Orange- to green-emitting Li(Sr,Ca)₄(BO₃)₃:Eu²⁺ phosphor: emission-tunable properties and white light emitting diode application

Abstract

An emission-tunable phosphor, Eu²⁺-activated LiSr_(4−x−y)Ca_x(BO₃)₃:yEu²⁺ phosphor, was synthesized by high temperature solid state reaction. By combining with Reitveld refinement analysis, the X-ray powder diffraction test confirmed the formation of a crystalline phase. The optical spectral properties, concentration quenching properties, decay curves and thermal stability of the LiSr_(4−x−y)Ca_x(BO₃)₃:yEu²⁺ phosphor were investigated in detail. The results showed that with the Ca²⁺ doping concentration increasing, the emission maxima is shifted from 618 to 501 nm and the color hue can be tuned from orange red to blue green. The effects of Ca²⁺ substitutions into Sr²⁺ sites on the emission band change were investigated. A white LED was fabricated using a blue 450 nm chip pumped by LiSr_3.92Ca_0.06(BO₃)₃:0.02Eu²⁺ phosphor film. Under 100 mA driven currents, the white LED device has a Commission International de I'Eclairage (CIE) color coordinate of (0.3563, 0.3576) at a white light (correlated color temperature = 5083 K) and an excellent color rendering indices of 85.3.

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1. Introduction

In recent years, the white light-emitting diode (w-LED) has received wide attention from all over the world as a new generation of light source.^1–3 It has many advantages, such as, high luminous efficiency, small volume, saving energy and so on. At present, the major commercial white LEDs are phosphor-converted LEDs which consist of a blue InGaN chip and a yellow phosphor, (Y,Gd)₃(Al,Ga)₅O₁₂:Ce³⁺. Unfortunately, the w-LEDs packaged with yellow phosphors exhibit high color temperature and poor color rendering index (CRI), due to the lack of red components.⁴ To improve the rendering index, w-LEDs that use a near ultraviolet (n-UV) LED chip with three primary color emission phosphors have been widely investigated. The w-LEDs have several benefits, including high color rendering index, high luminous efficiency, and a stable light color that are almost independent of the changed current. However, compared with 1-pc-LED (one phosphor-converted LED), mixing powders and finding high-efficiency compounds are more difficult for this type of device. Furthermore, low efficiency of the red phosphors and the need for complex coating technology, restrict their development.⁵

Recently, several w-LEDs modules fabricated using n-UV chip coupled with a blend of tunable emitting (blue-to-yellow) and red-emitting phosphor. This combination mode presented favourable properties, for example, excellent CRI values, tunable CIE chromaticity coordinates and tunable correlated color temperature.^6–10 Phosphor with tunable colors from the same luminescent component are highly desirable. As we know, the emission colour tuning range of the phosphor with a single activated ion is only limited to blue to yellow. In order to broaden the tuning range, the realization of color tunable phosphors from red to yellow has been a challenge.

As orange-emitting borate phosphor, LiSr₄(BO₃)₃:Eu²⁺ phosphor has been reported in the literatures.^11–13 To the best of our knowledge, the tunable luminescence properties of LiSr₄(BO₃)₃:Eu²⁺ phosphor has not yet been reported in literature. In this study, we investigated the tunable luminescent properties of Eu²⁺-doped LiSr₄(BO₃)₃:Eu²⁺ by alkali-earth metal ions (Sr²⁺ and Ca²⁺) mutual substitution in detail. The phosphor can be excited by n-UV and blue LED chips and shows tunable orange red to blue green emission. In addition, the LED device using Li(Sr,Ca)₄(BO₃)₃:Eu²⁺ phosphor with a n-UV chip were fabricated to demonstrate the potential applications of Li(Sr,Ca)₄(BO₃)₃:Eu²⁺ phosphor as a novel color conversion material.

2. Experimental

2.1 Materials and synthesis

The LiSr_4−x−yCa_x(BO₃)₃:yEu²⁺ phosphor samples were synthesized by a traditional high-temperature solid-state reaction method. The SrCO₃ (99.99%), CaCO₃ (99.99%), H₃BO₃ (99.99%), Li₂CO₃ (99.9%) and Eu₂O₃ (99.999%) were used as beginning materials. According to the stoichiometric ratio, the beginning materials were weighed, and the mixed thoroughly in an agate mortar. Finally the mixture was sintered at 950 °C with reducing atmosphere (5% H₂/95% N₂) for 5 h in a tube furnace.

2.2 Materials characterization

The X-ray diffraction (XRD) pattern was collected on an X-ray diffractometer (Bruker Axs D2 PHASER diffractometer). Rietveld refinenements on the X-ray diffraction data were performed using the software TOPAS. The excitation and emission spectrum of phosphors were recorded by a PL3-211-P spectrometer (HORIBA JOBIN YVON, America) and a 450 W xenon lamp was used as the excitation source. The diffuse reflectance spectrums of the undoped and doped Eu²⁺ samples were tested by a UV-3600 UV-Vis spectrometer (Shimadzu, Japan). The spectral, photometric and colorimetric quantities of the LEDs were measured by a HAAS-2000 Light & radiation measuring instrument for packaged LEDs (Everfine, China).

2.3 LED lamp fabrication

We mixed the LED packaging epoxy resin and LiSr_3.92Ca_0.06(BO₃)₃:0.02Eu²⁺ phosphor together, then spread the mixture on a substrate which can be used to control the shape and thickness of film. And then, the film was placed into an oven and baked at 60 °C for 4 h. Finally a luminescent thin film was obtained, and packaged on a 450 nm blue LED chip.

3. Results and discussion

3.1 Crystal structure

The phase purity and structure change of all phosphor samples were verified by XRD analysis. Fig. 1 shows XRD pattern for a Li(Sr_3.98−xCa_x)(BO₃)₃:0.02Eu²⁺ sample as a function of substituted Ca²⁺ ions content (x). All the diffraction peaks of the obtained samples can be indexed to the standard data of LiSr₄(BO₃)₃ with the ICSD file no. 170861, indicating that these samples are single-phase in our experimental doping-content ranges and that the doping ions Ca²⁺/Eu²⁺ are incorporated in the host lattice. Additionally, the diffraction peaks exhibit a slight shift toward higher angles with increasing content of the Ca²⁺ ion, which may be related to the substitution of the larger Sr²⁺ by the smaller Ca²⁺.

Fig. 1 XRD patterns of Li(Sr_3.98−xCa_x)(BO₃)₃:0.02Eu²⁺ samples. The standard data for LiSr₄(BO₃)₃ (ICSD-170861) is shown as a reference.

Fig. 2 showed the Rietveld refinement results of LiSr_3.98(BO₃)₃:0.02Eu²⁺ phosphors. ICSD 170861 (LiSr₄(BO₃)₃) was performed as the standard data to refine the experimental crystal structures. LiSr_3.98(BO₃)₃:0.02Eu²⁺ crystallized in a cubic unit cell with the same space group Ia3̄d(230). For Li(Sr_3.98Eu_0.02)(BO₃)₃, the lattice constants (a, b, c) are 15.0710 (Å) and cell volume (V) is 3423.15 (ų). The detailed refinement results were showed in Table 1.

Fig. 2 Observed (black), calculated (red), and difference (green) XRD profiles for the Rietveld refinement of LiSr_3.98(BO₃)₃:0.02Eu²⁺. Bragg reflections are indicated with tick marks.

Table 1 Rietveld refinement and crystal data of LiSr_3.98(BO₃)₃:0.02Eu²⁺ phosphors

Compound	x = 0
Formula	LiSr3.98(BO3)3:0.02Eu^2+
Cryst. syst.	Cubic
Space group	La3̄d(230)
Crystal density (g cm^−3)	6.642
Unit, Z	16
a (Å) = b (Å) = c (Å)	15.0710
V (Å^3)	3423.15
Rexp (%)	4.61
Rwp (%)	5.13
Rp (%)	4.46
GOF	1.57

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The results of structure refinement show that there are two crystallographically different sites for Sr atoms in the crystal structure of Li(Sr_3.98Eu_0.02)(BO₃)₃. The Sr(1) atom occupies the center of octahedral SrO6 polyhedron (CN = 6) and Sr(2) in the center of hexagonal bipyramidal SrO8 polyhedral (CN = 8) (see the inset in Fig. 2). The ionic radius of Sr²⁺ was 1.18 Å (CN = 6) and 1.26 Å (CN = 8). The ionic radii of Eu²⁺ were 1.17 Å (CN = 6) and 1.25 Å (CN = 8). Because of the similarity in ionic radius, the Sr²⁺ ions sites would be occupied randomly by the Eu²⁺ ions in the LiSr₄(BO₃)₃ host. Meanwhile, when Eu²⁺ ions occupied Sr²⁺ ions sites, two different types of emission spectra could be obtained.

The unit cell constants a and V of Li(Sr_3.98−xCa_x)(BO₃)₃:0.02Eu²⁺ samples were calculated by refining the powder XRD data as a function of the Ca²⁺ content. It can be seen from Fig. 3 that the lattice constants a and V decrease linearly when x is gradually increased from 0 to 0.15, which is in good agreement with Vegard's law.¹⁴ The change in lattice constants also indicates that the Ca²⁺ ions are doped into the host and that the solid solution is well formed in the LiSr₄(BO₃)₃ crystal.

Fig. 3 Unit cell parameters of LiSr_3.98−xCa_x(BO₃)₃:0.02Eu²⁺ samples obtained from Rietveld refinement data present both a contraction in the (a) lattice parameter and (b) cell volume as the concentration of Ca²⁺ increases, reflecting the effects of the cations substitutions.

3.2 Diffuse reflection spectra

The diffuse reflection spectra of the undoped LiSr_4−xCa_x(BO₃)₃ powders were illustrated in Fig. 4(a). In the visible range (400–700 nm) the sample of LiSr_4−xCa_x(BO₃)₃ showed a high reflection. The undoped LiSr_4−xCa_x(BO₃)₃ samples showed absorption band between about 200 and 425 nm range. The absorption edge shifted toward long wavelength with the increasing of Ca²⁺ ions content.

Fig. 4 (a) Diffuse reflection spectra of undoped LiSr_4−xCa_x(BO₃)₃ samples, (b) absorption spectra of LiSr_4−xCa_x(BO₃)₃ (x = 0 and 1.5) as calculated by the Kubelka–Munk formula.

The optical band gap of LiSr_4−xCa_x(BO₃)₃ hosts had been calculated by following Kubelka–Munk function:¹⁵

F(R) = K/S = (1 − R)²/2R (1)

here, S, K and R are the scattering coefficient, absorption coefficient and reflection coefficient, respectively. The absorption spectra of LiSr_4−xCa_x(BO₃)₃ derived with the Kubelka–Munk function are shown in Fig. 4(b). The value of the optical band gap is calculated to be from 5.35 to 5.85 eV with the increasing content of Ca²⁺ ions in the LiSr_4−xCa_x(BO₃)₃ host by extrapolating the Kubelka–Munk function to K/S = 0.

The diffuse reflectance spectra of LiSr_3.4−yCa_0.6(BO₃)₃:yEu²⁺, y = 0–0.08 are given in Fig. 5, the strong absorption band is presents at 360 nm, and one more weak absorption band is observed at 310 and 400 nm; all the bands are attributed to the 4f⁷–4f⁶5d¹ electronic transitions of Eu²⁺. These bands are absent in the diffuse reflectance spectrum of the parent phase (unsubstituted LiSr_3.4Ca_0.6(BO₃)₃). The band at 260 nm observed for all compositions is due to host absorption. Also, the absorption edge gradually extends to longer wavelengths, and the absorption gets enhanced with higher Eu²⁺ concentration.

Fig. 5 Diffuse reflectance spectra of LiSr_3.4−yCa_0.6(BO₃)₃:yEu²⁺ (y = 0–0.10).

3.3 Excitation and emission properties

Fig. 6 depicts the excitation and emission spectra of LiSr_3.38Ca_0.6(BO₃)₃:0.02Eu²⁺ phosphor, and the deconvoluted Gaussian components. The excitation spectrum of LiSr_3.38Ca_0.6(BO₃)₃:0.02Eu²⁺ monitored by 534 and 585 nm presents a broad hump between 300 and 450 nm, respectively, except for the difference of peak shapes and intensities, which means the phosphor can be well excited by the n-UV LED chips. The excitation spectra contains two excitation bands peaking at 360 (380) and 410 nm, which can be attributed to the 4f⁷–4f⁶5d¹ transitions of Eu²⁺ from the ground state to the different splitting levels of the 5d state.

Fig. 6 Excitation (left) and emission (right) spectra of LiSr_3.38Ca_0.6(BO₃)₃:0.02Eu²⁺ phosphors, (a) λ_ex = 515 nm, λ_em = 360 nm, (b) λ_ex = 588 nm, λ_em = 380 nm, fitted curve (red dashed line) and deconvoluted Gaussian components (green solid lines).

Upon 360 nm (27 777 cm⁻¹) and 380 nm (26 315 cm⁻¹) excitation, the phosphor shows the different broad band emission centered at 534 nm (18 726 cm⁻¹) and 585 nm (17 094 cm⁻¹), respectively, which is attributed to the electric-dipole-allowed transition from the lowest level of the 5d excited state (4f⁶5d) to the 4f ground state (4f⁷(⁸S_7/2)) of the doped Eu²⁺ ions. The unsymmetrical emission spectrum can be deconvoluted into two Gaussian peaks centered at 515 nm (19 417 cm⁻¹) and 588 nm (17 006 cm⁻¹) (shown as two green dotted curves in Fig. 6), respectively.¹⁶ It is also observed that the excitation spectrum monitoring by 515 and 588 nm exhibits a different spectral profile, which means that the two emission peaks should be ascribed to two different Sr²⁺ sites occupied by Eu²⁺ ions, which is also in accordance with the above analysis on the crystal structure.

The decay curves of LiSr_3.38Ca_0.6(BO₃)₃:0.02Eu²⁺ phosphors are showed in Fig. 7, the corresponding luminescence decay time could be best fitted with a double exponential functions as I = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂), where I is the luminescence intensity, A₁ and A₂ are constants, t is the time, τ₁ and τ₂ are rapid and slow lifetimes for exponential components, respectively. The average decay times were calculated to be 411.36 ns (monitored at 517 nm) and 523.58 ns (monitored at 587 nm). The double exponential decay dynamic is in agreement with the fact that two types of cation sites exist in the host lattice (Fig. 8).

Fig. 7 Decay curves of Eu²⁺ emission in LiSr_3.92Ca_0.06(BO₃)₃:0.02Eu²⁺ phosphor monitored at (a) 517 nm and (b) 587 nm.

Fig. 8 The 5d energy levels of free Eu²⁺, Eu²⁺(1) and Eu²⁺(2) in LiSr₄(BO₃)₃.

According to the report of Van Uitert, the energy (E) of the lower d-band edge for Eu²⁺ can be estimated using the following equation:^17,18

where E represents the real position of the d band edge in energy for the Eu²⁺ ion (cm⁻¹), Q represents the position in energy for the lower d-band edge of the Eu²⁺ free ions (34 000 cm⁻¹), V represents the valence of the Eu²⁺ ion, n represents the number of anions in the immediate shell about this ion, E_a represents the electron affinity of the atoms that form anions, r is the radius of the host cation replaced by the Eu²⁺ ion (in Å). There are two Sr²⁺/Ca²⁺ sites in LiSr_3.92Ca_0.06(BO₃)₃:0.02Eu²⁺ phosphor. It is apparent that the value of E (cm⁻¹) is directly proportional to the product of n and r. Thus, we can conclude that the band peaked at 515 nm (19 417 cm⁻¹) is ascribed to the 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ ions occupying the eight-coordinated Sr1/Ca1O8 site, while the other emission peaks centered at 588 nm (17 006 cm⁻¹) attribute to Eu²⁺ occupying six-coordinated Sr2/Ca2O6 ions site, respectively.

The energy of the lowest 5d excited level of Eu²⁺ is influenced by centroid shift (ε_c) and crystal field splitting (ε_cfs). The 5d centroid shift for Eu²⁺ can be expressed by the following equation¹⁹

where R_i is the distance (pm) between Eu²⁺ and anion i in the undistorted lattice, 0.6ΔR is correction for lattice relaxation around Eu²⁺, and ΔR is the difference between the radii of Eu²⁺ and cation sites that Eu²⁺ ions occupy. The summation is over all N anions that coordinate Eu²⁺. α_spⁱ (in units of 10⁻³⁰ m⁻³) is the spectroscopic polarizability of anion i. For oxides, , χ_av is the electro-negativity of the cations. For LiSr₄(BO₃)₃ χ_av = [χ(Li) + 8χ(Sr) + 9χ(B)]/18 = 1.36, and one obtains α^O_sp = 2.93 × 10⁻³⁰ m⁻³. The ε_c value of Eu²⁺(1) and Eu²⁺(2) are calculated to be 1.36 and 1.20 eV, respectively.

The crystal field splitting of the 5d levels can be expressed as²⁰

ε_cfs = β_poly^QR_av⁻² (4)

where β_poly^Q is a constant that depends on the type of coordination polyhedron, Q is 2+ for Eu²⁺ ions. The ratio β_poly(6)²⁺ : β_poly(8)²⁺ : β_poly(12)²⁺ equals 1 : 0.89 : 0.42, namely, β_poly^Q decreases with increasing coordination number. The value of β_poly(6)²⁺ for Eu²⁺ is 135 466 eV pm².²⁰ The value of β_poly(8)²⁺ for 8 O-coordinated Sr²⁺ polyhedron is estimated to be 0.89 times of β_poly(6)²⁺ according to the above ratio. R_av is close to the average distance between anions and cations that is replaced by Eu²⁺, which can be expressed as follow:The R_av values for Sr1–O and Sr2–O in LiSr₄(BO₃)₃ are 261.5 and 253.0 pm, respectively. Therefore, the crystal field splitting values for 5d levels of Eu²⁺(1) and Eu²⁺(2) are 1.76 and 2.12, respectively. That is to say, Eu²⁺(1) have smaller ε_cfs than that of Eu²⁺(1). Therefore, the lowest excited 5d level of Eu²⁺(1) is at a higher energy (corresponding to a shorter emission wavelength, 515 nm) than that of Eu²⁺(2) (588 nm). Fig. 7 shows the 5d energy levels of free Eu²⁺ and Eu²⁺ on Sr sites based the above analyses.

In addition, the quantum efficiency of the phosphor was recorded using an integrating sphere attached to the PL3-211-P spectrometer, the internal (η_i) and external (η_ex) quantum efficiencies (QEs) of the LiSr_3.92Ca_0.06(BO₃)₃:0.02Eu²⁺ phosphor were calculated using following equations:²¹

where Ex(λ) and R_e(λ) are the spectra of the excitation light without and with the phosphor in the integrating sphere, respectively; Em(λ) is the emission spectrum of the phosphor in the integrating sphere. The η_i and η_ex values of LiSr_3.92Ca_0.06(BO₃)₃:0.02Eu²⁺ under 360 nm excitation are calculated to be ∼53.7% and ∼36.5%, respectively.

Fig. 9 shows the concentration dependence of the integrated emission intensity of LiSr_3.92−xCa_0.06(BO₃)₃:xEu²⁺ with various concentration of Eu²⁺ ions under 360 and 380 nm excitation, respectively. For LiSr_3.92−xCa_0.06(BO₃)₃:xEu²⁺ phosphors, the emission intensity increases with the increase in Eu²⁺ content up to a maximum x value of about 0.02 mol, and then it decreases because of interaction between Eu²⁺ ions. According to the percolation model, concentration quenching of the compound can occur by two mechanisms: (1) interactions between the Eu²⁺ ions, which results in energy reabsorption among neighboring Eu²⁺ ions in the rare earth sublattice; (2) energy transfer from a percolating cluster of Eu²⁺ ions to killer centers.

Fig. 9 (a) Concentration dependence of the relative integrated emission intensity of LiSr_3.92−xCa_0.06(BO₃)₃:xEu²⁺ (x = 0.005–0.08) under 380 and 360 nm excitation, respectively. (b) Relationships of log(I/xEu²⁺) versus log(xEu²⁺) in LiSr_3.92−xCa_0.06(BO₃)₃:xEu²⁺ phosphor.

Hence we have calculated the critical distance between the Eu²⁺ ions for energy transfer by using the relation given by Blasse²²

where V is the volume of the unit cell, x_c is the critical concentration of activator, Z is the number of formula units per unit cell. For LiSr₄(BO₃)₃ host, using Z = 16, x_c = 0.02 and V = 3243.3 ų. The obtained R_c is 26.84 Å. For the exchange interaction, the critical distance is about 5 Å, which is much smaller than the obtained result (26.84 Å), thus, the exchange interaction between the Eu²⁺ ions may be ruled out in LiSr₄(BO₃)₃ host.

The non-radiative energy transfer between Eu²⁺ ions is caused by electric multipole–multipole interactions, which depend on distance according to Dexter's theory.²³ The strength of the multipole–multipole interactions can be determined from the change in the emission intensity if the energy transfer occurs between the same types of activators. The emission intensity (I) per activator ion is expressed by the formula given in the following:^24,25

here x is Eu²⁺ ions concentration above the quenching contents; k and β are constants for a given host crystal; and θ is the energy transfer parameter. θ = 3 correspond to the energy transfer among the nearest-neighbor ions, in addition, θ = 6, 8, or 10 correspond to dipole–dipole, dipole–quadrupole, or quadrupole–quadrupole interactions, respectively. Fig. 9(b) presents that the relationships of log(I/x_Eu²⁺) versus log(x_Eu²⁺) is linear and the slope is −1.8. By using the eqn (9), the value of θ was 5.4, which is approximately equal to 6. The result indicates that the dipole–dipole is the main mechanism of concentration quenching of the central Eu²⁺ emission in LiSr_3.92−yCa_0.06(BO₃)₃:yEu²⁺ phosphors.

Fig. 10 shows the normalized emission spectra of LiSr_3.98−xCa_x(BO₃)₃:0.02Eu²⁺ under the 360 nm excitation. From the spectrum can be observed that there is an obvious blue shift from 618 to 501 nm. At the same time, the peak of excitation spectra is shifted to the short wavelength (Fig. S2†). The peaks of excitation and emission wavelength, Stokes shift, crystal field splitting and the full-width at half maximum (FWHM) for LiSr_3.98−xCa_x(BO₃)₃:0.02Eu²⁺ are summarized in Table S1 (ESI†). From Fig. 6 and 10, two emission peaks should be ascribed to two different sites occupancy of Eu²⁺ ions. In order to analyze the occupancy of Eu²⁺ after entering LiSr₄(BO₃)₃ host, Gauss imitating decomposition was undertaken successfully to the emission spectra of LiSr_3.98−xCa_x(BO₃)₃:0.02Eu²⁺ phosphors (see Fig. S2†). The peak value and normalized intensity of the two emission bands are summarized in Table S2 (ESI†). The results indicated that the two emission bands are blue shifted with the content of Ca²⁺ increased in the LiSr_3.98−xCa_x(BO₃)₃:0.02Eu²⁺. But the intensity changes of the two emission bands are not consistent: the intensity of the short wavelength emission band increases gradually, while the long wavelength emission band is just the opposite. The intensity and peak position of the sub emission bands were obviously changed, and the final performance was the blue shift of the whole emission.

Fig. 10 Normalized emission spectra of LiSr_3.98−xCa_x(BO₃)₃:0.02Eu²⁺ (λ_ex = 360 nm).

With the introduction of Ca²⁺ substituted Sr²⁺ ions, the crystal structure of the LiSr₄(BO₃)₃ system would change to a certain extent, thus, the local environment of the Eu²⁺-substituted sites are significantly change due to bond-length changes between the activator cation and the ligand anion, according to the equation:

where D_q is the crystal field strength, Z is the charge or valence of the anion, e is the charge of the electron, r is the radius of the d wavefunction and R is the distance between the central ion and its ligands. The gradually replace of Sr²⁺ by the smaller Ca²⁺ ions is expected to be followed by a shorter metal–ligand distance, leading to a stronger crystal field environment, which can be proved by the decreasing lattice parameters and volumes, as shown in Fig. 3. Thus, there is theoretical causing a red shift in the emission, meanwhile, decreasing the emission energy. Out of our expectation, there are such abnormal blue shift emission properties of the sub emission bands which should be worth further study.

From the crystal structure of LiSr₄(BO₃)₃, the centre Sr²⁺ ions are surrounded by coordinated O²⁻ ions which also linked with other neighboring cations, such as B³⁻, and outer Sr²⁺ ions. When an activator ion was introduced, Eu²⁺ would randomly occupy the Sr²⁺ site in LiSr₄(BO₃)₃, thus, causing the orange emitting. When Sr²⁺ are replaced by smaller Ca²⁺ at the nearest neighbour sites of Eu²⁺, blue shift is evidently shown in emission spectrum, suggesting that the introduction of Ca²⁺ ions could change the crystal field of Eu²⁺ ions. As for how to change, the ionic potential should be considered. The attractive force of the central cations towards the anions can be estimated using the following formula:²⁶

where Z is the electric charge number of ion, and r is the ion radius (pm). As is known, the ionic radius of Ca²⁺ is smaller than Sr²⁺. Thus; Ca²⁺ has the higher attractive force towards O²⁻ than the Sr²⁺ ions. When Sr²⁺ is substituted by the smaller Ca²⁺ ion, the coordinated O²⁻ ions could be attracted by strong attraction, then closer to Ca²⁺ ions. As a result, the bond length between Eu²⁺ and O²⁻ becomes longer, then the magnitude of the crystal field strength decreases, thus, the blue shift occurs in the sub emission bands with the increase of the content of Ca²⁺ in LiSr_3.98−xCa_x(BO₃)₃:0.02Eu²⁺.

Fig. 11 shows the temperature dependence of emission intensity for LiSr_3.98−xCa_x(BO₃)₃:0.02Eu²⁺ phosphors. With the increasing of the Ca²⁺ contents in LiSr_3.98−xCa_x(BO₃)₃:0.02Eu²⁺ phosphors, the thermal stability of the phosphors increase gradually. Furthermore, the activation energy ΔE of the activator ion, can be expressed by:^27,28

here k is the Boltzmann constant (8.629 × 10⁻⁵ eV K⁻¹), A is a constant, I₀ and I_T are the luminescence intensity of at room temperature and the resting temperature. From eqn (12), ΔE was obtained to be 0.162 eV for LiSr_3.98(BO₃)₃:0.02Eu²⁺ and 0.276 eV for LiSr_2.48Ca_1.5(BO₃)₃:0.02Eu²⁺. The calculated value ΔE of LiSr_2.48Ca_1.5(BO₃)₃:0.02Eu²⁺ phosphor is bigger than that of LiSr_3.98(BO₃)₃:0.02Eu²⁺ phosphor. Therefore, the thermal stability of LiSr_2.48Ca_1.5(BO₃)₃:0.02Eu²⁺ phosphor is better than LiSr_3.98(BO₃)₃:0.02Eu²⁺ phosphor.

Fig. 11 The relationship between temperature and emission intensity of the LiSr_3.98−xCa_x(BO₃)₃:0.02Eu²⁺ phosphors.

The difference in thermal stability is related to the change in the Stokes shift, which can be explained by the configuration coordinate diagram in Fig. 12. R represents the Stokes shift, which is in close relation with the interatomic distance between the Eu²⁺ and the coordination anions. After absorption of the excitation energy, a non-radiative relaxation occurs at the bottom of the excited state, and then emission takes place by radiative transitions returning to ground state. However, under a high temperature, thermal activation can happen due to the electron–phonon coupling and the energy reaches the crossing point (C) between the excited and ground states. In this case, non-radiative relaxation occurs by heat dissipation rather than radiation emission, which could quench the luminescence. With the decreasing in the Stokes shift, the thermal activation energy increases, that means that a smaller Stokes shift results in a lower thermal quenching behaviour. Therefore, doping of Ca²⁺ is beneficial to improve the thermal stability of the LiSr_3.98(BO₃)₃:0.02Eu²⁺ system.

Fig. 12 Configuration coordinate diagram of LiSr_3.98−xCa_x(BO₃)₃:0.02Eu²⁺ phosphor.

Fig. 13 shows the CIE chromaticity coordinates for the LiSr_3.98−xCa_x(BO₃)₃:0.02Eu²⁺ phosphors with different x values. The color hue can be tuned from orange red (point 1) to blue green (point 8), and the corresponding chromaticity coordinates (x, y) varying from (0.5524, 0.4270) to (0.1895, 0.4718) summarized in the inset table. The inset photos show luminescence of LiSr_3.98−xCa_x(BO₃)₃:0.02Eu²⁺ (0 ≤ x≤ 1.5) phosphors under excitation at 360 nm.

Fig. 13 Variation in CIE chromaticity coordinates as function x in LiSr_3.98−xCa_x(BO₃)₃:0.02Eu²⁺.

3.4 Application to white LEDs

Fig. 14 shows the electroluminescence (EL) spectrum of the white LED which is packaged from the LiSr_3.92Ca_0.06(BO₃)₃:0.02Eu²⁺ phosphor film and a 450 nm blue LED chip under a applied current of 100 mA. The inset shows the photograph of (a) LiSr_3.92Ca_0.06(BO₃)₃:0.02Eu²⁺ phosphor film, (b) the phosphor film excited at 365 nm in a UV box, (c) as-fabricated LED and (d) LED in operation, respectively. The white LED had CIE color coordinates of (0.3563, 0.3576) with a color-rendering index of 85.3 around the correlated color temperature of 5083 K which is higher than a YAG:Ce – based white LED (R_a = 75–78) at 20 mA,²⁹ and β-Y_0.95Ce_0.05FS-based white LED (R_a = 66–77) at 50–250 mA,³⁰ and is similar to γ-Ca₂SiO₄:Ce³⁺,Li⁺-based white LEDs (R_a = 86).³¹ The white LEDs exhibited a luminous efficacy (η_L) of 22.4 lm W⁻¹. This result confirms that the LiSr_3.92Ca_0.06(BO₃)₃:0.02Eu²⁺ have potential for application in white LED.

Fig. 14 Electroluminescence spectrum of white LED using (Ca_3.85Ba_0.10)(PO₄)₂O:0.05Eu²⁺ under applied currents 100 mA.

4. Conclusions

In summary, we have synthesized the emission-tunable phosphor Eu²⁺-doped Li(Sr,Ca)₄(BO₃)₃ by high temperature solid state method and investigated its tunable luminescence performance. The results show that there are the different broad band emission centered at 534 and 585 nm in Li(Sr,Ca)₄(BO₃)₃:Eu²⁺ phosphors, respectively, and its emission can be adjusted from orange red to blue green by changing Ca²⁺ doping concentration. Applying a blend of LiSr_3.92Ca_0.06(BO₃)₃:0.02Eu²⁺ phosphor film on blue chip, we can obtain a white LED device with high CRI value of 85.3 and CCT value of 5083 K. With the interesting tunable emission property, Li(Sr,Ca)₄(BO₃)₃:Eu²⁺ phosphor has great application potential as a good color conversion material for solid state lighting.

Acknowledgements

This research is supported by Zhejiang Provincial Natural Science Foundation of China under Grant No. LR14A040002, LQ13E020003 and LZ14F050001, the Project of the National Nature Science Foundation of China under Grant No. 51402077, 51272243, 61370049 and 61405185.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13029a

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