1. Introduction

Due to the increased demand of lighting worldwide, new energy efficient and environmentally friendly lighting devices are strongly required. Solid state lighting systems are gradually replacing the old technologies thanks to their excellent stability, fast response, longer life expectancy, environmentally friendly characteristics and high luminous efficiency [1, 2].

White light can be produced by the combination of the fundamental colours (red, green and blue) coming from three individual LEDs, or combining only one LED with suitable phosphors. For this second approach, commercially available blue and near ultraviolet (NUV) LEDs can be used to excite either a yellow phosphor or a green and red phosphor (using blue LEDs), or to excite blue, green and red phosphors (using NUV LEDs) [2]. Both approaches offer the production of white-LEDs with comparable brightness. Nevertheless, in the three converter system, the blue emission efficiency is low due to strong re-absorption of the blue light. As a consequence, the development of novel single-phased white-emitting phosphors is attracting a lot of attention over the last years [2–5].

Inorganic phosphors doped with rare earth ions have drawn much interest because of their intense emission efficiencies due to 4f-4f and 4f-5d electronic transitions, and are currently used in many technological fields. Silicate phosphors present high interest from the spectroscopic point of view due to their good transparency in the UV/VIS region and chemical stability, and are to be considered as efficient hosts for doping with luminescent trivalent lanthanide ions [6, 7].

In this paper, the room temperature luminescence spectra and decay kinetics of Ca₃A_1.98Tb_0.02Si₃O₁₂ (A = Gd, Y), Ca₃A_1.98Eu_0.02Si₃O₁₂ (A = Gd, Y), Ca₃Gd_2-x-yTb_xEu_ySi₃O₁₂ and Ca₃Y_2-x-yTb_xEu_ySi₃O₁₂ phosphors belonging to silico-carnotite family have been systematically studied for various Tb³⁺ and Eu³⁺ doping concentration, upon UV excitation.

2. Experimental methods and structural characterization

Polycrystalline samples of Ca₃A_1.98Tb_0.02Si₃O₁₂ (A = Gd, Y), Ca₃A_1.98Eu_0.02Si₃O₁₂ (A = Gd, Y), Ca₃Gd_2-x-yTb_xEu_ySi₃O₁₂ (x = 0.02, y = 0.04; x = 0.02, y = 0.01; x = 0.01, y = 0.01 and x = 0.01, y = 0.005) and Ca₃Y_2-x-yTb_xEu_ySi₃O₁₂ (x = 0.02, y = 0.005; x = 0.015, y = 0.003; and x = 0.01, y = 0.005) were prepared by solid state reaction at high temperature (1450 ° C), following the procedures previously described [8,9].

Powder X-ray diffraction (XRD) measurements were performed to analyze the structure of the synthesized compounds. Experiments were carried out with a Thermo ARL X’TRA powder diffractometer, operating in the Bragg-Brentano geometry and equipped with a Cu-anode X-ray source (Kα, λ = 1.5418 A), using a Peltier Si (Li) cooled solid state detector. The patterns were collected with a scan rate of 0.04°/s in the 5-90° 2θ range. The phase identification was performed with the PDF-4 + 2013 database provided by the International Centre for Diffraction Data (ICDD). Polycrystalline samples were ground in a mortar and then put in a low-background sample holder for the data collection. All the obtained materials are single phase. They are isostructural with silico-carnotite Ca₅(PO₄)₂SiO₄ (orthorhombic, space group, Pnma) [10]. The structural formula can be represented as AB₂C₂(EO₄)₃ where A, B and C refer, respectively, to nine-, eight-, and seven-coordinate cationic sites, characterized by very low site symmetries (C₁ or C_s). E could be Si, P or both. It has been shown previously that the Ca₃RE₂Si₃O₁₂ (RE-rare earth ions) family of compounds possess orthorhombic Pnma crystal structure, and the distribution of the cations on the three available crystal sites is strongly dependent on the nature of the rare earth ion [11,12].

Room temperature luminescence spectra and decay curves were measured with a Fluorolog 3 (Horiba-Jobin Yvon) spectrofluorometer, equipped with a Xe lamp, a double excitation monochromator, a single emission monochromator (mod.HR320) and a photomultiplier in photon counting mode for the detection of the emitted signal.

3. Results and discussion

3.1 Ca₃Gd_2-x-yTb_xEu_ySi₃O₁₂

Figure 1(a) shows the room temperature emission spectra of Ca₃Gd_1.98Tb_0.02Si₃O₁₂ measured upon excitation at 377 nm (in ⁵D₃ level of Tb³⁺) and of Ca₃Gd_1.98Eu_0.02Si₃O₁₂ recorded upon excitation at 393 nm. For Ca₃Gd_1.98Tb_0.02Si₃O₁₂ the emission presents various bands in the ranges 400-480 and 500-650 nm. These bands are due to the ⁵D₃ → ⁷F_J (J = 5, 4, 3) and ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) transitions. Among them, the ⁵D₄ → ⁷F₅ green emission at 542 nm is the most intense.

 

Fig. 1 (a) Room temperature emission spectra of Ca₃Gd_1.98Tb_0.02Si₃O₁₂ (λ_exc = 377 nm) and Ca₃Gd_1.98Eu_0.02Si₃O₁₂ (λ_exc = 393 nm). The spectra are normalized to Tb^{3+ 5}D₄→⁷F₅ emission, and Eu^{3+ 5}D₀→⁷F₂ emission respectively. (b) Room temperature emission spectra of Ca₃Gd_2-x-yTb_xEu_ySi₃O₁₂ measured upon excitation at 378 nm. All spectra are normalized to Tb^{3+ 5}D₃→⁷F₅ emission.

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As expected in diluted terbium materials, no quenching of the blue emission is observed. This is because the cross relaxation processes (CR) that transfer population from ⁵D₃ to ⁵D₄, are concentration dependent [13–15], and therefore they are not efficient at low terbium concentration levels. Taking into account that the vibrational cut-off of the Ca₃Y₂Si₃O₁₂ silico-carnotite host is about 1050 cm⁻¹, and the energy gap between ⁵D₃ and ⁵D₄ levels is about 5750 cm⁻¹ [7], the gap can be bridged by 4-5 phonons. Since for the Ca₃Gd₂Si₃O₁₂ host similar vibrational cut-off values are expected, in this system it is possible to populate ⁵D₄ from ⁵D₃ by multiphonon relaxation (MPR). The presence of these processes explains the origin of the green emission bands observed in the spectrum measured upon ⁵D₃ excitation. In the case of Ca₃Gd_1.98Eu_0.02Si₃O₁₂ five bands in the 590 −730 nm range are observed upon excitation at 393 nm in the ⁵L₆ level. These emission bands correspond to the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transitions of Eu³⁺, the most intense being the hypersensitive ⁵D₀ → ⁷F₂ red emission at 612 nm, as expected due to the distorted geometry of the sites accommodating the impurity [16].

In view of these results, it is interesting to consider the possibility of combining the blue, green and red emissions observed in the Ca₃Gd₂Si₃O₁₂ phosphors doped with Tb³⁺ and Eu³⁺. In order to study this phenomenon, a series of Ca₃Gd_2-x-yTb_xEu_ySi₃O₁₂ (x = 0.02, y = 0.04; x = 0.02, y = 0.01; x = 0.01, y = 0.01 and x = 0.01, y = 0.005) samples have been prepared. The excitation populates the ⁵D₃ and ⁵L₇ levels of Tb³⁺ and Eu³⁺, respectively. The emission spectra of the powders (λ_exc = 378 nm) are presented in Fig. 1(b). The spectra present various emission bands originating from the ⁵D₃ and ⁵D₄ levels of Tb³⁺ (blue to yellow range), and from the ⁵D₀ level of Eu³⁺ (orange to red range). It is observed that the relative intensities of the ⁵D₃→⁷F_J and ⁵D₄→⁷F_J transitions do not vary significantly when modifying Tb³⁺ concentration. This is because, as discussed before, the doping levels are low (from 1 to 0.5 mol%), so the non-radiative relaxation from ⁵D₃ to ⁵D₄ is not enhanced. The relative intensities of the blue/green bands respect to the red ones, however, are strongly affected by the Tb³⁺/Eu³⁺ concentration ratio: it is observed that if the concentration of Tb³⁺ is equal or lower than the concentration of Eu³⁺, the intensity of the red bands is almost double than for blue-green ones. On the contrary, if Tb³⁺ concentration is higher than Eu³⁺, the relative intensities tend to be identical. Controlling the doping ratio thus allows changing the final emission colour of the material from red to pinkish-white. To clarify this point, the CIE XYZ chromaticity coordinate diagram is shown in Fig. 2. The calculated values of CIE coordinates are provided in Table 1. For completeness, CIE values for Ca₃Gd_1.98Tb_0.02Si₃O₁₂ and Ca₃Gd_1.98Eu_0.02Si₃O₁₂ are also included.

 

Fig. 2 CIE diagram coordinates of Ca₃Gd_1.98Tb_0.02Si₃O₁₂, Ca₃Gd_1.98Tb_0.02Si₃O₁₂ and Ca₃Gd_2-x-yTb_xEu_ySi₃O₁₂ excited at 377, 378 and 393 nm respectively.

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Table 1. Calculated values for CIE coordinates of Ca₃Gd_1.98Tb_0.02Si₃O₁₂ and Ca₃Gd_2-x-yTb_xEu_ySi₃O₁₂ excited at 378 nm and Ca₃Gd_1.98Eu_0.02Si₃O₁₂ excited at 393 nm.

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For all the compounds, the emission bands are relatively broad due to the structural disorder present in the host, as previously observed in other silico-carnotite materials doped with Tb³⁺ and Eu³⁺ [9].

The excitation spectra of all samples are shown in Fig. 3. The spectrum of Ca₃Gd_1.98Tb_0.02Si₃O₁₂ is composed of overlapping bands in the UV ranging from 300 nm to almost 400 nm, and a band at 480 nm. The transitions have been assigned as ⁷F₆ → ⁵G₃ at 340 nm, ⁷F₆ → ⁵L₈ + ⁵G₄ at 350 nm, ⁷F₆ → ⁵D₃ at377 nm and ⁷F₆ → ⁵D₄ at 480 nm. For the Ca₃Gd_2-x-yTb_xEu_ySi₃O₁₂ phosphors, the excitation spectra when monitoring the emission of Tb³⁺ (λ_emi = 542 nm) are composed of the same bands observed for Ca₃Gd_1.98Tb_0.02Si₃O₁₂. In the case of the samples with less Tb³⁺ doping, it is possible to observe that the intensity of the ⁷F₆ → ⁵D₄ band is comparable to the⁷F₆ → ⁵D₃ band.

 

Fig. 3 (a) Room temperature excitation spectra of Ca₃Gd_1.98Tb_0.02Si₃O₁₂ (λ_emi = 542 nm), Ca₃Gd_1.98Eu_0.02Si₃O₁₂ (λ_emi = 612 nm) and Ca₃Gd_2-x-yTb_xEu_ySi₃O₁₂ monitoring both emission of Tb³⁺ and Eu³⁺ (λ_emi = 542 nm, and λ_emi = 611 nm). The spectra are normalized to Tb^{3+ 7}F₆→⁵D₃ band, and Eu^{3+ 7}F₀→⁵D₃ band respectively.

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The spectrum of Ca₃Gd_1.98Eu_0.02Si₃O₁₂ is composed of various bands ranging from 360 nm to 430 nm and other bands at 460 nm and 530 nm. In this case the transitions have been assigned as ⁷F₀ → ⁵D₄ at 360 nm, ⁷F₀ → ⁵L₇ at 393 nm, which it is the most intense, ⁷F₀ → ⁵D₃ at 412 nm, ⁷F₀ → ⁵D₂ at 460 nm and ⁷F₀ → ⁵D₁ at 525 nm and ⁷F₁ → ⁵D₁ at 530 nm. In the case of the co-doped samples, the excitation spectra when monitoring the emission of Eu³⁺ (λ_emi = 611 nm) present no differences respect to one the observed for Ca₃Gd_1.98Eu_0.02Si₃O₁₂. No evidence of Tb³⁺-Eu³⁺ energy transfer is found in the spectra of these materials. This can be explained on the basis of the fact that in the co-doped systems the Tb³⁺ and the Eu³⁺ concentrations are low, and then the average distance between Ln³⁺ ions is estimated to lie between 10 and 16 Å, which is larger than the typical critical distance required for electric dipole-electric dipole energy transfer processes involving Tb³⁺ and Eu³⁺ (<10 Å) [17]. Moreover, this implies that there is no fast migration among Tb³⁺ ions. We have reported that in silico-carnotite materials, the energy migration in the Tb³⁺ subset plays a relevant role in the Tb³⁺-Eu³⁺ energy transfer process [9]. Since in the present case Tb³⁺-Tb³⁺ energy migration is not efficient, the absence of Tb³⁺-Eu³⁺ energy transfer can be also expected.

Decay curves for Tb³⁺ and Eu³⁺ emission were measured upon excitation at 378 nm. Figure 4(a) shows decay curves for Tb³⁺ emission from ⁵D₄ in the various co-doped phosphors (diluted compounds). All the curves are plotted together. To compare the differences, a concentrated Tb³⁺ compound (Ca₃Tb₂Si₃O₁₂) is also included. The obtained time constants are reported in Table 2, together with the decay constant for Ca₃Gd_1.98Tb_0.02Si₃O₁₂. In the co-doped phosphors there is a clear rise at short times, which is not present in Ca₃Tb₂Si₃O₁₂. As reported in our previous work [9], this behaviour of Ca₃Tb₂Si₃O₁₂ is an effect of the high Tb³⁺ concentration, which allows easy ⁵D₃ → ⁵D₄ cross relaxation in Tb³⁺ ions. As it was explained previously, in the co-doped materials there is no migration among Tb³⁺ ions and therefore the observed decay in this case is slow, as energy transfer to killer centres does not occur. The slow initial build up is due to the MPR processes involved in the population of ⁵D₄, that are slower than the CR processes that are present in Ca₃Tb₂Si₃O₁₂. Due to the low levels of doping, the shapes of the curves and the decay constants are not affected by Eu³⁺ concentration. This fact, together with the absence of energy migration in the Tb³⁺ subset and from Tb³⁺ to Eu³⁺, explains why the decay curves present an almost identical behaviour in all the different Ca₃Gd_2-x-yTb_xEu_ySi₃O₁₂ phosphors. As a result, the curves appear superimposed in the graph. The decay curves for Tb³⁺ emission from ⁵D₃ (blue bands) have been also measured for Ca₃Gd_1.98Tb_0.02Si₃O₁₂ and the co-doped phosphors. The curves are not exponential due to the presence of structural disorder. Decay constant is in practice not affected by changes in Eu³⁺ concentration, and its value is about 1 ms for all the samples.

 

Fig. 4 (a) Room temperature decay curves of the ⁵D₄ Tb³⁺ emission excited at 377 nm (Ca₃Tb₂Si₃O₁₂) and at 378 nm (Ca₃Gd_2-x-yTb_xEu_ySi₃O₁₂). (b) Room temperature decay curves of the ⁵D₀ Eu³⁺ emission excited at 378 nm (Ca₃Gd_2-x-yTb_xEu_ySi₃O₁₂) and 393 nm (Ca₃Eu₂Si₃O₁₂).

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Table 2. Decay data for the luminescent levels of Tb³⁺ and Eu³⁺ in several oxide hosts upon UV excitation

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Figure 4(b) presents the decay curves for Eu^{3+ 5}D₀ emission for the various co-doped samples. All the curves are plotted together. Ca₃Eu₂Si₃O₁₂ is also included to compare the behaviour of the ion. The time constants calculated from the decay curves are also reported in Table 2. For the Eu³⁺-doped samples, the curves are well fitted by a single exponential function. This behaviour is explained because there is no energy transfer from Tb³⁺ to Eu³⁺, and thanks to the fast relaxation from the excited level (⁵L₆ for the diluted compounds, ⁵L₇ for the concentrated sample) to ⁵D₀ due to the small energy gaps between the intermediate levels, which require less than three phonons to be bridged. In the case of Ca₃Eu₂Si₃O₁₂, decay time is shortened, presumably due to concentration quenching as a result of energy migration to quenching impurities. No significant changes are observed when varying Eu³⁺ concentration in the diluted samples. All the curves present an almost identical behaviour, and therefore they appear superimposed in the graph.

3.2 Ca₃Y_2-x-yTb_xEu_ySi₃O₁₂

Due to the interesting properties observed for the gadolinium-based phosphors, it is worth to explore other similar hosts starting from these results. In the present section the luminescence of various yttrium-based silico-carnotite are discussed.

Figure 5(a) shows the room temperature emission spectra of Ca₃Y_1.98Tb_0.02Si₃O₁₂ measured upon excitation at 378 nm (in ⁵D₃ level of Tb³⁺) and the emission spectrum of Ca₃Y_1.98Eu_0.02Si₃O₁₂ recorded upon excitation at 393 nm. In both cases, the spectra are similar to the ones presented in the previous section for the gadolinium-based samples. In the case of Ca₃Y_1.98Tb_0.02Si₃O₁₂, the emission spectrum presents various bands in the ranges of 400-480 and 500-650 nm, the most intense being the ⁵D₄ → ⁷F₅ green emission at 542 nm. These results include the blue emission originating from the ⁵D₃ level, which was erroneously not shown and discussed in ref [7]. published by some of us. In the case of Ca₃Y_1.98Eu_0.02Si₃O₁₂ five bands in the 590-730 nm range are observed. The most intense is the ⁵D₀ → ⁷F₂ red emission at 612 nm. The changes of the structure due to the presence of one metal cation or another, i.e. the difference in the site distribution of Ca²⁺ and Ln³⁺ (including the luminescent dopant ones), lead to small differences of the average crystal field of the cations in each material [18]. These variations are responsible for the little changes observed in the blue/green (B/G) and orange/red (O/R) ratios of Tb³⁺ and Eu³⁺ emissions respectively, thus affecting the emission colour of the material. In Table 3 the B/G and O/R ratios calculated from the spectra for both gadolinium and yttrium-based compounds are presented.

 

Fig. 5 (a) Room temperature emission spectra of Ca₃Y_1.98Tb_0.02Si₃O₁₂ (λ_exc = 377 nm) and Ca₃Y_1.98Eu_0.02Si₃O₁₂ (λ_exc = 393 nm). The spectra are normalized to Tb^{3+ 5}D₄→⁷F₅ emission, and Eu^{3+ 5}D₀→⁷F₂ emission respectively. (b) Room temperature emission spectra of Ca₃Y_2-x-yTb_xEu_ySi₃O₁₂ measured upon excitation at 378 nm. All spectra are normalized to Tb^{3+ 5}D₃→⁷F₅ emission.

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Table 3. Calculated values for B/G and O/R ratios for the Ca₃A_1.98Tb_0.02Si₃O₁₂ (A = Gd, Y), Ca₃A_1.98Eu_0.02Si₃O₁₂ (A = Gd, Y) phosphors.

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Starting from the results obtained for the gadolinium silico-carnotite, a series of Ca₃Y_2-x-yTb_xEu_ySi₃O₁₂ (x = 0.02, y = 0.005; x = 0.015, y = 0.003 and x = 0.01, y = 0.005) samples have been prepared. The emission spectra of the powders (λ_exc = 378 nm) are presented in Fig. 5(b). As it was observed for the gadolinium-based phosphors, the spectra present emission bands originating from the ⁵D₃ and ⁵D₄ levels of Tb³⁺ (blue to yellow range), and from the ⁵D₀ level of Eu³⁺ (orange to red range).

The CIE XYZ chromaticity coordinates diagram is shown in Fig. 6. The calculated values of CIE coordinates are reported in Table 4. As done before and for completeness, CIE values for Ca₃Y_1.98Tb_0.02Si₃O₁₂ and Ca₃Y_1.98Eu_0.02Si₃O₁₂ are also included in both diagram and table. In this case, control over the doping ratio allows obtaining a final emission colour that varies from pinkish-white to yellowish-white in the co-doped materials.

 

Fig. 6 CIE diagram coordinates of Ca₃Y_1.98Tb_0.02Si₃O₁₂, Ca₃Y_1.98Tb_0.02Si₃O₁₂ and Ca₃Y_2-x-yTb_xEu_ySi₃O₁₂ excited at 377, 378 and 393 nm respectively.

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Table 4. Calculated values for CIE coordinates of Ca₃Y_1.98Tb_0.02Si₃O₁₂ and Ca₃Y_2-x-yTb_xEu_ySi₃O₁₂ excited at 378 nm and Ca₃Y_1.98Eu_0.02Si₃O₁₂ excited at 393 nm.

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The excitation spectra present no significant differences with the ones measured for the gadolinium-based materials. Also in this case, no evidence of Tb³⁺-Eu³⁺ energy transfer process has been found. The same considerations made for the gadolinium phosphors apply here.

Decay curves for Tb³⁺ and Eu³⁺ emission were measured upon excitation at 378 nm. The obtained time constants are reported in Table 5. In the co-doped phosphors there is a clear rise at short times in the decay curves for Tb³⁺ emission from ⁵D₄, as observed in the gadolinium samples, and the decay constant is also much longer than in concentrated Tb³⁺ systems. Since in these compounds the Tb³⁺ concentration is also low, all the considerations discussed in the case of the gadolinium host apply in this case. The decay curves for Tb³⁺ emission from ⁵D₃ have been also measured. As observed previously for the gadolinium phosphors, curves are not exponential and the decay constant is about 1 ms. Again, and due to the low levels of doping, decay constant is not affected by Eu³⁺ concentration (not for blue or green emission).

Table 5. Decay data for the luminescent levels of Tb³⁺ and Eu³⁺ in several yttrium hosts upon UV excitation.

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Regarding decay curves for Eu^{3+ 5}D₀ emission for the co-doped samples, the curves are also well fitted by a single exponential function, and the time constants calculated from them are similar to the obtained for the gadolinium phosphors. The values are reported in Table 5. As well as it was observed before, there are no significant changes when varying Eu³⁺ concentration.

4. Conclusions

In this work the luminescence spectra and decay curves of several Ca₃Gd_2-x-yTb_xEu_ySi₃O₁₂ and Ca₃Y_2-x-yTb_xEu_ySi₃O₁₂ powders have been systematically studied. Experiments for various Tb³⁺ and Eu³⁺ doping, and co-doping concentration have been performed. The obtained results indicate that the ⁵D₄ level can be populated from ⁵D₃ via a multiphonon relaxation process, while ⁵D₃-⁵D₄ cross relaxation does not pay a relevant role. Therefore, in this case quenching of blue emission is avoided. No evidence of Tb³⁺-Eu³⁺ energy transfer process has been found in any of the co-doped systems as a result of the low doping levels, that do not favour the Tb³⁺-Tb³⁺ energy migration, nor direct Tb³⁺-Eu³⁺ transfer. The obtained results show that for the present samples the emission colour strongly depends on the Tb-Eu ratio, and not on the total doping concentration of each ion. Controlling the doping ratio leads to close-to-white emission in various silico-carnotite compounds, opening an interesting opportunity in the search for novel phosphors for white-LED excited in the UV.