Synthesis, structure and luminescent properties of a new blue-green-emitting garnet phosphor Ca₂LuScZrAl₂GeO₁₂:Ce³⁺

Abstract

A series of new garnet phosphors, Ca₂Lu_1−xZrScAl₂GeO₁₂:xCe³⁺ (0.005 ≤ x ≤ 0.1), have been successfully synthesized through a conventional high-temperature solid-state reaction method. The crystal structure and electronic structure of the host, and the morphology, luminescence property, diffuse reflectance spectra, fluorescence lifetime and thermal stability of the phosphors are investigated in this article. The crystal structure was characterized by X-ray diffraction Rietveld refinement and it belongs to the Ia3̄d(230) space-group. The band gap of this matrix is about 5.15 eV, which is much narrower than that of YAG, enhancing the possibility of photo-ionization. The optimized phosphor can be excited by UV or near-UV light in the range of 320–420 nm and exhibits a broad blue-green emission centering at 472 nm under 405 nm excitation. In addition, the concentration quenching mechanism and differentiation of dodecahedral sites were discussed in detail.

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1 Introduction

White light-emitting diodes (w-LEDs) have been widely used in the illumination and display fields due to their excellent properties, such as energy saving, long lasting life, fast start up etc.^1–3 Nowadays, the combination of blue InGaN LED chips with yellow garnet phosphor YAG:Ce³⁺ is the main route to fabricate w-LEDs.^4,5 However, these lamps are inherently limited to low color-rendering index (CRI) and high correlated color temperature (CCT) due to the red emission deficiency in the visible spectrum.⁶ Meanwhile, the application of w-LEDs has an intense blue emission, which easily causes sleep disorders and harms human health.⁷ In order to solve the above problems, combining tricolor phosphors with near ultraviolet (n-UV, 380–420 nm) LED chips to fabricate w-LEDs is considered as a promising route to cover indoor lighting needs for their better color-rendering ability, lower correlated color temperature, and tunable blue emission intensity.^8–10

It is well known that Ce³⁺ activated garnet phosphors always have strong and broad absorption in the blue or n-UV light region ascribed to transition of 4f to 5d parity allowed electric dipole. Moreover, the photoluminescence properties of Ce³⁺ strongly depend on crystal-field splitting of the 5d state and the nephelauxetic effect, which lead to the emission peak of Ce³⁺ varying from blue to red region.¹¹ Furthermore, the garnet compound has an outstanding physicochemical stability and an opened framework, for which, it is extensively investigated as the matrix material for phosphor. It can be demonstrated by a general formula {A}₃[B]₂(X)₃O₁₂, where A, B and X indicate cations which are coordinated by eight, six and four O²⁻ ions forming dodecahedron, octahedron and tetrahedron, respectively.^12–14 Moreover, A, B and X sites can be substituted by different cations, thus a variety of new matrix materials for phosphors have been designed by single- or multi-ions substitution, for instance, substituting Y³⁺ by other rare earth cations (Lu³⁺, Gd³⁺, Tb³⁺, etc.),^15–19 Mg²⁺–Si⁴⁺/Ge⁴⁺ replacing Al³⁺–Al³⁺,^13,20,21 or Si⁴⁺–N³⁻ superseding Al³⁺–O²⁻ in the YAG:Ce³⁺,²² etc. Thereupon, more and more efficient phosphors have been successively developed, such as YAG:Ce³⁺,⁵ Ca₃Sc₂Si₃O₁₂:Ce³⁺,¹² Lu₂CaMg₂(Si,Ge)₃O₁₂:Ce³⁺,¹³ etc.

Motivated by the above investigations, some Ce³⁺ doped new garnet phosphors, which can be excited by near-UV LED chip, have been successively designed and synthesized.^23,24 Typically, Zhong et al. replaced Sn⁴⁺ with Zr⁴⁺ in garnet host Ca₃Sn₂SiGa₂O₁₂ and obtained a new blue-green-emitting phosphor Ca₃Zr₂SiGa₂O₁₂:Ce³⁺,²⁵ and Wang et al. reported a new cyan-green garnet phosphor Ca₂YZr₂Al₃O₁₂:Ce³⁺,²⁶ etc.

Zhong et al. summarized the B site tends to be filled by a single kind of atom, which is helpful to maintain the cubic structure with minimum lattice stress.¹¹ However, in this work, we extend another approach, applying the Diagonal Relationship theory, to design a new garnet matrix. As we all known, Sc/Zr are diagonal elements in periodic table of the elements, respectively, which have the similar chemical properties and ionic radius based on the Diagonal Relationship theory.²⁷ Herein, the A site is introduced with Ca and Y; B site with Sc and Zr, and X sites with Al and Ge. However, A site with larger cations generally destabilize the garnet structure.²¹ So we replaced Y³⁺ with smaller Lu³⁺ ions. Finally, a new blue-green-emitting garnet phosphor Ca₂LuScZrAl₂GeO₁₂:Ce³⁺ was successfully synthesized. The structures, electronic structure, photoluminescence properties and related mechanisms of the phosphor were investigated in detail.

2 Experimental

2.1 Materials and synthesis

The powder samples Ca₂Lu_1−xScZrAl₂GeO₁₂:xCe³⁺ (0 ≤ x ≤ 0.1) were synthesized by high-temperature solid-state reaction method. The raw materials CaCO₃ (99.9%), Lu₂O₃ (99.99%), Sc₂O₃ (99.99%), ZrO₂ (99.99%), Al₂O₃ (99.99%), GeO₂ (99.99%) and CeO₂ (99.99%), were all weighed out as the stoichiometry, and thoroughly mixed in an agate mortar. The powder mixtures were put in an alumina crucible and continually fired at 1400 °C in a reducing atmosphere (CO) for 4 h. Finally, the samples were cooled to room temperature, and ground in an agate mortar.

2.2 Calculation methods

In our work, density functional theory (DFT) method is used for structural relaxation and electronic structure calculation. The ion–electron interaction is treated by the projector augmented-wave²⁸ technique as implemented in the Vienna ab initio simulation package (VASP).²⁹ The exchange-correlation potential is treated by PBE.³⁰ For structural relaxation, all the atoms are allowed to relax until atomic forces are smaller than 0.01 eV Å⁻¹. The k-mesh is generated by the Monkhorst–Pack scheme, which depends on the lattice constant.

2.3 Characterization methods

The phase formation and crystal structure of the as-prepared samples were analyzed by the X-ray powder diffractometer (Rigaku, Japan) with Co-Ka radiation (λ = 0.17889 nm). The data were collected over a 2θ range from 10° to 90° with scanning speed of 6° 2θ min⁻¹ for the series Ca₂Lu_1−xScZrAl₂GeO₁₂:xCe³⁺ (0 ≤ x ≤ 0.1) samples and 10–90° at intervals of 0.02° with a count time of 5 s per step for the structural refinement data (with Cu-Ka radiation (λ = 0.15418 nm)). The structural parameters of Ca₂LuScZrAl₂GeO₁₂ garnet was refined by the Rietveld method using the Fullprof software.³¹ The morphology of the samples were obtained using scanning electron microscopy (SEM; Hitachi SUI 510).

The photoluminescence spectra were measured by using a FLS 920 spectrofluorometer of Edinburgh Instruments with a monochromated Xe lamp as an excitation source. The thermal quenching were measured by a spectrofluorometer (Fluoromax-4, Edison, U.S.A.), which are composed of a Xe high-pressure arc lamp, a photomultiplier tube, and a heating apparatus. The reflectance spectra were measured by a Shimadzu UV-3600 Plus UV-VIS-NIR Spectrophotometer with an integrated sphere attachment, and BaSO₄ was used as a white standard. Quantum efficiency was measured using the integrating sphere on the QE-2100 quantum yield measurement system (Otsuka Electronics Co., Ltd., Japan), and a Xe lamp was used as an excitation source and white BaSO₄ powder as a reference. The fluorescent lifetime was conducted on a LifeSpec Red Spectrometer with excitation wavelength at 405 nm.

3 Results and discussion

3.1 Phase and crystal structure analysis

Fig. 1 shows the X-ray diffraction (XRD) patterns of as-prepared Ca₂Lu_1−xScZrAl₂GeO₁₂:xCe³⁺ (0 ≤ x ≤ 0.1) phosphors. The XRD results show that all samples are in agreement with the garnet structure, which indicates that Ce³⁺ have been doped into the crystal lattices of the cubic Ca₂LuScZrAl₂GeO₁₂ instead of forming the impurity phase or generating significant structure changes in the host. Furthermore, the peaks of the synthesized material exhibit a slight shift towards lower 2θ angles with the increasing of Ce³⁺ content. This reflects the expansion of lattices when bigger Ce³⁺ ions substitute for smaller Lu³⁺ ions on dodecahedral sites.¹¹ Rietveld refinement of XRD pattern of the Ca₂LuScZrAl₂GeO₁₂ garnet is displayed in Fig. 2. The refined residual factors, atoms coordinates and unit cell parameters are summarized in Table 1. The refinement is convergent well with residual factors R_wp = 9.93, R_p = 9.16 and χ² = 3.35. And the cell parameters are obtained to be a = b = c = 12.43660(4) Å and V = 1923.55(9) ų.

Fig. 1 Powder XRD patterns of Ca₂Lu_1−xZrScAl₂GeO₁₂:xCe³⁺ (0 ≤ x ≤ 0.10). The panel on the right details the zoomed diffraction peak with varied Ce³⁺ contents near 2θ = 37.6°.

Fig. 2 Rietveld refinement XRD patterns of the Ca₂LuZrScAl₂GeO₁₂ host; inset shows the crystal structure of Ca₂LuZrScAl₂GeO₁₂ (a) and the coordination environment of cations (b).

Table 1 Results of structure refinement of Ca₂LuZrScAl₂GeO₁₂

Formula	Ca2LuZrScAl2GeO12
Symmetry	Cubic
Space group	Ia3̄d
a = b = c (Å)	12.43660(4)
α = β = γ	90°
V (Å^3)	1923.55(9)
Z	8
Rp	9.16
Rwp	9.93
χ^2	3.35

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Atom	Site	x	y	z	Occu.
Ca	24c	0.12500	0.00000	0.25000	0.1351(4)
Lu	24c	0.12500	0.00000	0.25000	0.0869(7)
Zr	16a	0.00000	0.00000	0.00000	0.0969(9)
Sc	16a	0.00000	0.00000	0.00000	0.0500(2)
Al	24d	0.37500	0.00000	0.25000	0.3307(7)
Ge	24d	0.37500	0.00000	0.25000	0.0157(6)
O	96h	0.28449	0.09539	0.19698	1.0003(6)

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The inset of Fig. 2 presents a schematic diagram of the Ca₂LuScZrAl₂GeO₁₂ garnet structure according to the Rietveld refinement. Ca²⁺/Lu³⁺, Sc³⁺/Zr⁴⁺, Al³⁺/Ge⁴⁺ cations are randomly distributed on the dodecahedral, tetrahedral, octahedral sites, connecting with eight, six and four surrounding O²⁻, respectively.³² Furthermore, the [(Sc/Zr)O₆] octahedra are shared edges with [(Ca/Lu)O₈] dodecahedra and [(Al/Ge)O₄] tetrahedra, respectively. The [(Ca/Lu)O₈] dodecahedra are connected with part of [(Al/Ge)O₄] tetrahedra by O²⁻ points, and the other tetrahedra by edges.³³ That is to say, different ratio of Zr⁴⁺/Sc³⁺ or Al³⁺/Ge⁴⁺ surrounding dodecahedron can lead differentiated coordination environment of the dodecahedral sites, namely, differentiation of the dodecahedral sites, thereby affecting the luminescent properties of Ce³⁺.²⁵

The Ca₂LuZrScAl₂GeO₁₂, band structure (BS) and density of states (DOS) of the pure Ca₂LuZrScAl₂GeO₁₂ host are investigated by first-principles calculation, presented in Fig. 3 and 4, respectively. The calculated band gap of the pure Ca₂LuZrScAl₂GeO₁₂ host corresponding to the direct transition is 3.297 eV, which is much narrower than that of YAG 5.0 eV calculated by same method.³⁴ Many studies reported that Ce³⁺-activated Ge⁴⁺-containing garnet phosphors always have a fatal drawback of relatively low luminescence intensity, which is most likely attributed to Ce³⁺ photo-ionization in Ge⁴⁺ series garnet matrices.^13,35,36 This phenomenon can be explained by the fact that 5d levels at higher energy of Ce³⁺ ion is already in the conductive band of matrix which have a small band gap, and then the excited 5d electron of Ce³⁺ ion can enter this band easily.³⁷ From our studies, the relatively low band gap may enhance the photo-ionization effect of this Ce³⁺-activated phosphors.

Fig. 3 Band structures of Ca₂LuScZrAl₂GeO₁₂.

Fig. 4 Density of states of Ca₂LuScZrAl₂GeO₁₂.

In Fig. 4, the DOS shows that the top edge of the valence band mainly consists of the O 2p orbital. And the 4s orbital of Ge and the 4d orbital of Zr are the major contributors to the bottom edge of the conductive band, which indicate that the dominant role in photo-ionization process is not only played by the Ge 4s orbital, but also by the Zr 4d orbital. Perhaps that is the reason for Ce³⁺-activated Ge-containing and Zr-containing series garnet phosphors always have relatively low luminescence efficiency.^{23–25,35,36}

The Fig. 5 exhibits the SEM image of the Ca₂Lu_0.99ScZrAl₂GeO₁₂:0.01Ce³⁺ phosphor. It reveales that the grains trend to form spherical shape and smooth morphology, and the size is in the range of 2–10 μm, which also indicates that the well-crystallized powders have been obtained.

Fig. 5 SEM image of the Ca_1.99LuZrScAl₂GeO₁₂:0.01Ce³⁺.

3.2 Photoluminescence properties of the Ca₂LuScZrAl₂GeO₁₂:Ce³⁺ phosphor

The emission (PL) and excitation (PLE) spectra of the Ca₂Lu_0.99ZrScAl₂GeO₁₂:0.01Ce³⁺ at room temperature are depicted in the Fig. 6. The PLE spectrum monitored at 472 nm, ranges from 300 to 450 nm, and exhibits two main broad bands at 348 nm and 405 nm. Moreover, the position of the lowest Ce³⁺ 4f¹–5d¹ absorption transition in Ca₂Lu_0.99ZrScAl₂GeO₁₂:0.01Ce³⁺ is at a much higher energy level than that of classical garnet phosphor YAG:Ce³⁺, which indicates much weaker crystal field strength in this sample. Additionally, the emission spectrum of Ca₂Lu_0.99ZrScAl₂GeO₁₂:0.01Ce³⁺ under excitation at 405 nm, exhibits a blue-green emission band with a typical asymmetrical spectra from 425 nm to 600 nm peaking at 472 nm. We can decompose the emission band into two Gaussian profiles (the dashed lines in Fig. 6) with maximum at 464 nm (21 552 cm⁻¹) and 494 nm (20 243 cm⁻¹), respectively. The energy difference Δk is about 1310 cm⁻¹, which is different with the theoretical energy difference between ²F_5/2 and ²F_7/2 levels of the Ce³⁺ ground state (∼2000 cm⁻¹).³⁸ It indicates that Ce³⁺ ions not only enter one specific site, which shows agreement with the structure analysis that a differentiation appears to the dodecahedral site of Ca₂LuZrScAl₂GeO₁₂ host.²⁵ So the emission band should be decomposed into multiple Gaussian profiles rather than two for this phosphor.

Fig. 6 PLE (λ_em = 472 nm) spectra and PL (λ_ex = 405 nm) spectra of the Ca₂Lu_0.99ZrScAl₂GeO₁₂:0.01Ce³⁺ phosphor.

Fig. 7 illustrates the dependence of the emission spectra with Ce³⁺ doping concentration for Ca₂Lu_1−xZrScAl₂GeO₁₂:xCe³⁺ (0.005 ≤ x ≤ 0.1) samples under 405 nm excitation. Additionally, the inset of Fig. 7 shows the Ce³⁺ concentration dependent emission intensity and peak position. With the increasing of Ce³⁺ concentration, the emission peak position has a red shift from 469 nm to 493 nm. This phenomenon can be explained by the change of crystal field strength. The crystal field splitting (D_q) can be determined by the following eqn (1):³⁹

where D_q relates a measure of the energy level separation, Z is the anion charge, e is the electron charge, r represents the radius of the d wavefunction, and R is the bond length. With the increasing concentration of bigger Ce³⁺ entering into smaller dodecahedral sites, it is possible that the Ce³⁺ will suffer intensive compression from other neighboring atoms in the rigid garnet structure, which results in a closer band length between Ce³⁺ and O²⁻. Moreover, the crystal field splitting is proportional to 1/R⁵, shorter Ce³⁺–O²⁻ distance can lead to the enhancement of crystal field surrounding the Ce³⁺ ion, thus results in a larger crystal field splitting of the 5d level of Ce³⁺ with a red shift in the emission spectra.

Fig. 7 PL spectra (λ_ex = 405 nm) of Ca_2−xLuZrScAl₂GeO₁₂:xCe³⁺ with varying Ce³⁺ concentration. Inset shows the emission intensity and emission peak wavelength as a function of the Ce³⁺ content.

In the Fig. 7, the emission intensity of samples first increases with Ce³⁺ content reaching the maximum intensity at x = 0.01, and then decreases gradually with x beyond the critical concentration. Obviously, the concentration quenching occurs when the Ce³⁺ concentration exceeds 1 mol%. The critical distance (R_c) of energy transfer among Ce³⁺ ions in Ca₂LuZrScAl₂GeO₁₂ can be demonstrated by the flowing eqn (2):⁴⁰

here, V corresponds to the unit cell volume; X_c equals the critical concentration of dopant ions Ce³⁺ here; and N represents the number of host cations in the unit cell. Here, the values V = 1923.55(9) ų, N = 24, X_c = 0.01, then we can obtain the critical distance R_c of energy transfer is about 24.8 Å. Apparently, this value is much longer than 5 Å,⁴¹ indicating little possibility of the nonradiative energy transfer between different Ce³⁺ ions in the Ca₂LuZrScAl₂GeO₁₂ host via the exchange interaction mechanism. Generally, the other two mechanisms of the nonradiative energy transfer are multipolar interactions and radiation reabsorption.⁴² The multipolar interactions between Ce³⁺ ions can be identified by the following formula (3) according to the Dexter's theory,⁴³where I is the emission intensity, x is the concentration of the Ce³⁺ ion, β and k are constants for the given host under the same excitation conditions, θ is a function of the multipole–multipole interaction. According to the Van Uitert's report, θ = 6, 8, and 10 represents dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions, respectively. In order to get the value of θ, the relationship between lg(I/x) and lg(x) with a slope of −1.39 by linear fitting illustrated in Fig. 8. So the calculated θ = 4.2, which is roughly close to 6, suggesting that the dipole–dipole (d–d) interaction is not the only reason of the nonradiative energy transfer in Ca₂LuZrScAl₂GeO₁₂:Ce³⁺ phosphor. On the other hand, we can't ignore the overlap between the excitation band and emission band in Fig. 6. In fact, Ce³⁺ excitation garnet phosphors always have such a reabsorption problem in nonradiative energy transfer process.^11,44 So that the reabsorption effect should be taken into consideration in the nonradiative energy transfer process for Ca₂LuZrScAl₂GeO₁₂:Ce³⁺ phosphor.

Fig. 8 The linear fitting of the relationship of lg(I/x) vs. lg(x).

Fig. 9 presents the temperature dependent emission spectra of the optimized Ca₂Lu_0.99ZrScAl₂GeO₁₂:0.01Ce³⁺ phosphor under excitation at 405 nm and the inset shows the relative emission intensities as a function of temperature. It reveales that the relative PL intensity decreases with the increase of temperature, and remains only 40% of the corresponding initial value (at room temperature) when the temperature was raised up to 100 °C, which is far lower than YAG:Ce³⁺ (no less than 90%).¹¹ Generally, the thermal quenching process can be explained by the enhanced phonon–electron interactions with the increase of temperature and the thermally activated photo-ionization of the luminescent process.³⁹ To verify the origin of temperature-dependent emission intensity I_T, the activation energy ΔE for the electrons excited from the ground-state level 4f to 5d¹ of the Ce³⁺ ions can be ascribed in the following equation:³⁹

where I₀ represent the initial emission intensity at room temperature and I_T is the intensity at different temperature, c is a constant for a certain host, and k is the Boltzmann constant (8.629 × 10⁻⁵ eV). By linear fitting the relationship of In[(I₀/I_T) − 1] vs. 1000/T for the Ca₂Lu_0.99ZrScAl₂GeO₁₂:0.01Ce³⁺ phosphor with a slop of −3.535 (as shown in Fig. 10), the activation energy is calculated to be 0.305 eV, which is quite lower than that of YAG:Ce³⁺ (0.81 eV).⁴⁵ Thus, the thermally activated photo-ionization of the luminescent process should be accounted for the thermal quenching of Ca₂Lu_0.99ZrScAl₂GeO₁₂:0.01Ce³⁺. As the electronic structure study of matrix in Fig. 3 and 4, photo-ionization effect may be easy to occur when the matrix contains Ge⁴⁺ or Zr⁴⁺ in garnet phosphor.

Fig. 9 Temperature-dependent PL spectra of Ca₂Lu_0.99ZrScAl₂GeO₁₂:0.01Ce³⁺ phosphor. Inset shows the emission intensity as a function of temperature.

Fig. 10 Arrhenius fitting of the emission intensity of Ca₂Lu_0.99ZrScAl₂GeO₁₂:0.01Ce³⁺ phosphor and the calculated activation energy for thermal quenching.

Fig. 11 exhibits the diffuse reflectance spectra (DRS) of Ce³⁺-doped and undoped Ca₂LuZrScAl₂GeO₁₂. The undoped Ca₂LuZrScAl₂GeO₁₂ shows a drop in reflection in the range about 450 nm, corresponding to the transition from the valence to conduction band of the host lattice. The intense reflection in the visible spectral range is in agreement with the grey-white daylight colour of undoped Ca₂LuZrScAl₂GeO₁₂. Compared with the undoped sample, the Ce³⁺-doped Ca₂LuZrScAl₂GeO₁₂ displays a pronounced absorption band peaking at about 405 nm which is attributed to the absorption by the Ce³⁺ ions. The band gap energy can be determined by extrapolating the absorption edge onto the energy axis, as shown in the inset of Fig. 11.⁴⁶ And the conversion of the reflectance to absorbance data can be obtained by the Kubelka–Munk function (K–M)^46,47

where R is the reflectance; F(R) is proportional to the extinction coefficient. The band gap energy of undoped Ca₂LuZrScAl₂GeO₁₂ by reflection spectrum analysis is estimated to be 5.15 eV (as shown in Fig. 6 inset), which is much lower than YAG (6.5 eV).¹¹ Compared with calculated band gap energy of the host, the result indicates that the first principle calculation method could qualitatively analyze and predict the luminescence properties of phosphor. It is conceivable that the photo-ionization effect is more likely to occur in these Ge⁴⁺- or Zr⁴⁺-containing garnet phosphors.

Fig. 11 Diffuse reflectance spectra of the Ca_1.99LuZrScAl₂GeO₁₂:0.01Ce³⁺ and Ca₂LuZrScAl₂GeO₁₂ host at room temperature. The inset is the calculated energy of host.

Fig. 12 illustrates the florescence decay curves of different Ca₂Lu_1−xZrScAl₂GeO₁₂:xCe³⁺ phosphors under 405 nm excitation and monitored at 475 nm. Normally the decay curves of luminescent materials are represented as a single exponential function, i.e., lg(t) vs. t presents a linear relation, for the phosphor which has only one type of luminescent centre, such as YAG:Ce.⁴⁸ However, the single exponential function is not able to deal with the decay curves of Ca₂Lu_1−xZrScAl₂GeO₁₂:xCe³⁺ phosphors. But a three exponential function (6) can be well fitted.⁴⁹

where I is emission intensity, A, A₁, A₂ and A₃ are constants, t is time, τ₁, τ₂ and τ₃ are decay time for the exponential components, respectively. The average lifetime of the Ca₂Lu_1−xZrScAl₂GeO₁₂:xCe³⁺ phosphors were determined to be 9.84 ns, 8.81 ns, 7.14 ns, 5.34 ns, 4.08 ns, 3.40 ns and 3.13 ns by the following formula (7),⁵⁰

Fig. 12 Decay curves and calculated lifetimes of Ca_2−xLuZrScAl₂GeO₁₂:xCe³⁺ (0.005 ≤ x ≤ 0.1) phosphor with varying Ce³⁺ concentration.

The decay time decreases gradually with increasing of Ce³⁺ concentration, which indicates that the nonradiative transition has been enhanced.⁵¹ Nevertheless, the decay curves fit well with a three exponential function and the relationship of lg I(t) vs. t gradually deviates from a straight line with the increase of Ce³⁺ concentration, which reveals that more and more Ce³⁺ ions occupy the differentiated dodecahedral sites.

The Fig. 13 illustrates the CIE value of the Ca₂Lu_1−xZrScAl₂GeO₁₂:xCe³⁺ phosphors excited at 405 nm. The emission color of Ca₂Lu_1−xZrScAl₂GeO₁₂:xCe³⁺ phosphors varies from blue to green and the chromaticity index can be tuned from (0.1386, 0.2287) to (0.1600, 0.3644) by adjusting the concentration of Ce³⁺ ions from 0.5% to 1%. Additionally, the internal quantum efficiency (QE) of Ca_1.99LuZrScAl₂GeO₁₂:0.01Ce³⁺ phosphor is measured to be 18.9% under 405 nm excitation at room temperature. The relatively low QE value corroborates the opinion that photo-ionization effect may be easy to occur in luminescence process of these Ce³⁺-activated phosphors. And this can be further improved by synthesis and composition optimization, especially, making further substitutions, such as Si⁴⁺ replacing Ge⁴⁺, which is conducive to reduce the photo-ionization process.¹³

Fig. 13 The CIE coordinates of Ca₂Lu_1−xZrScAl₂GeO₁₂:xCe³⁺ (0.005 ≤ x ≤ 0.1) phosphors under excitation at 405 nm.

4 Conclusions

In summary, we applied the Diagonal Relationship theory to phosphor design and successfully synthesized a group of new garnet Ca₂Lu_1−xZrScAl₂GeO₁₂:xCe³⁺ (0.005 ≤ x ≤ 0.1) phosphors by high-temperature solid-state reaction. The XRD and Rietveld refinement results confirm that this phosphor belong to garnet structure. The band gap between conductive band and valance band is about 5.15 eV, which is quite narrow that may cause a photo-ionization process. Both the 4s orbital of Ge and the 4d orbital of Zr play a dominant role in photo-ionization process which conjectured by the PDOS of the matrix. The SEM image reveals that the sample trends to form spherical shape and smooth morphology with the size at the range of 2–10 μm. In addition, the Ca₂LuZrScAl₂GeO₁₂:Ce³⁺ shows two main broad excitation bands with the peaks at 348 nm and 405 nm. As the Ce³⁺ content increases, the emission spectra of Ca₂LuZrScAl₂GeO₁₂:Ce³⁺ phosphor varies from 469 nm to 493 nm under 405 nm excitation, which mainly due to the increasing crystal field splitting. The optimal Ce³⁺ doping concentration is x = 0.01. Moreover, the critical distance of energy transfer among Ce³⁺ ions in Ca₂LuZrScAl₂GeO₁₂ is 24.8 Å, and the corresponding concentration quenching mechanism is d–d interaction and radiation reabsorption.

Acknowledgements

This present work was financially supported by the National Basic Research Program of China (No. 2014CB643801).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11258g

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