X-ray Diffraction and Fluorescence Spectroscopy Analysis of Sm3+ in Lithium Calcium Silicate

Trivalent samarium–doped lithium calcium silicate phosphors with the nominal formulas of Li₂Ca_1-xSm_xSiO₄ were prepared with a conventional solid-state reaction technique and the dopant concentration was varied in the range of 0.01 ≤ x ≤ 0.10. The phase and structure of the as-prepared samples were characterized using X-ray diffraction. The excitation and emission spectra were measured to characterize the luminescent properties of the Li₂CaSiO₄:Sm³⁺ phosphors. The excitation and emission spectra at room temperature show the typical f–f transitions of Sm³⁺. The dominant excitation line is around 408 nm because of the ⁶ H_5/2→⁴K_11/2 transition. The emission spectrum of all phosphors exhibited four sharp emission peaks corresponding to the ⁴G_5/2→⁶H_5/2 (565 nm), ⁴ G_5/2→⁶H_7/2 (601 nm), ⁴G_5/2→⁶H_9/2 (654 nm), and ⁴G_5/2→⁶H_11/2 (708 nm) transitions of Sm³⁺. The emission intensity depended on the Sm³⁺ concentration and it was observed that the optimum concentration of Sm³⁺ in phosphor was 4 mol% for the highest emission intensity at 601 nm.

Luminescence materials are important for modern technology because of their ability to convert electromagnetic radiation (that is, ultraviolet [UV] and infrared [IR] light) into visible light (1). Recently, the development of rare earth, ion-activated, novel luminescent materials have attracted more attention because of their potential applications in optical display systems (2–9). Because the 4f electrons in rare earth ions are shielded by the outer 5s and 5p electrons, the intra-4f emission spectra of rare earth ions are characterized by narrow lines with high color purity (10).

The Sm³⁺ ion is widely used as an activator of reddish orange emission because of its ⁴ G_5/2→⁶H_J (J = 5/2, 7/2, 9/2, and 11/2) transitions (11–16), which is the most suitable source for lighting and display, from a practical viewpoint. Moreover, Sm³⁺ ions act as a spectroscopic probe of local structure around rare-earth ions in condensed matter. This is because of the electric-dipole (ED) character of the ⁴ G_5/2→⁶H_9/2 hypersensitive transition, whose intensity increases as the environmental symmetry of the luminescent site decreases (17). Those investigations of Sm³⁺ -doped materials mainly focused on the host matrixes, which included borates (18), silicates (19), and vanadates (20).

In this article, Li₂CaSiO₄:Sm³⁺ reddish orange phosphors were synthesized by a solid-state reaction method. The photoluminescence of the phosphors was systematically investigated using a spectrofluorometer at room temperature. Also, the dependence of the emission intensity on the Sm³⁺ concentration for the Li₂Ca_1-xSm_xSiO₄ (0.01 ≤ x ≤ 0.10) was studied in detail.

Experimental

Li₂Ca_1-xSm_xSiO₄ (x = 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, and 0.10) phosphors were prepared according to a standard solid-state technique (Table I). The starting materials were Li₂CO₃, CaCO₃, SiO₂, and Sm₂O₃ (all materials were analytical grade). The raw materials were weighed according to the stoichiometric proportions. The chemicals were ground in an agate mortar at room temperature and later were collected into an alumina crucible for heating. First, the powder mixtures were calcined at 450 °C for 1 h. After grinding and the homogenization of preannealed mixtures, the mixtures were heat treated at 600 °C for 1 h and 850 °C for 2 h.

Table I: Sm3+ concentration (x) for Li2Ca1-xSmxSiO4

The structural characterization of the fine powders was performed by X-ray diffraction (XRD) analysis using a Bruker AXS D8 Advance diffractometer. The system was run at 20–60 kV and 6–80 mA, 2Θ = 10–90° and a step of 0.002° using the CuKα line of 1.5406 A° (quartz [PDF 46-1045] was used as reference material for XRD). Scanning electron microscopy (SEM) images were taken with a FEI Quanta 250 FEG model scanning electron microscope using an accelerating voltage of 20 kV.

All the luminescence characteristics of these phosphors were investigated at room temperature using a Thermo Scientific Lumina fluorescence spectrometer equipped with a 150-W xenon lamp.

Results and Discussion

The XRD pattern of pure and Sm³⁺-doped Li₂CaSiO₄ is presented in Figure 1, which is in agreement with the XRD data of Li₂CaSiO₄ presented by Liu and colleagues (21). The result indicates that the diffraction peak positions of the prepared sample are well matched with the standard powder diffraction file (Contrast JCPDS-27-0290). In the Li₂CaSiO₄ crystal structure, the Ca atom occupies the 8-coordinated distorted dodecahedral site. The divalent calcium site is substituted by the trivalent samarium ions because the Sm³⁺ ions (0.96 Å) are expected to occupy the Ca²⁺ (0.99 Å) sites in the Li₂CaSiO₄ host and no Sm³⁺ ion is expected to occupy the Li⁺ sites (0.68 Å) (22,23). According to the XRD phase analysis, the Sm³⁺ substitutes Ca²⁺ without disturbing the Li₂CaSiO₄ crystal lattice.

Figure 1: XRD patterns (CuKα radiation) obtained for Li2Ca1-xSmxSiO4 (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.10).

When trivalent rare earth ions are incorporated into the host material, it is usually considered that rare earth ions occupy mainly divalent M²⁺ (Ca, Sr, and Ba) sites (24–32). Therefore, the substitution on Ca²⁺ sites in Li₂CaSiO₄ needs charge compensation mechanisms, which can be described as

Figure 2 shows the images obtained from the SEM of the phosphors calcined at 850 °C for 2 h using solid-state reactions. The microstructures of the phosphor consisted of regular fine grains.

Figure 2: The SEM images of (a) Li2CaSiO4, (b) Li2CaSiO4:2%Sm3+, (c) Li2CaSiO4:4%Sm3+, (d) Li2CaSiO4:6%Sm3+, (e) Li2CaSiO4:8%Sm3+, and (f) Li2CaSiO4:10%Sm3+ phosphors.

Figure 3 shows the excitation spectra of undoped Li₂CaSiO₄ and 4 mol% Sm³⁺ :Li₂CaSiO₄ in the range of 190–500 nm by monitoring the 604-nm emission. In the longer wavelength region, the f–f transitions within the Sm³⁺ 4f⁵ configuration can be detected, which is assigned as transition from the ⁶H_5/2 ground state to the corresponding excited state of Sm³⁺ listed in Table II. Among the sharp lines, the strongest peak centered at 408 nm is assigned to ⁶H_5/2→⁴K_11/2 (33).

Figure 3: Excitation spectra of undoped and 4 mol% Sm3+: Li2CaSiO4 phosphor (λem = 604 nm).

Figure 4 shows the emission spectra of Li₂Ca_1-xSm_xSiO₄ (x = 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, and 0.10) under the excitation of 407 nm at room temperature. The emission spectrum of all phosphors exhibited four sharp emission peaks corresponding to ⁴G_5/2→⁶H_5/2 (565 nm), ⁴G_5/2→⁶H_7/2 (601 nm), ⁴G_5/2→⁶H_9/2 (654 nm), and ⁴G_5/2→⁶H_11/2 (708 nm) transitions of Sm³⁺. When Sm³⁺ occupies a site coinciding with a center of symmetry, ⁴G_5/2→⁶H_7/2 is forbidden as an electric dipole, but allowed as a magnetic dipole (34). The ⁴G_5/2→⁶H_9/2 transition is allowed as an electric dipole while the Sm³⁺ ion is situated at a site that lacks the inversion symmetry (35).

Table II: The excitation peak in the excitation spectra of Li2CaSiO4:Sm3+ phosphors at room temperature

The dependence of the emission intensity on the Sm³⁺ concentration for the Li₂Ca_1-xSm_xSiO₄ (x = 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, and 0.10) is shown in Figure 5. It is well-known that in phosphor materials, photon emission intensity of rare earth activator ions has an optimum concentration (36). It is difficult for the phosphor materials to exhibit efficient emission when the rare earth activator ion concentration is too low. On the other hand, the cross-relaxation interaction between the nearest rare earth activator ions will quench the emission if the rare earth activator ions exceed their optimum concentration. The experimental result revealed that the luminescence intensity of Sm³⁺ -doped Li₂CaSiO₄ increased with the Sm³⁺ dopant concentration. The luminescence intensity reached its maximum when the concentration of Sm³⁺ was at about 4% (Figure 3) and then decreased with concentration because the increase of the Sm³⁺ dopant concentration increased the cross-relaxation between two neighboring Sm³⁺ ions and therefore quenched the emission. The process of cross-relaxation results from the nonradiative electric dipole–electric quadrupole transition from the ⁴G_5/2 energy level.

Figure 4: Photoluminescence spectra of the Li2Ca1-xSmxSiO4 (0.01 ≤ x ≤ 0.10) phosphors (λex = 407 nm).

Conclusions

In summary, we have successfully synthesized Li₂CaSiO₄:Sm³⁺ reddish orange phosphors by using a solid-state reaction method at 850 °C and investigated their structural properties using X-ray powder diffraction analysis. Undoped Li₂CaSiO₄ was not shown to have the photoluminescence property. The relationship between the luminescent intensity and the concentration of activated Sm³⁺ ions shows that there is concentration quenching of Sm³⁺ in the host Li₂CaSiO₄, and the critical concentration is 4 mol%.

Figure 5: Emission intensity as a function of Sm3+ concentration (x) for Li2Ca1-xSmxSiO4 (0.01 ≤ x ≤ 0.10).

Acknowledgment

The authors would like to thank Professor Orhan Türkoğlu (Erciyes University) for the use of the XRD instrument.

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Ertuğrul Erdoğmuş and Í. Pekgözlü are with the Faculty of Engineering in the Department of Environmental Engineering Bartin University in Bartin, Turkey. Esra Korkmaz is with the Faculty of Science and Arts in the Department of Chemistry at Bozok University in Yozgat, Turkey. Direct correspondence to: ertugrulerdogmus67@gmail.com