Intense photoluminescence in CaTiO₃:Sm³⁺ phosphors, effect of co-doping singly, doubly and triply ionized elements and their applications in LEDs

Abstract

In this work, Sm³⁺-doped and Sm³⁺/Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ co-doped CaTiO₃ phosphors were synthesized by a solid-state reaction method at 1473 K. The phase of phosphors was identified to be orthorhombic with space group Pnma (62) by XRD measurements. The morphological properties of the prepared samples were studied by SEM measurements. The average crystallite and particle sizes were found to increase in the presence of modifiers and they follow the trend Li⁺ > Mg²⁺ > Gd³⁺ > K⁺ > Bi³⁺ > Ba²⁺. EDX measurements were used to verify the presence of Ca, Ti, O, Sm, K, Mg, Ba, Gd and Bi atoms in the prepared phosphor samples. The Sm³⁺ ion shows emission peaks at 564, 599 and 646 nm due to ⁴G_5/2 → ⁶H_5/2, ⁶H_7/2 and ⁶H_9/2 transitions upon 407 nm excitation, among which the peak situated at 599 nm has maximum emission intensity. Concentration quenching was observed above 2 mol% of Sm³⁺ ions in this host. However, the emission intensity of Sm³⁺ peaks can be enhanced using different modifier (Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺) ions. It was found that the size (ionic radii) and charge compensation of the ion together play a dominant role. The enhancement is more after co-doping with smaller radius ions (Li⁺, Mg²⁺ and Gd³⁺), among which Li⁺ shows the largest enhancement. This is because ions of smaller size will be able to go closer to the activator ion and the charge imbalance causes a larger field. The CIE color coordinates, correlated color temperature (CCT) and color purity of the phosphors were calculated and show orange-red color emissions with a maximum color purity of ∼93% in the case of CaTiO₃:2Sm³⁺/1.0Li⁺ phosphor. The lifetime value is increased in the presence of these ions. It is again maximum for the Li⁺ co-doped CaTiO₃:2Sm³⁺ phosphor sample. Thus, the prepared phosphor samples are suitable sources for orange-red light.

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1. Introduction

Rare earth-doped perovskite phosphor materials are chemically and thermally stable and give intense photoluminescence at suitable excitation wavelengths.^1–3 These materials are used for various applications such as color tunable devices, display devices, light-emitting devices, plasma display panels, temperature sensors, optical heaters, bio-imaging devices, plant growth and solar cells.^4–10 The rare earth ions possess ladder-like energy levels due to which they show multi-modal behaviours such as upconversion (UC), downshifting (DS) and quantum cutting (QC) depending on different excitation detection techniques.^11–14

Downshifting is a Stokes emission process, in which a high energy photon is converted into a low energy photon via different relaxation processes. Among the rare earth ions, the Sm³⁺ ion emits orange-red emissions due to ⁴G_5/2 → ⁶H_j (j = 5/2, 7/2 and 9/2) transitions under n-UV excitation.^15–17 Ha et al. studied the structure and photoluminescence properties of the Sm³⁺-doped CaTiO₃ phosphor and observed intense orange-red emissions due to the ⁴G_5/2 → ⁶H_j transition under 408 nm excitation.¹⁸ Shivaram et al. synthesized Sm³⁺-doped CaTiO₃ by a low-temperature solution combustion method and reported an intense emission peak at 601 nm due to the ⁴G_5/2 → ⁶H_7/2 transition under 407 nm excitation.¹⁹ Generally, the orange/red-emitting phosphors show poor luminescence efficiency as compared to green, yellow and blue-emitting phosphors and need to be improved using different sensitizer/modifier ions.^20–22 The emission intensity of activator ions may be enhanced in two ways: the first one is by co-doping with surface modifier ions such as Li⁺, Na⁺, Sr²⁺, Ca²⁺, Ba²⁺, Mg²⁺, Bi³⁺, Gd³⁺ and the second one is via energy transfer from the sensitizer to activator ions.^23–28 Cao et al. observed an enhancement in the emission intensity of Sm³⁺-doped CaTiO₃ phosphors via the addition of Na⁺ and H₃BO₃.²⁹ Shanbhag et al. have reported the photoluminescence properties of CaTiO₃:Sm³⁺/Li⁺.³⁰ Pamuluri et al. have tried to enhance the photoluminescence properties of Sm³⁺ by energy transfer from Dy³⁺ to Sm³⁺ in Dy³⁺/Sm³⁺ co-doped Lu₃Ga₅O₁₂ nano-garnets.³¹ Zhu et al. also observed an enhancement in the emission intensity of Sm³⁺ ions via energy transfer from Tb³⁺ to Sm³⁺ in Na₃Bi(PO)₄.³² Dhananjaya et al. have reported the PL properties of Eu³⁺-doped Gd₂O₃ phosphors in the presence of alkali ions (M⁺ = Li⁺, Na⁺ and K⁺).³³ They have observed that the PL intensity of phosphors is increased in the presence of alkali ions, which is due to the modification in the local crystal field around the activator ion. In our previous work, we have observed an enhancement in the emission intensity of Eu³⁺-doped CaTiO₃ phosphors in the presence of alkali ions (Li⁺, Na⁺ and K⁺).³⁴ The enhancement in the emission intensity in the presence of alkali ions is due to the modification in crystal field around the activator ion, which increases the average crystallite and particle size. Singh et al. have studied the photoluminescence properties of Eu³⁺-doped MSiO₃ in the presence of alkaline earth ions (M = Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) and found an intense emission of Eu³⁺ bands due to the modification in the crystal field around the activator ion.³⁵ Wang et al. reported the enhancement in the luminescence properties of SrIn₂O₄:Eu³⁺ phosphors in the presence of Gd³⁺ ions.³⁶ It is clear from these examples that ions of smaller as well as larger size with single, double and triple ionization state separately have been used to enhance the emission intensity of different rare earth ions in different hosts. All these ions modify the crystal field around the activator ions due to which enhancement in emission intensity is observed. However, from all these studies, it is not clear whether the ionization state or size of the ion or both plays a dominant role in enhancing the photoluminescence emission intensity of the activator ions. Therefore, it will be interesting to study these in detail. The idea is to see whether the ionization state or size of ions and charge compensation behaviors are more effective in enhancing the emission intensity of activators.

In this work, we studied the structural and optical behaviors of Sm³⁺-doped CaTiO₃ phosphors in more detail in the presence of different types of surface modifier ions like singly, doubly and triply ionized elements with smaller and larger ionic radii in a single platform and tried to realize which one is more effective. The Sm³⁺-doped CaTiO₃ and Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ co-doped CaTiO₃:2Sm³⁺ phosphor materials were prepared by a solid-state reaction method at 1473 K. The XRD measurements showed that phosphor materials have an orthorhombic phase with the Pnma (62) space group. The SEM measurements were carried out to know the effect of different modifier ions on the particle size of the prepared samples. The photoluminescence excitation and emission spectra of the Sm³⁺-doped CaTiO₃ phosphors were studied by taking λ_em = 599 nm and λ_ex = 407 nm, respectively. The concentration of Sm³⁺ was optimized for optimum emission. To improve the emission intensity further, Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ ions (smaller and larger ionic radii of singly, doubly and triply ionized elements) were co-doped in the CaTiO₃:2Sm³⁺ phosphor and their concentrations were varied to obtain the optimum emission. It was found that the enhancement is more in the case of smaller radius ions irrespective of their ionization state and it is optimum for Li⁺ ions. We also calculated the CIE, CCT and color purity of the Sm³⁺ ions in the presence of Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ ions. The value of color purity is larger in the presence of smaller radius (Li⁺/Mg²⁺/Gd³⁺) ions and they follow the trend Li⁺ > Mg²⁺ > Gd³⁺. The lifetime measurements were carried out for the ⁴G_5/2 level of Sm³⁺ ions in the absence and presence of these modifier ions. It was found that the lifetime of the ⁴G_5/2 level of Sm³⁺ ions increased in the presence of modifier ions and their order is τ_Li > τ_k ∼ τ_Gd > τ_Mg > τ_Bi > τ_Ba.

2. Preparation of phosphor samples and characterization

2.1. Synthesis of materials

The phosphor samples were synthesized by a solid-state reaction technique at 1473 K. The starting materials were calcium carbonate (CaCO₃, 99.9%), titanium dioxide (TiO₂, 99.9%), samarium oxide (Sm₂O₃, 99.9%), lithium carbonate (Li₂CO₃, 99%), potassium carbonate (K₂CO₃, 99%) magnesium oxide (MgO, 97%), barium carbonate (BaCO₃, 99.9%), gadolinium oxide (Gd₂O₃, 99.9%) and bismuth oxide (Bi₂O₃, 99.9%). Initially, a series of cSm³⁺ (c = 1.0, 1.5, 2.0, 2.5, 3.0 and 5.0 mol%) doped CaTiO₃ phosphor samples were synthesized to find the Sm³⁺ concentration for maximum photoluminescence. The Sm³⁺ ion was found to give optimum emission at 2 mol%. Further, in order to enhance the PL intensity of Sm³⁺, xLi⁺ (where x = 0.5, 1.0 and 3.0 mol%), xK⁺ (where x = 1.0, 3.0, 5.0 and 7.0 mol%), yMg²⁺ (where y = 1.0, 3.0, 5.0 and 10 mol%), yBa²⁺ (where y = 3.0, 5.0 and 10 mol%), zGd³⁺ (where z = 0.1, 0.5 and 0.7 mol%) and zBi³⁺ (where z = 3.0, 5.0 and 10 mol%) were co-doped in CaTiO₃:2 mol% Sm³⁺ phosphors separately to get the optimum emission of Sm³⁺ ions in the presence of these ions.

The weighed materials were carefully mixed for one hour in an agate mortar with acetone as a mixing medium. The final mixtures were heated at 1473 K for 4 hours in a programmable electric furnace. The phosphor samples thus obtained were further crushed in an agate mortar to obtain a fine powder for further characterizations.

2.2. Instrumentation

The phase identification of phosphor materials was carried out by XRD measurements using CuK_α radiation (λ = 0.15406 nm) with MiniFlex600 (Rigaku, Japan). The morphology of the materials was studied by SEM using a Zeiss, Evo 18 Research system. The Fourier transform infrared measurements were done to know the phonon frequency of phosphor samples using a PerkinElmer I-Frontier system in the 400–3000 cm⁻¹ region. The photoluminescence excitation and emission spectra of the samples were recorded using a Fluorolog-3 spectrophotometer with a 450 W xenon lamp source (Horiba Jobin Yvon). We also measured the lifetime of the ⁴G_5/2 level of Sm³⁺ ions in the absence and presence of Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ ions using a 25 W pulsed xenon lamp attached with the same unit (only corresponding to optimized samples).

3. Results and discussion

3.1. Structural characterization

3.1.1. XRD measurements. The powder X-ray diffraction patterns of CaTiO₃, CaTiO₃:2Sm³⁺, CaTiO₃:2Sm³⁺/1.0Li⁺, CaTiO₃:2Sm³⁺/5.0Mg²⁺, CaTiO₃:2Sm³⁺/0.5Gd³⁺, CaTiO₃:2Sm³⁺/5.0K⁺, CaTiO₃:2Sm³⁺/5.0Ba²⁺ and CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphor samples were monitored in the range of 20–80°, 2θ angles, and they are shown in Fig. 1(a)–(h). The phase of the phosphors were identified to be orthorhombic with space group Pnma (62). There were no extra peaks due to substituent ions; however, the XRD peaks show shifts due to substitution. The sharp diffraction peaks also confirm the crystalline nature of the prepared samples. When the lower radius ions (Li⁺/Mg²⁺/Gd³⁺) are co-doped in the CaTiO₃:2Sm³⁺ phosphor at Ca²⁺ sites, a shifting of peaks in the higher angle side was observed in the diffraction peaks. However, on doping of higher radius ions (K⁺/Ba²⁺/Bi³⁺) at the Ca²⁺ site, a shifting of peaks in the lower angle side was observed in the diffraction peaks. Upon doping of Sm³⁺ (0.095) at Ca²⁺ sites, the diffraction peaks are shifted slightly towards a higher 2θ angle side. These shifts observed in different cases are shown in Fig. 1(i) for intense peak (121) in the range of 32.4 to 33.6°, 2θ angle.

Fig. 1 XRD patterns of (a) CaTiO₃, (b) CaTiO₃:2Sm³⁺, (c) CaTiO₃:2Sm³⁺/1.0Li⁺, (d) CaTiO₃:2Sm³⁺/5.0Mg²⁺, (e) CaTiO₃:2Sm³⁺/0.5Gd³⁺, (f) CaTiO₃:2Sm³⁺/5.0K⁺, (g) CaTiO₃:2Sm³⁺/5.0Ba²⁺, (h) CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphors and (i) shifting in the (121) peak in the range of 32.4 to 33.6°, 2θ angle. (j) Rietveld refinements for the CaTiO₃:2Sm³⁺ phosphor.

The ionic radii of Li⁺, Mg²⁺ and Gd³⁺ are 0.076, 0.072 and 0.093 nm, while the ionic radii of Ca²⁺ is 0.100 nm. Therefore, on substitution of Li⁺, Mg²⁺ and Gd³⁺ ions at the Ca²⁺ site, the crystal lattice shrinks due to which the XRD peaks are shifted towards a higher 2θ angle side. Wu et al. have also observed the shift in peaks to a higher 2θ angle side on co-doping of Li⁺ (smaller ionic radii) at the Ca²⁺ site in CaTiO₃:Eu³⁺ phosphors.³⁷ However, the ionic radii of K⁺, Ba²⁺ and Bi³⁺ are 0.138, 0.135 and 0.103 nm, which are larger than that of Ca²⁺ ionic radii. Therefore, on substitution of K⁺, Ba²⁺ and Bi³⁺ at the place of Ca²⁺ the crystal lattice expands due to which the XRD peaks are shifted towards a lower 2θ angle side.

The Rietveld refinements for the CaTiO₃:2Sm³⁺ phosphor sample were carried out, and they are shown in Fig. 1(j). The lattice parameters are a = 5.4420, b = 7.6463, and c = 5.3840, with volume V = 224.0342, and conventional Rietveld parameters were R_p = 14.2, R_wp = 23.7, R_exp = 18.7, and χ² = 1.59.³⁸

The average crystallite size (D) of the phosphor samples was calculated using the Debye–Scherrer (D–S) equation:³⁹

where D is the average crystallite size, k is the shape factor (0.89), λ is the X-ray wavelength, β is the full width at half maximum (FWHM) and θ is the diffraction angle. The average crystallite size values were found to be 30.0, 31.02, 35.51, 33.40, 33.05, 32.80, 31.50 and 32.20, nm for the CaTiO₃, CaTiO₃:2Sm³⁺, CaTiO₃:2Sm³⁺/1.0Li⁺, CaTiO₃:2Sm³⁺/5.0Mg²⁺, CaTiO₃:2Sm³⁺/0.5Gd³⁺, CaTiO₃:2Sm³⁺/5.0K⁺, CaTiO₃:2Sm³⁺/5.0Ba²⁺ and CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphor samples, respectively. This shows that the crystallite size corresponding to Li⁺, Mg²⁺, and Gd³⁺ are slightly larger than that of K⁺, Ba²⁺, Bi³⁺. The dislocation density was used to know the optical efficiency of the phosphor materials. The dislocation density was calculated from the crystallite size using the following relation:³⁴

The dislocation density values for CaTiO₃, CaTiO₃:2Sm³⁺, CaTiO₃:2Sm³⁺/1.0Li⁺, CaTiO₃:2Sm³⁺/5.0Mg²⁺, CaTiO₃:2Sm³⁺/0.5Gd³⁺, CaTiO₃:2Sm³⁺/5.0K⁺, CaTiO₃:2Sm³⁺/5.0Ba²⁺ and CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphor samples were found to be 11.1 × 10⁻⁴, 10.3 × 10⁻⁴, 7.9 × 10⁻⁴, 8.9 × 10⁻⁴, 9.1 × 10⁻⁴, 9.3 × 10⁻⁴, 10.0 × 10⁻⁴ and 9.6 × 10⁻⁴ nm⁻², respectively. Thus, the dislocation density values were found to decrease and the average crystallite size increase in case of CaTiO₃:2Sm³⁺/1.0Li⁺, CaTiO₃:2Sm³⁺/5.0Mg²⁺, CaTiO₃:2Sm³⁺/0.5Gd³⁺, CaTiO₃:2Sm³⁺/5.0K⁺, CaTiO₃:2Sm³⁺/5.0Ba²⁺ and CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphors. Therefore, the PL intensity of Sm³⁺ ions is expected to enhance in the presence of Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ ions due to the increase in average crystallite size.

3.1.2. SEM and EDX measurements. The surface morphology of the prepared phosphor samples was studied by SEM measurements. Fig. 2(a)–(g) show the SEM images of CaTiO₃:2Sm³⁺, CaTiO₃:2Sm³⁺/1.0Li⁺, CaTiO₃:2Sm³⁺/5.0K⁺, CaTiO₃:2Sm³⁺/5.0Mg²⁺, CaTiO₃:2Sm³⁺/5.0Ba²⁺, CaTiO₃:2Sm³⁺/0.5Gd³⁺ and CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphor samples, respectively. However, the enlarged picture of a small part of these images in all the cases is shown in the right top corner. In almost all cases, particles are slightly agglomerated to each other and nearly spherical or slightly elongated in shape. It is clear from the enlarged part of the images that the particle sizes are larger in the cases of Li⁺, Mg²⁺ and Gd³⁺ compared to K⁺, Ba²⁺ and Bi³⁺ ions. The average particle size of the prepared phosphors was calculated by the SEM images from the histogram using the ImageJ software and these values were found to be 1.02, 2.19, 1.69, 1.77, 1.24, 1.73 and 1.66 μm for CaTiO₃:2Sm³⁺, CaTiO₃:2Sm³⁺/1.0Li⁺, CaTiO₃:2Sm³⁺/5.0K⁺, CaTiO₃:2Sm³⁺/5.0Mg²⁺, CaTiO₃:2Sm³⁺/5.0Ba²⁺, CaTiO₃:2Sm³⁺/0.5Gd³⁺ and CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphor samples, respectively [see Fig. 3(a)–(g)]. Thus, the average particles size was found to increase in the presence of all the ions and it is maximum for CaTiO₃:2Sm³⁺/1.0Li⁺ phosphors. The Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ ion actually acts as a surface modifier, which modifies the local crystal field around the Sm³⁺ ions in the CaTiO₃ matrix, and therefore, improves the emission intensity in the presence of these modifier ions. It is found maximum in the case of Li⁺ ions.

Fig. 2 SEM images of (a) CaTiO₃:2Sm³⁺, (b) CaTiO₃:2Sm³⁺/1.0Li⁺, (c) CaTiO₃:2Sm³⁺/5.0K⁺, (d) CaTiO₃:2Sm³⁺/5.0Mg²⁺, (e) CaTiO₃:2Sm³⁺/5.0Ba²⁺, (f) CaTiO₃:2Sm³⁺/0.5Gd³⁺, and (g) CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphor samples.

Fig. 3 Particle size distribution of (a) CaTiO₃:2Sm³⁺, (b) CaTiO₃:2Sm³⁺/1.0Li⁺, (c) CaTiO₃:2Sm³⁺/5.0K⁺, (d) CaTiO₃:2Sm³⁺/5.0Mg²⁺, (e) CaTiO₃:2Sm³⁺/5.0Ba²⁺, (f) CaTiO₃:2Sm³⁺/0.5Gd³⁺ and (g) CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphor samples.

Energy-dispersive X-ray spectroscopic (EDX) measurements were used to know the elements present in the prepared phosphor samples. The EDX patterns of CaTiO₃:2Sm³⁺, CaTiO₃:2Sm³⁺/1.0Li⁺, CaTiO₃:2Sm³⁺/5.0K⁺, CaTiO₃:2Sm³⁺/5.0Mg²⁺, CaTiO₃:2Sm³⁺/5.0Ba²⁺, CaTiO₃:2Sm³⁺/0.5Gd³⁺ and CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphor samples are shown in Fig. 4(a)–(g). The spectra show the presence of Ca, Ti, O, Sm, K, Mg, Ba, Gd and Bi elements in the prepared phosphor samples except the Li element. Because of its very light nature, it was ionized in the SEM chamber during interaction with the incident beam, and therefore, it could not be seen in the spectrum.³⁴ This confirmed that all the elements which were used in the sample preparation are present in the prepared samples.

Fig. 4 EDX spectra of (a) CaTiO₃:2Sm³⁺, (b) CaTiO₃:2Sm³⁺/1.0Li⁺, (c) CaTiO₃:2Sm³⁺/5.0K⁺, (d) CaTiO₃:2Sm³⁺/5.0Mg²⁺, (e) CaTiO₃:2Sm³⁺/5.0Ba²⁺, (f) CaTiO₃:2Sm³⁺/0.5Gd³⁺ and (g) CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphor samples.

3.2. Optical characterization

3.2.1. FTIR studies. The Fourier transform infrared measurements were carried out to know the phonon frequencies (vibrational bands frequencies) of the phosphor samples. The FTIR spectra of CaTiO₃:2Sm³⁺ and CaTiO₃:2Sm³⁺/1.0Li⁺ phosphors in the spectral region 400–3000 cm⁻¹ are shown in Fig. 5. The vibrational bands are observed at 430 and 545 cm⁻¹ due to Ca–O and Ti–O groups.^34,39

Fig. 5 FTIR spectra of CaTiO₃:2Sm³⁺ and CaTiO₃:2Sm³⁺/1.0Li⁺ phosphors.

The spectra corresponding to modifier ions (Li⁺) are exactly identical except that there is a change in the intensity of the bands. The following conclusions could be drawn on the basis of these measurements. First, the phonon frequency of the phosphor materials is low, and hence, the non radiative relaxation process in this case will be poor. This means that the radiative emission in this case is expected to be high. Second, the vibrational frequency of the bands remains unchanged upon co-doping of the modifier. Only their intensity is affected due to scattering of photons with the modifier ion.

3.2.2. Photoluminescence excitation spectrum and emission spectra of CaTiO₃:cSm³⁺ phosphors. The photoluminescence excitation spectrum (PLE) of CaTiO₃:2Sm³⁺ phosphor sample was monitored with λ_em = 599 nm in the 350–525 nm region, as shown in Fig. 6(a). The spectrum of CaTiO₃:2Sm³⁺ phosphor contained a number of peaks, situated at 365, 380, 407, 422, 440, 467 and 481 nm due to ⁶H_5/2 → ⁴D_3/2, ⁴D_1/2, ⁴F_7/2, ⁴P_5/2, ⁴G_9/2, ⁴I_13/2 and ⁴I_11/2 transitions of the Sm³⁺ ion, respectively.^15–17 The peak at 407 nm shows maximum intensity and lies in the region of the n-UV LED chip. Hence, this is important for LED applications.

Fig. 6 (a) Photoluminescence excitation (PLE) spectrum of the CaTiO₃:2Sm³⁺ phosphor sample with λ_em = 599 nm and (b) photoluminescence emission (PL) spectra of CaTiO₃:cSm³⁺ (where c = 1.0, 1.5, 2.0, 2.5, 3.0 and 5.0 mol%) phosphor samples with λ_ex = 407 nm.

The photoluminescence emission (PL) spectra of CaTiO₃:cSm³⁺ (where c = 1.0, 1.5, 2.0, 2.5, 3.0 and 5.0 mol%) phosphors with λ_ex = 407 nm in the spectral region 525–700 nm and they are depicted in Fig. 6(b). In the PL spectra, we observed three intense emission peaks at 564, 599, and 646 nm due to ⁴G_5/2 → ⁶H_5/2, ⁶H_7/2 and ⁶H_9/2 transitions of Sm³⁺ ions.^15–17 The most intense peak is situated at 599 nm (⁴G_5/2 → ⁶H_7/2). The peak at 564 nm is due to the magnetic dipole transition. The most intense emission peak at 599 nm appears partially due to magnetic and partially due to forced electric dipole transitions. However, the peak at 646 nm is found due to electric dipole transition.

The Sm³⁺ ions present in the ground state (⁶H_5/2), are promoted to the ⁴F_7/2 excited state by absorption of 407 nm photons. The Sm³⁺ ions from ⁴F_7/2 excited state relax non-radiatively to the ⁴G_5/2 excited state, which give multi-transition emission to different sublevels of the ground state, which lie in orange to red regions. The energy level diagram of Sm³⁺ ions is shown in Fig. 7(a). The photoluminescence emission intensity was found to increase with the concentrations of Sm³⁺ ions, and it was found optimum at 2 mol% in this host. The emission intensity was found to decrease for higher concentrations due to concentration quenching. The variation in emission intensity with different concentrations of Sm³⁺ ions is shown in Fig. 7(b). Two mechanisms are generally found to involve in concentration quenching. One is the exchange interaction and the other is the multipolar interaction. The two mechanisms depend on the critical distance between the activator ions. If the value of critical distance between the two ions is ≤5 Å, the concentration quenching would be due to exchange interaction. However, if it is ≥5 Å, it would be due to multipolar interaction.

Fig. 7 (a) Schematic energy level diagram of Sm³⁺ and the process involved in excitation and emission. (b) Variation of emission intensity with the concentration of Sm³⁺ ions. (c) Dual logarithmic plot between PL emission intensity per Sm³⁺ ion concentration [Log(I/x)] versus Sm³⁺ concentration [Log(x)].

The value of critical distance between Sm³⁺–Sm³⁺ ions could be calculated using the following equation:³⁴

where V is the volume of unit cell, x is the optimum concentration of Sm³⁺ ions for which the emission intensity is maximum and N is the number of Ca²⁺ sites occupied by Sm³⁺ ions. In the present case, the value of V = 224.0342 ų and x = 0.02. The number of Ca sites, i.e. N = 4. This resulted in an R_c value of 17.44 Å, a value much larger than 5 Å. Hence, the concentration quenching in CaTiO₃:cSm³⁺ phosphors was found to be multipolar interaction.

The multipolar interactions can be recognized using the following formula:³⁴

where I is the PL emission intensity, x is the Sm³⁺ ion concentration, k and β are the constants for a given host. The value of θ decides the actual nature of interaction between Sm³⁺ ions. Depending on the value of θ equal to 6, 8 or 10, the nature of interaction would be dipole–dipole (d–d), dipole–quadrupole (d–q) or quadrupole–quadrupole (q–q), respectively. A plot between Log(I/x) versus Log(x) for CaTiO₃:cSm³⁺ (where c = 1.0, 1.5, 2.0, 2.5, 3.0 and 5.0 mol%) phosphors with λ_ex = 407 nm and λ_em = 599 nm is shown in Fig. 7(c). The slope value of the curve was found by fitting the plot Log(I/x) versus Log(x). The observed slope value (equal to −θ/3) was found to be 1.04582, from which the value of θ ∼ 3.13, close to 3. If the value of θ is less than 6, the energy transfer is due to the interaction between the adjacent ions.

3.2.3. Effect of co-doping of singly, doubly and triply ionized ions on the photoluminescence emission intensity of CaTiO₃:2Sm³⁺ phosphor. The PL emission spectra of CaTiO₃:2Sm³⁺/xLi⁺ (where x = 0.5, 1.0 and 3.0 mol%), xK⁺ (where x = 1.0, 3.0, 5.0 and 7.0 mol%), yMg²⁺ (where y = 1.0, 3.0, 5.0 and 10 mol%), yBa²⁺ (where y = 3.0, 5.0 and 10 mol%), zGd³⁺ (where z = 0.1, 0.5 and 0.7 mol%) and zBi³⁺ (where z = 3.0, 5.0 and 10 mol%) phosphors were monitored under excitation at 407 nm, and they are given in Fig. 8(a)–(f). The emission intensity is maximum for 1.0 mol% Li⁺, 5.0 mol% K⁺, 5.0 mol% Mg²⁺, 5.0 mol% Ba²⁺, 0.5 mol% Gd³⁺ and 5.0 mol% Bi³⁺ co-doped CaTiO₃:2Sm³⁺ phosphors. As mentioned earlier, we selected two types of ions: one with a smaller size (Li⁺/Mg²⁺/Gd³⁺ with singly, doubly and triply ionized states) and the other with a larger size (K⁺/Ba²⁺/Bi³⁺ with singly, doubly and triply ionized states), and monitored the photoluminescence emission spectra under the same conditions. It was found that the emission wavelengths of the Sm³⁺ ion are the same. However, the photoluminescence emission intensity is increased in the presence of all these modifier ions. Moreover, it is larger in the case of ions with smaller ionic radii. The enhancement in the intensity of Sm³⁺ ions follow the trend I_Li⁺ > I_Mg²⁺ > I_Gd³⁺ > I_K⁺ > I_Bi³⁺ > I_Ba²⁺. Several researchers have used these ions to improve the emission intensity of rare earth ions in different host matrices.^23–27 For example, Wu et al. have studied the photoluminescence properties of Eu³⁺/Li⁺ co-doped CaTiO₃ phosphors.³⁷ They found that the emission intensity of Eu³⁺ ions is increased in the presence of Li⁺ ions. They have explained it to be the result of the increase in average particle size and charge compensation. Our group have also observed an enhancement in the emission intensity of Tm³⁺/Yb³⁺ co-doped ZnWO₄ phosphors in the presence of Mg²⁺ ions due to the modification in the local crystal field.⁴⁰ Linga et al. have reported an enhancement in the photoluminescence intensity of (Ca_1−x−y,Ln_y)MoO₄:xEu³⁺ (Ln = Y and Gd) phosphors in the presence of Y³⁺ and Gd³⁺ ions.⁴¹

Fig. 8 Photoluminescence emission (PL) spectra of (a) CaTiO₃:2Sm³⁺/xLi⁺ (where x = 0, 0.5, 1.0 and 3.0 mol%), (b) CaTiO₃:2Sm³⁺/xK⁺ (where x = 0, 1.0, 3.0, 5.0 and 7.0 mol%), (c) CaTiO₃:2Sm³⁺/yMg²⁺ (where y = 0, 1.0, 3.0, 5.0 and 10 mol%), (d) CaTiO₃:2Sm³⁺/yBa²⁺ (where y = 0, 3.0, 5.0 and 10 mol%), (e) CaTiO₃:2Sm³⁺/zGd³⁺ (where z = 0, 0.1, 0.5 and 0.7 mol%) and (f) CaTiO₃:2Sm³⁺/zBi³⁺ (where z = 0, 3.0, 5.0 and 10 mol%) phosphor samples with λ_ex = 407 nm.

The emission intensity of CaTiO₃:2Sm³⁺/1.0Li⁺, CaTiO₃:2Sm³⁺/5.0K⁺, CaTiO₃:2Sm³⁺/5.0Mg²⁺, CaTiO₃:2Sm³⁺/5.0Ba²⁺, CaTiO₃:2Sm³⁺/0.5Gd³⁺ and CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphors was enhanced by 6.3, 4.0, 5.1, 1.4, 5.0 and 2.5 times (for 599 nm peak) compared to the CaTiO₃:2Sm³⁺ phosphor.

These observations indicate that the ions with smaller size (ionic radii) are more effective in enhancing the emission intensity of Sm³⁺ ions. This is due to the reason that ions with a smaller ionic size will be able to reach closer to activator ions. The charge compensation will create a larger field to enhance the emission intensity. In the case of larger radius ions, whatever may be the ionization state (K⁺, Ba²⁺, and Bi³⁺), they will be struck out by the host atoms/ions before they reach closer to the activator ion (i.e. Sm³⁺) due to their larger size, and hence, the field created will be smaller. Thus, the ionic radii of Li⁺, Mg²⁺ and Gd³⁺ ions are 0.076, 0.072 and 0.093 nm, which are smaller than the Ca²⁺ ionic radii (0.100 nm) at which these ions are substituted, while the ionic radii of K⁺, Ba²⁺ and Bi³⁺ are 0.138, 0.135 and 0.103 nm, which are larger than the Ca²⁺ ionic radii. In all these, Li⁺, Mg²⁺ and Gd³⁺ ions produce larger enhancement and in that Li⁺ is the largest one. Another reason for the enhancement in emission intensity is if we compare the crystallite and particle size, they also follow the same trend, i.e. (Li⁺) > (Mg²⁺) > (Gd³⁺) > (K⁺) > (Bi³⁺) > (Ba²⁺). As larger the particle size, it contains a large number of activators which give large emission intensity. The increase in average crystallite and particle sizes in the presence of Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ ions improves the population of the ⁴G_5/2 level. Therefore, the emission intensity is enhanced in the presence of Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ ions.⁴² The emission intensity is maximum in the case of CaTiO₃:2Sm³⁺/1.0Li⁺ phosphor and it is due to its smaller ionic radii and used as a charge compensator as well as it has the largest average crystallite and particle size.

3.2.4. Color coordinates, color purity and correlated color temperature calculations. The CIE (Commission Internationale de I'Eclairage) diagram is an excellent tool to verify the color emitted from the phosphors. The values of CIE coordinates were calculated for different samples, and they are shown in Fig. 9 for CaTiO₃:2Sm³⁺ (0.58, 0.42), CaTiO₃:2Sm³⁺/1.0Li⁺ (0.60, 0.41), CaTiO₃:2Sm³⁺/5.0Mg²⁺ (0.59, 0.41), CaTiO₃:2Sm³⁺/0.5Gd³⁺ (0.59, 0.41), CaTiO₃:2Sm³⁺/5.0K⁺ (0.58, 0.42), CaTiO₃:2Sm³⁺/5.0Ba²⁺ (0.58, 0.42), and CaTiO₃:2Sm³⁺/5.0Bi³⁺ (0.58, 0.42), phosphors. The values of CIE coordinates are nearly the same in the presence of different ions and they lie in the orange red region.

Fig. 9 CIE diagram of CaTiO₃:2Sm³⁺, CaTiO₃:2Sm³⁺/1.0Li⁺, CaTiO₃:2Sm³⁺/5.0Mg²⁺ and CaTiO₃:2Sm³⁺/0.5Gd³⁺ phosphor samples upon 407 nm excitation.

The color purity of the phosphor materials is another important parameter to recognize phosphor as a good source of light for a particular color for solid-state lighting applications. The color purity of the light source can be calculated using the following relation:⁴³

where (x, y) are the CIE coordinates of the phosphor, (x_d, y_d) are the CIE coordinates of the dominant wavelength and (x_i, y_i) are the CIE coordinates for the standard light source. The values of color purity in the present case were found to be 87.80% for CaTiO₃:2Sm³⁺, CaTiO₃:2Sm³⁺/5.0K⁺, CaTiO₃:2Sm³⁺/5.0Ba²⁺ and CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphors and 89.88% for CaTiO₃:2Sm³⁺/5.0Mg²⁺ and CaTiO₃:2Sm³⁺/0.5Gd³⁺ phosphors. The color purity for the CaTiO₃:2Sm³⁺/1.0Li⁺ phosphor is 93%. The color purity was also found to improve in the presence of ions of smaller ionic radius (Li⁺/Mg²⁺/Gd³⁺) co-doping and it is maximum for 1.0 mol% Li⁺ co-doping in the CaTiO₃:2Sm³⁺ phosphor.

We have also calculated the correlated color temperature (CCT) to evaluate the nature of emitted light. The CCT values for the different phosphor samples were evaluated using McCammy's equation as follows:⁴³

T = −449(n³) + 3525(n²) − 6823.3(n) + 5520.33 (6)

where n = (x − 0.3320)/(y − 0.1858) and (x, y) refer to CIE coordinates. The calculated CCT values were found to be 1975 K, for CaTiO₃:2Sm³⁺, CaTiO₃:2Sm³⁺/5.0K⁺, CaTiO₃:2Sm³⁺/5.0Ba²⁺ and CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphors and 1880 K CaTiO₃:2Sm³⁺/5.0Mg²⁺ and CaTiO₃:2Sm³⁺/0.5Gd³⁺ phosphors. The CCT value is 1805 K for CaTiO₃:2Sm³⁺/1.0Li⁺, phosphors. Thus, the CCT values were found to lie in the warm orange-red light region. Therefore, Sm³⁺-doped Sm³⁺/Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ co-doped CaTiO₃ phosphor samples were found to be suitable for orange-red lighting devices.

3.2.5. Lifetime measurements. The lifetime of the ⁴G_5/2 level of Sm³⁺ ions was measured using the ⁴G_5/2 → ⁶H_7/2 transition emitting at 599 nm under 407 nm excitation, and the decay curves are shown in Fig. 10(a)–(g). The decay curves of the ⁴G_5/2 level of Sm³⁺ ions in different cases were found to fit well using a single exponential relation:³⁸

I(t) = I₀ exp(−t/τ) (7)

where I₀ and I(t) are the PL emission intensities at time zero and t seconds, respectively and ‘τ’ is the lifetime.

Fig. 10 Decay curves for the ⁴G_5/2 level of Sm³⁺ ions under λ_ex = 407 nm and λ_em = 599 nm for (a) CaTiO₃:2Sm³⁺, (b) CaTiO₃:2Sm³⁺/1.0Li⁺, (c) CaTiO₃:2Sm³⁺/5.0K⁺, (d) CaTiO₃:2Sm³⁺/5.0Mg²⁺, (e) CaTiO₃:2Sm³⁺/5.0Ba²⁺, (f) CaTiO₃:2Sm³⁺/0.5Gd³⁺ and (g) CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphors.

The value of lifetime for the ⁴G_5/2 level of Sm³⁺ in the case of CaTiO₃:2Sm³⁺, CaTiO₃:2Sm³⁺/1.0Li⁺, CaTiO₃:2Sm³⁺/5.0K⁺, CaTiO₃:2Sm³⁺/5.0Mg²⁺, CaTiO₃:2Sm³⁺/5.0Ba²⁺, CaTiO₃:2Sm³⁺/0.5Gd³⁺ and CaTiO₃:2Sm³⁺/5.0Bi³⁺ phosphors were found to be 0.95, 1.02, 1.00, 0.99, 0.96, 1.00 and 0.97 ms, respectively. From this, it is clear that the value of lifetime is increased in the presence of these modifiers (Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ ions) and it is optimum in the case of Li⁺ ions. The lifetime of the ⁴G_5/2 level of Sm³⁺ ions follows the trend, i.e. τ_Li > τ_k ∼ τ_Gd > τ_Mg > τ_Bi > τ_Ba. The increase in the average crystallite and particle size and the modification in the crystal field around the activator Sm³⁺ ion in the presence of Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ ions improve the population in ⁴G_5/2 levels, and therefore, the value of lifetime of ⁴G_5/2 is increased.⁴² This might be one of the reasons for enhancement in the PL emission intensity.

4. Conclusions

Sm³⁺-doped and Sm³⁺/Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ co-doped CaTiO₃ phosphors have been synthesized by a solid-state reaction method at 1473 K. The structural and morphological properties of the prepared samples were studied by XRD and SEM measurements. The average crystallite and particle sizes were found to increase in the presence of modifiers and they follow the trend Li⁺ > Mg²⁺ > Gd³⁺ > K⁺ > Bi³⁺ > Ba²⁺. EDX measurements were carried out to verify the elements present in the respective phosphor samples. The infrared measurements of the phosphors showed the presence of Ca–O and Ti–O vibrational bands at 430 and 545 cm⁻¹, respectively, indicating that the phosphor has a low phonon frequency. The emission spectra of Sm³⁺ ions showed an intense emission peak at 599 nm due to ⁴G_5/2 → ⁶H_7/2 transition upon excitation at 407 nm wavelength and the emission intensity is maximum for 2 mol% of Sm³⁺ ion. The emission intensity is quenched for a higher concentration of Sm³⁺ ions. Therefore, the surface modifiers were used to further enhance the emission intensity. It was found that co-doping 1.0 mol% Li⁺, 5.0 mol% K⁺, 5.0 mol% Mg²⁺, 5.0 mol% Ba²⁺, 0.5 mol% Gd³⁺ and 5.0 mol% Bi³⁺ ions enhances the emission intensity by 6.3, 4.0, 5.1, 1.4, 5.0 and 2.5 times respectively as compared to the 2Sm³⁺-doped CaTiO₃ phosphor. The increase in emission intensity is due to the modification of the crystal field around the Sm³⁺ ions in the CaTiO₃ host as well as the increase in average crystallite and particle sizes in the presence of these ions. It was found that the size (ionic radii) and charge compensation of the ion together play a dominant role. The enhancement is more after co-doping with smaller radius ions (Li⁺, Mg²⁺ and Gd³⁺), among which Li⁺ shows the largest enhancement. This is because ions of smaller size will be able to go closer to the activator ion and the charge imbalance causes a larger field. The emission intensity is maximum in the case of the CaTiO₃:2Sm³⁺/1.0Li⁺ phosphor due to its smaller ionic radii and used as a charge compensator as well as it has the largest average crystallite and particle size. The CIE color coordinates and correlated color temperature (CCT) showed the orange-red color with a color purity as high as 93% in the case of the CaTiO₃:2Sm³⁺/1.0Li⁺ phosphor. The lifetime of the ⁴G_5/2 level of Sm³⁺ ions was also found enhanced in the presence of Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ ions and it follows the trend τ_Li > τ_k ∼ τ_Gd > τ_Mg > τ_Bi > τ_Ba. From these studies, it is suggested that Sm³⁺-doped and Sm³⁺/Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ co-doped CaTiO₃ samples may be suitable for display devices and for LEDs under n-UV excitation.

Conflicts of interest

This work does not have any conflict of interest.

Acknowledgements

Ms Priti Singh wishes to acknowledge University Grants Commission (UGC), India for providing research fellowship. Authors are also thankful to the Institute of Eminence (IOE), BHU -6031 grant for chemicals.

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