Tunable photoluminescence in Lu₃Al₅O₁₂–Lu₂CaMg₂Si₃O₁₂ solid solution phosphors manipulated by synchronous ions co-substitution

Abstract

Developing solid-solution phosphors with diversified luminescence and consistent host structure can effectively improve the uniformity of the powder during the mixing process and thus enhance the performance of LEDs. Herein, “synchronous ions co-substitution” is applied as a principle to design and fabricate a series of Ce³⁺ activated Lu₃Al₅O₁₂–Lu₂CaMg₂Si₃O₁₂ solid-solution phosphors. The emission peaks of the as-prepared phosphors show a continuous red shift from 511 to 585 nm which can be easily tuned by adjusting the proportion of ions co-substitution, and the corresponding red shifting mechanism has been discussed in detail. The quantum yields of the phosphors are calculated to be 30–60%, and show excellent thermal photoluminescence stability up to 200 °C. The color-tunable Lu₃Al₅O₁₂–Lu₂CaMg₂Si₃O₁₂ solid-solution phosphors manipulated by synchronous ions co-substitution can be regarded as promising luminescent candidates for LEDs.

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1. Introduction

Solid state lighting (SSL) is considered to be one of the most important revolutions in the 20th century; it dramatically changed the lighting and display industries and produces light sources with outstanding performance and functionality.^1–3 This revolution also stimulates the development of numerous potential phosphors for SSL applications. Particularly, garnet phosphors are definitely the most important and useful family because of their excellent physical stability and high luminescent efficiency.^4–6 Generally, the commercial white light emitting diode (LED) is composed of a blue chip and a yellow Y₃Al₅O₁₂:Ce³⁺ garnet phosphor.^7,8 However, most of the garnet phosphors emit green-yellow light in the relatively narrow wavelength range of 520–550 nm, such as Gd₃Al₅O₁₂:Ce³⁺,⁹ Ca₃Sc₂Si₃O₁₂:Ce³⁺,^10,11 Lu₃Al₅O₁₂:Ce³⁺,^12,13 etc. As a result, many strategies to extend the emission scope of garnet phosphors were proposed. For example, Setlur etc.¹⁴ prepared Lu₂CaMg₂Si₃O₁₂:Ce³⁺ garnet phosphor and found that this phosphor can maintain almost the same structure with Y₃Al₅O₁₂, while the emission band showed an obvious red shift to ∼600 nm. This is an encouraging sign for developing new garnet phosphors to meet various spectral demands. However, the host composition design principle of this new matrix and why the matrix with such a different chemical composition can maintain almost the same structure as Y₃Al₅O₁₂ still have not been solved.

Recently, the solid solution phosphors have attracted intense attention. Notably, the phosphors often show a variety of luminescence properties due to the evolution of their compositions and crystal structures. Xia etc. have reported several kinds of solid solution phosphors with linear structural evolution and tunable photoluminescence properties, such as Lu_xSr_2−xSiN_xO_4−x:Eu²⁺,¹⁵ La₅(Si_2+xB_1−x)(O_13−xN_x):Ce³⁺ (ref. 16) and (Na_1−xCa_x)(Sc_1−xMg_x)Si₂O₆:Eu²⁺.¹⁷ In particular, they proposed a new host design principle as a universal strategy named “chemical unit co-substitution”, referring to the replacement of a structural unit where the overall sum of the oxidation state of the new unit is the same, and verified this based on experimental and theoretical analysis.¹⁸ This is a new strategy toward forming solid solutions, which can be further used to discover new phases and tune the properties of existing materials.

In view of the structure and luminescence properties of the Ce³⁺ activated Lu₂CaMg₂Si₃O₁₂ phosphor, we propose that it will be a promising candidate in a garnet solid solution system. The host composition design principle can be identified to be “ions co-substitution”, which refers to replacing two ions concurrently with another two ions based on charge balance. For instance, Katelnikovas etc. co-substituted “Al³⁺–Al³⁺” ions with “Mg²⁺–Si⁴⁺” ions in Y₃Al₅O₁₂:Ce³⁺ phosphor to maintain the garnet structure and resulted in a giant long-wavelength shift of the emission band.¹⁹ Jia etc. replaced “Lu³⁺–Al³⁺” ions with “Mg²⁺–Si⁴⁺” ions in Lu₃Al₅O₁₂ to form a new single garnet phase, Mg_1.5Lu_1.5Al_3.5Si_1.5O₁₂, which presents approximately the same crystallographic data as Lu₃Al₅O₁₂ while a blue shift luminescence can be observed when activated by Ce³⁺.²⁰ However, the specific host design scheme and the structural and luminescence evolution of the Lu₂CaMg₂Si₃O₁₂ garnet solid solution system remains a great challenge.

In this paper, we deduce that the Lu₂CaMg₂Si₃O₁₂ matrix is derived from the Lu₃Al₅O₁₂ garnet matrix and the two are end members of a Lu₃Al₅O₁₂–Lu₂CaMg₂Si₃O₁₂ solid-solution system. The host design scheme we propose is “synchronous ions co-substitution”. The general stoichiometric formula of the garnet is {A}₃[B]₂(C)₃O₁₂, where {A}, [B], and (C) denote sites with dodecahedral, octahedral and tetrahedral coordination, respectively. For Lu₃Al₅O₁₂, Lu³⁺ ions occupy dodecahedral sites, two of the Al³⁺ ions occupy octahedral sites and three of the Al³⁺ ions occupy tetrahedral sites. Considering the relationship between the compositions of Lu₃Al₅O₁₂ and of Lu₂CaMg₂Si₃O₁₂, we constructed two different types of ions co-substitution synchronously: substitute “Al³⁺(octahedral)–Al³⁺(tetrahedral)” ions with “Mg²⁺–Si⁴⁺” ions and substitute “Lu³⁺(dodecahedral)–Al³⁺(tetrahedral)” ions with “Ca²⁺–Si⁴⁺” ions. Accordingly, a series of Ce³⁺ activated Lu₃Al₅O₁₂–Lu₂CaMg₂Si₃O₁₂ solid solution garnet phosphors were successfully prepared based on the “synchronous ions co-substitution” scheme. Both regular structure evolvement and tunable photoluminescence by adjusting the proportion of ions co-substitution were observed. The relationships of the structure and the photoluminescence properties of these phosphors were also further investigated.

2. Experimental section

Chemicals and materials

CaCO₃, Mg(OH)₂·4MgCO₃·6H₂O, Al(OH)₃, H₃BO₃, LiF, were purchased from Guangzhou Chemical Reagent Co., Ltd. (Guangzhou, China), SiO₂ (99.99%), Lu₂O₃ (99.9%) and CeO₂ (99.99%) were purchased from Aladdin Industrial Inc. (Shanghai, China). All the reagents are of analytical grade and were used directly without further purification.

Synthesis of solid-solution phosphors

All of the samples were synthesized by solid state (SS) reactions. Stoichiometric amounts of raw materials were mixed and ground in an agate mortar, 5 wt% of H₃BO₃ and LiF (1 : 1 weight ratio) were added as a flux. Then the mixture was sintered at 1250–1550 °C for 5 h in a reducing atmosphere created by burning activated carbon. The as-synthesized phosphors were cooled down to room temperature and crushed into powder. The chemical formula of the target compound turns out to be Lu_1.97Ce_0.03Lu_1−xCa_xAl_4−2yMg_ySi_yAl_1−xSi_xO₁₂ (0 ≤ x ≤ 1, 0 ≤ y ≤ 2) and chemical compositions of the prepared samples are listed in Table 1.

Table 1 The chemical compositions and the results of Rietveld refinement of the prepared samples

	x	y	a, Å	Rwp, %	Rp, %
Lu3Al5O12	0	0	11.893	10.55	7.35
Lu2.77Ce0.03Ca0.2Al4Mg0.4Si0.6O12	0.2	0.4	11.916	8.82	6.20
Lu2.57Ce0.03Ca0.4Al3Mg0.8Si1.2O12	0.4	0.8	11.929	9.22	6.50
Lu2.37Ce0.03Ca0.6Al2Mg1.2Si1.8O12	0.6	1.2	11.941	12.28	8.25
Lu2.17Ce0.03Ca0.8AlMg1.6Si2.4O12	0.8	1.6	11.949	13.50	8.99
Lu1.97Ce0.03CaMg2Si3O12	1.0	2.0	11.961	13.60	9.32

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Characterization

X-ray powder diffraction (XRD) patterns of the products were recorded on a PANalytical PW3040/60 X-ray power diffractometer with Cu Kα radiation (λ = 1.5405 Å). Rietveld analysis of XRD profiles was made by the general structure analysis system (Materials Studio) program. The surface morphology and crystal microstructure of the samples were examined using a field emission scanning electron microscope (FE-SEM, JEOL JSM-7001F) and a transmission electron microscope (TEM, JEM 2100F). Quantum yields, photoluminescence (PL) and photoluminescence excitation (PLE) signals of the samples at room temperature were detected by a HITACHI F-7000 spectrometer. PL spectra at high temperatures were measured by a FLS920 combined fluorescence lifetime and steady state spectrometer from EDINBURGH Inc. The UV-vis diffuse reflectance spectra (UV-vis DRS) of samples were obtained over a UV-vis spectrophotometer (Cary 300) using BaSO₄ as a reference. Thermogravimetric analysis (TGA, SDT 2960) was used to estimate the thermal stability of the as-prepared samples.

3. Results and discussion

As presented in Table 1, the chemical composition of the matrix turned from Lu₃Al₅O₁₂ to Lu₂CaMg₂Si₃O₁₂ with an increasing proportion of ions co-substitution. However, the diffraction peaks of all the products can be indexed with the standard XRD patterns of Lu₃Al₅O₁₂ (PDF#73-1368), as shown in Fig. 1, indicating the products are single-phase and a Lu₃Al₅O₁₂–Lu₂CaMg₂Si₃O₁₂ solid-solution system was formed. Therefore, it is reasonable to believe that the “Mg²⁺–Si⁴⁺” ions and “Ca²⁺–Si⁴⁺” ions are entirely dissolved in the garnet matrix, where Ca²⁺ ions occupy the dodecahedral sites,¹⁴ and Mg²⁺ and Si⁴⁺ occupy the octahedral and tetrahedral sites, respectively. Thus, we deduce that the Lu₂CaMg₂Si₃O₁₂ matrix can be derived from the original Lu₃Al₅O₁₂ matrix by substituting “Al³⁺(octahedral)–Al³⁺(tetrahedral)” ions and “Lu³⁺(dodecahedral)–Al³⁺(tetrahedral)” ions with “Mg²⁺–Si⁴⁺” ions and “Ca²⁺–Si⁴⁺” ions synchronously on the basis of charge balance. Furthermore, the strongest XRD peaks of the samples shift towards a lower 2 theta angle when the proportion of ions co-substitution increases, indicating the gradual expansion of the unit cell. This is also consistent with the results of Rietveld refinement shown in Table 1: the lattice parameters of all the prepared samples increase step by step from the Lu₃Al₅O₁₂ end member to the Lu₂CaMg₂Si₃O₁₂ end member; the stepwise shifting of the lattice parameters can also demonstrate the continuous structure evolution of the Lu_1.97Ce_0.03Lu_1−xCa_xAl_4−2yMg_ySi_yAl_1−xSi_xO₁₂ (0 ≤ x ≤ 1, 0 ≤ y ≤ 2) solid solution samples. In addition, with the increasing proportion of ions co-substitution, the crystallinity of the samples gradually decreases, which can be inferred from the gradual decline of the X-ray diffraction peak intensities as shown in Fig. 1. This can be explained by the substitution of the larger Ca²⁺ (99 pm) for the smaller Lu³⁺ (84.8 pm) at the dodecahedral sites and the larger Mg²⁺ (72 pm) for the smaller Al³⁺ (53.5 pm) at the octahedral sites. Due to the difference of the ion radii, the larger structural distortion can be expected,²¹ so the phase structure of the samples becomes more and more expanded and incompact from Lu₃Al₅O₁₂ to Lu₂CaMg₂Si₃O₁₂.

Fig. 1 XRD patterns of the Lu_1.97Ce_0.03Lu_1−xCa_xAl_4−2yMg_ySi_yAl_1−xSi_xO₁₂ (0 ≤ x ≤ 1, 0 ≤ y ≤ 2) samples.

The morphology and detailed microstructure information of typical samples (x = 0, 0.6, 1.0) were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The samples show a granular morphology with obvious agglomeration (Fig. S1†) because the hardness of the SS samples was extremely high, and the particle morphology of phosphor was seriously destroyed during the grinding treatment. The high-resolution transmission electron microscopy (HRTEM) images and the fast Fourier transform (FFT) images are shown in Fig. 2; the distinct lattice fringe reveals the relatively high crystallinity of our samples. It can be found that the lattice fringes with a d spacing of 0.3198, 0.4923, 0.2707 nm, which can be assigned respectively to the (321), (211), (420) planes of the Lu_1.97Ce_0.03Lu_1−xCa_xAl_4−2yMg_ySi_yAl_1−xSi_xO₁₂ samples with x = 0 (Fig. 2(c)), 0.6 (Fig. 2(f)) and 1 (Fig. 2(i)). In order to investigate the relationship in the microstructures of the samples, we tried to transfer the d spacing in different orientations into the same orientation of the (420) plane based on the classic Bragg equation.¹⁷ The values were calculated to be 0.2675, 0.2696, 0.2707 nm for the samples with the composition of x = 0, 0.6 and 1. The increasing d spacing with the x value can be ascribed to the increase of the cell volume, which is consistent with the XRD analysis. Furthermore, the continuous structure evolution of the Lu_1.97Ce_0.03Lu_1−xCa_xAl_4−2yMg_ySi_yAl_1−xSi_xO₁₂ (0 ≤ x ≤ 1, 0 ≤ y ≤ 2) solid solution samples can also be demonstrated by the gradually increasing d spacing.

Fig. 2 (a, d and g) TEM images; (b, e and h) high-resolution TEM images and (c, f and i) fine lattice fringes with FFT images (insets) of Lu_1.97Ce_0.03Lu_1−xCa_xAl_4−2yMg_ySi_yAl_1−xSi_xO₁₂ (x = 0, 0.6, 1) samples.

A regular and specific variation of the photoluminescence properties related to the unique structural characteristics is illustrated in Fig. 3. In a typical luminescence process when Ce³⁺ is applied as a luminous center in a garnet matrix, the 5d state of Ce³⁺ is strongly affected by the crystal field of the host matrix and splits into several 5d sub-bands;^22,23 the stronger crystal field and the wider splitting gap of the 5d state can be observed. So the excitation spectrum of Ce³⁺ in the Lu₃Al₅O₁₂ matrix always contains two bands, a weaker band at ∼350 nm and a stronger band at ∼450 nm, which results from the absorption of electrons from the 4f¹ ground state to the upper and lower excited 5d levels (²D_3/2, ²D_5/2).²⁴ From Fig. 3(a) and (c), we can notice that the weaker band of the excitation spectra of these samples presents a blue shift whereas the stronger band shows a red shift with an increasing proportion of ions co-substitution. It can be calculated from the difference of the two excitation peaks that the energy gap between the excited 5d levels becomes wider as the proportion of ions co-substitution increases, which can serve as an important proof of a stronger crystal field splitting brought by the ions co-substitution. Usually, the emission spectra of Ce³⁺ in the Lu₃Al₅O₁₂ matrix consist of a broad asymmetric band from 450 to 625 nm with a maximum at 511 nm, which can be decomposed into two transitions from the lowest 5d excitation state to ²F_5/2 and ²F_7/2 ground states, respectively.²⁵ In this work, an obvious red shift of the emission bands can be observed after ions co-substitution, and the emission peak shifts from 511 nm to 585 nm in the whole solid-solution range from the Lu₃Al₅O₁₂ end member to the Lu₂CaMg₂Si₃O₁₂ end member, whereas the photoluminescence intensity decreases, as shown in Fig. 3(b) and (d). The red shift can be ascribed to a lower transition energy between the lowest 5d state and the 4f ground state. It is well known that the position of the lowest 5d level is dependent on two factors: the centroid shift (CS) of the 5d state from the free ion level and the crystal field splitting (CFS) of the 5d excitation state.^26,27 The Ce³⁺ centroid shift in Lu₂CaMg₂Si₃O₁₂ was estimated to be ∼13 300 cm⁻¹, smaller than ∼14 300 cm⁻¹ for Lu₃Al₅O₁₂.¹⁴ So we deduced that the wavelength variation of the photoluminescence may be mainly attributed to the stronger crystal field splitting of the 5d excitation state, which originates from the structure evolution after ions co-substitution. Accordingly, the energy levels of Ce³⁺ in the solid-solution range are listed in Fig. 4. From Lu₃Al₅O₁₂ to Lu₂CaMg₂Si₃O₁₂, the CF becomes decreasingly smaller while the CFS increases dramatically larger, so the position of the lowest 5d excitation level of Ce³⁺ shows a decreasing trend and the emission spectra shifts to a longer wavelength. The CIE chromaticity coordinates and photographs of these samples were also displayed in Fig. 5 and the variation of the emission colors depending on host composition can be clearly observed, suggesting that we can tune the emission from green to orange by manipulating the proportion of ions co-substitution conveniently according to different requirements.

Fig. 3 (a and c) PLE spectra (λ_em = 511–585 nm); (b) PL spectra (λ_ex = 445–468 nm); (d) the emission peak wavelength (marked by circles) and intensities (marked by squares) of Lu_1.97Ce_0.03Lu_1−xCa_xAl_4−2yMg_ySi_yAl_1−xSi_xO₁₂ (0 ≤ x ≤ 1, 0 ≤ y ≤ 2) samples.

Fig. 4 Energy level scheme of Ce³⁺ in Lu₃Al₅O₁₂–Lu₂CaMg₂Si₃O₁₂ solid-solution phosphors.

Fig. 5 CIE coordinates of Lu_1.97Ce_0.03Lu_1−xCa_xAl_4−2yMg_ySi_yAl_1−xSi_xO₁₂ samples ((a) x = 0, y = 0; (b) x = 0.2, y = 0.4; (c) x = 0.4, y = 0.8; (d) x = 0.6, y = 1.2; (e) x = 0.8, y = 1.6; (f) x = 1, y = 2).

Since quantum yield is always considered as an important criterion for potential application in LEDs, we have also measured the quantum yields of these as-prepared samples at room temperature, and the values are 56.9%, 45.0%, 41.2%, 36.6%, 33.1% and 31.1% for each phosphor with different x values, as shown in Fig. 6(h). The quantum yields are not as high as the commercially used Y₃Al₅O₁₂:Ce³⁺ garnet phosphor; however, it is notable that the quantum yields could be enhanced by optimizing the experimental process, improving the degree of crystallinity and controlling the morphology of the phosphors. Further development of the photoluminescence properties can be expected. It is found that the quantum yields of this series of solid-solution phosphors show a decreasing trend from the Lu₃Al₅O₁₂ end member to the Lu₂CaMg₂Si₃O₁₂ end member, just like the sequence of the photoluminescence intensity at room temperature, as shown in Fig. 3(b). In recent years, many researchers have confirmed that a high photoluminescence quantum yield of the phosphors requires the Ce³⁺ ion to be hosted in a rigid crystal structure, for example, Ca₃Zr₂SiGa₂O₁₂:Ce³⁺,²⁸ Li₄SrCa(SiO₄)₂:Ce³⁺ (ref. 29) and M[Mg₃SiN₄]:Ce³⁺/Eu²⁺ (M = Ca, Sr),³⁰ etc. As discussed above, the phase structure of the samples becomes increasingly expanded and incompact from Lu₃Al₅O₁₂ to Lu₂CaMg₂Si₃O₁₂. The unconsolidated and expanded structure will enhance the thermal vibration of the crystal lattice, then the probability of nonradiative transition will increase and result in the decline of the quantum yields.

Fig. 6 (a–f) Thermal quenching curves of the phosphors; (g) the relative intensity of the phosphors at 200 °C compared to the value at 20 °C; (h) the quantum yields of the phosphors at room temperature.

It is well known that when an LED is working, the temperature of the phosphor layer will increase to 120–150 °C because of the heat generated at the p–n junction in a LED chip.^31,32 So the chemical and photoluminescence stability of the phosphors at high temperature are considered as the most important parameters for evaluating the practical application possibilities of the luminescence materials used in LEDs. For our samples, TGA measurements exhibited that the weight of the as-prepared samples did not show any obvious reduction when the samples were heated from room temperature to 750 °C (Fig. S2†), which confirmed the excellent chemical stability of the samples at high temperature. In addition, the temperature dependent luminescence spectra of the Lu₃Al₅O₁₂–Lu₂CaMg₂Si₃O₁₂ solid-solution samples were also measured at a temperature range of 20–200 °C (Fig. 6(a)–(f)). As the temperature increases, the peaking wavelength of the emission spectra stays almost constant (Fig. S3†), suggesting the stability of the emission color when applied in a high power LED. However, the luminescence intensity of the samples decreases gradually with increasing temperature. At 200 °C, the relative emission intensity of the samples is found to be 75.4%, 71.9%, 70.4%, 66.3%, 64.4% and 57.9% of the initial value, as shown in Fig. 6(g). The quenching temperature T_q (the temperature at which the emission intensity is half of the initial value at room temperature ∼25 °C) must be above 200 °C, being higher than the LED working temperature. In other words, both the structure and photoluminescence stability of this series of phosphors can meet the candidate requirements for high power LED lamps.

It is worth noting that the thermal photoluminescence stability of the solid solution phosphors becomes increasingly degraded from Lu₃Al₅O₁₂ to Lu₂CaMg₂Si₃O₁₂. The host band gap (E_g) has been suggested to play an important role in thermal quenching of photoluminescence.^33,34 To ensure the relationship between the thermal quenching of photoluminescence and the width of the band gaps, the UV-vis diffuse reflectance of the representative phosphors in the solid solution system was measured in Fig. 7. The band gaps of these phosphors were approximately determined to be 2.67, 2.83, 2.95 eV for the samples with x = 0, 0.6, 1.³⁵ Combined with the thermal quenching properties, an empirical rule can be found that the thermal quenching of photoluminescence becomes increasingly enhanced as the width of the host band gap decreases. The result is also consistent with previous reports^36–38 and it can be explained like this: since the quenching of the 5d → 4f luminescence transition of Ce³⁺ or Eu²⁺ at high temperatures can be attributed to thermally activated ionization of the 5d electron to conduction band states,^39,40 the energy gap between the 5d electron excitation state and conduction band of the host crystal, designated as ΔE_dC in Fig. 8, plays an important role in estimating the thermal quenching properties. The wider the gap is, the weaker the photoluminescence quenching is as temperature rises. From this point, a relatively wider E_g is clearly advantageous for improving the thermal quenching properties of Ce³⁺ or Eu²⁺ in a series of phosphors with similar crystal structures and photoluminescence mechanism.

Fig. 7 UV-vis diffuse reflectance spectra of representative samples in the Lu₃Al₅O₁₂–Lu₂CaMg₂Si₃O₁₂ solid-solution system.

Fig. 8 The energy level scheme of the thermal quenching process, E_C and E_V refer to the energy levels of the conduction band and valance band of host crystal respectively, E_d, E_f refer to the energy levels of the 5d excitation state and the 4f ground state of the activated ion, respectively.

In order to further clarify the effect of ions co-substitution on photoluminescence properties, we have carried out the two types of ions co-substitutions separately and the results are shown in Fig. 9. As can be seen from the XRD diagram in Fig. 9(a), after the replacement of “Al³⁺(octahedral)–Al³⁺(tetrahedral)” ions with “Mg²⁺–Si⁴⁺” ions (x = 0, y = 2), the garnet structure is basically destroyed. Many previous works have also reported the phenomenon that the replacement of “Al³⁺(octahedral)–Al³⁺(tetrahedral)” ions with “Mg²⁺–Si⁴⁺” ions can make the emission spectra move towards a longer wavelength, but the most difficult problem is that the flexibility of the garnet lattice is limited, so that the introduction of “Mg²⁺–Si⁴⁺” ions often leads to serious deformation of the crystal lattice.^14,41,42 When co-substituting “Lu³⁺(dodecahedral)–Al³⁺(tetrahedral)” ions with “Ca²⁺–Si⁴⁺” ions (x = 1, y = 0), the situation is quite the opposite: not only can the original garnet crystal structure be maintained, but even some tiny impure phases in the original sample disappear. However, the PL spectra in Fig. 9(b) show there is no significant change in the profile of the emission spectra after “Ca²⁺–Si⁴⁺” ions replacement. That is to say, the co-substitution of “Ca²⁺–Si⁴⁺” ions is beneficial to the forming of the garnet crystal lattice, but has little effect on changing the emission color. The interesting thing is that when we launched the two kinds of ions co-substitution simultaneously (x = 1, y = 2), not only was the garnet crystal structure stable, but also a red shift of the luminescence could be observed (Fig. 9). Therefore, we believe that this is caused by the synergistic effect from the synchronous co-substitution of “Ca²⁺–Si⁴⁺” ions and “Mg²⁺–Si⁴⁺”ions; the introduction of “Ca²⁺–Si⁴⁺” ions may suppress the distortion of the crystal lattice and the problem brought by “Mg²⁺–Si⁴⁺” ions can be somehow resolved.

Fig. 9 (a) XRD patterns and (b) PL spectra of Lu₂Lu_1−xCa_xAl_4−2yMg_ySi_yAl_1−xSi_xO₁₂ samples.

4. Conclusions

A series of Ce³⁺ activated Lu₃Al₅O₁₂–Lu₂CaMg₂Si₃O₁₂ solid-solution phosphors has been successfully prepared by a SS method under the synchronous ions co-substitution principle and the properties of these phosphors, such as crystal structure, color and intensity of the photoluminescence, quantum yields and thermal photoluminescence stability, change in a regular sequence in the solid-solution range. The results indicate that tunable photoluminescence from green to orange can be easily manipulated by the proportion of synchronous ions co-substitution and this kind of solid-solution system can be regarded as a good example of the well-known structure–property relationship. It is beneficial for the design and fabrication of phosphor materials with different requirements and will somehow accelerate the discovery and improvement of various fluorescent materials.

Acknowledgements

This work was supported by the Natural Science Foundations of China (Grant No. 21306030, 21576056, and 21576057), Program Foundation of the second batch of innovation teams of the Guangzhou Bureau of Education (Grant No. 13C04), Scientific Research Project of the Guangzhou Municipal Colleges and Universities (Grant No. 1201431340, 1201410618), and the Innovation Training Project of College Students in Guangzhou University (201511078032).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05537k

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