu et al. reported the presence of Sm²⁺ emission lines in the absence of X-irradiation in nanocrystalline BaFCl:Sm³⁺ prepared by ball milling.[24].

In Figure 4 the photoluminescence spectra of nanocrystalline CaF₂:0.1%Sm³⁺ as a function of ball milling time are depicted. As follows from Figure 4a, the luminescence of Sm³⁺ increased with longer ball milling time. In contrast, the generation of Sm²⁺ upon X-irradiation gradually decreased with the ball milling time (Figure 4b). This may be due to a better embedding and charge compensation for longer ball milling times e.g. closer proximity of the charge compensators to the Sm³⁺ ions. It is also possible that with longer ball milling times, more defects are generated facilitating effective non-radiative deactivation paths for the Sm²⁺.

Photoluminescence spectra of nanocrystalline CaF₂:ySm³⁺ doped with different concentrations of Sm³⁺ (0.05 % ≤ y ≤ 5 %), and ball milled for 8 h are shown in Figure 5. As is seen in Figure 5a the intensity of Sm³⁺ luminescence lines of the as-prepared sample increased with the Sm³⁺ concentration for up to 1%, and then decreased with higher concentrations. Interestingly, the same trend was observed for the Sm²⁺ luminescence (upon 135 Gy X-irradiation) (Figure 5b). This concentration dependence is most liekly due to quenching for concenrations higher than 1% induced by rapid excitation energy transfer between the Sm ions that leads to non-radiative deactivation at trap sites.[38]

In Figure 6, the effect of post-annealing for 1 h at 200, 300, and 400 °C on the luminescence of nanocrystalline CaF₂:0.1%Sm³⁺ (ball-milled for 8 h) is summarized. The figure shows that both the Sm³⁺ luminescence of the as-prepared sample (Figure 6a) and the Sm²⁺ luminescence of the X-irradiated samples (Figure 6b) became significantly stronger with increasing annealing temperature. The normalized photoluminescence intensity of the Sm³⁺ and Sm²⁺ emissions followed a T^2.4 and T^2.6 power law, respectively. An increase in photoluminescence intensity of the Sm^3+/2+ with increased temperature was previously observed by Liu et al. for BaFCl:Sm³⁺.[39]

In Figure 7 a comparison is shown of the Sm²⁺ luminescence of X-irradiated (100 Gy) nanocrystalline CaF₂:0.5%Sm³⁺ prepared by co-precipitation (CPT) and as-prepared as well as annealed at 400 °C of CaF₂:0.5%Sm³⁺ prepared by 8 h of ball-milling (BM). As seen from the inset of the figure, the Sm²⁺ generation of BM CaF₂:0.5%Sm³⁺ showed a significantly increased by a factor 23 after annealed at 400 °C with crystallite size increased from 8 nm to 44 nm. In addition, both CPT CaF₂:0.5%Sm³⁺ and annealed BM CaF₂:0.5%Sm³⁺ had similar average crystallite sizes of 46 nm and 44 nm, respectively. However, in comparison with the CPT-sample, the Sm²⁺ luminescence intensity of the annealed BM-sample was lower by a factor of 3 after 100 Gy X-irradiation. This indicated a faster Sm³⁺ → Sm²⁺ conversion upon X-irradiation in the CPT sample compared to the BM samples. In the BM sample the trivalent Sm³⁺ may be more stabilized by a charge compensator due to the prolonged milling and annealing time, that enables ionic rearrangements of the lattice.[40]

4. Conclusions

We have reported a direct and facile mechanochemical preparation route for nanocrystalline CaF₂:Sm³⁺ by ball milling Ca(OAc), Sm(OAc)_2, and NH₄F at room temperature. The photoluminscence spectra of the as-prepared samples display the Sm^{3+ 4}G_J→ ⁶H_Jluminescence lines whereas X-irradiation generates Sm²⁺ with its characteristic luminescence around 708 nm at low temperatures. A ball milling period of 3 to 4 h was found to result in the best single phase whereas shorter and longer ball milling resulted in some impurity phases. A longer ball milling period such as 8 h reduced the efficacy of Sm²⁺ generation by X-irradiation. This is likely due to the stabilization of the trivalent state by embedding the charge compensator in the vicinity of the Sm ion as well as more effective non-radiative deactivation by the introduction of more defects. Maximum luminescence was observed for the sample with a 1 mol% Sm³⁺ concentration, and at higher concentration quenching was observed. Interestingly, post-annealing substantially increases the X-ray induced Sm³⁺ to Sm²⁺ conversion. It is noted here that attempts to anneal at higher temperatures such as 1100°C (in air) generated extra phases in the XRD pattern with an associated change of the Sm³⁺ luminescence spectrum. In comparison with the co-precipitation (CPT)-sample, the Sm³⁺ ion in the ball milling-sample (BM) is much more stable. The present results demonstrated that the X-ray storage efficiency of nanocrystalline CaF₂ can be controlled in the preparation process by varying parameters such as ball milling time, annealing temperature and rare earth ion concentrations.