A series of Sr_{0.8 −x}Ca_0.2Al₂Si₂O₈: xEu²⁺ (x = 0, 0.1%, 1%, 3%, 5%, 7%, 8%, and 10%) phosphors were synthesized in this work through the solid-state reaction with CaCO₃ (A.R.), Eu₂O₃ (99.99%), Al₂O₃ (A.R.), SrCO₃ (A.R.), and SiO₂ (A.R.) as raw materials. Stoichiometric mixtures of the raw materials were homogeneously mixed and ground, then placed in the alumina crucibles with covers and sintered at 1400 °C for 4 h under reductive atmosphere (95% H₂+5% N₂) in a high temperature furnace. After firing, the samples were cooled to room temperature, and ground into powder for subsequent use.

Phase purity of the samples was checked using a D8 x-ray diffractometer (XRD, 40 kV, 40 mA) with Cu Kα radiation (λ = 1.5418 Å). The XRD data were collected in a 2θ range from 10° to 80°, with the continuous scan mode at the speed of 0.1 s per step with a step size of 0.05°. Furthermore, the XRD patterns submitted for Rietveld refinement were acquired at a step size of 0.03° with a counting time of 0.5 s per step. Scanning electron micrograph (SEM) images were obtained using a Nova Nano SEM 650 instrument.

The excitation and emission spectra were measured using a Hitachi F4600 fluorescence spectrophotometer and a steady state fluorescence spectrometer equipped with a continuous xenon lamp (450 W), and both excitation and emission spectra were set up to be 0.2 nm with the width of the monochromator slits adjusted as 2.5 nm. The diffuse reflection spectra were measured with a Hitachi U4100 UV-VIS-NIR spectroscope, with the scanning wavelength from 200 nm to 800 nm at 240 nm/min. The thermoluminescence (TL) spectra of the samples were measured using an FJ-427Al TL dosimeter with a fixed heating rate of 1 °C/s within the range 25–300 °C. The luminescence decay curves of the samples were measured with a Horiba FL-1057 fluorescence spectrophotometer using an external LED (370 nm) as the excitation source. The thermally stable property was tested using a heating apparatus (TAP-02) in combination with a PL equipment. The color coordinates (X, Y) of the samples were obtained by a PMS-80 color analyzer at room temperature. The internal quantum yields (IQYs) of the phosphors were measured by a Horiba FL-1057.

The XRD patterns of SCAS: xEu²⁺ (x = 0, 0.1%, 1%, 3%, 5%, 7%, 8%, and 10%) samples are displayed in Fig. 1(a). It can be seen that all of the diffraction peaks can be indexed to the standard data of SrAl₂Si₂O₈ (JCPDS 38-1454), indicating that the doped ions are completely dissolved into the SrAl₂Si₂O₈ host without inducing significant changes of the crystal structure. The main magnified XRD patterns in the range of 35°–36.4° are shown in Fig. 1(b). It can be found that the main diffraction peaks of SCAS: xEu²⁺ first shift slightly to smaller angles before x = 3%, but shift to higher angles after x = 3%. On the basis of the effective ionic radii of the cations with different coordination numbers, the Eu²⁺ ions are expected to occupy smaller Ca²⁺ sites (n = 6, 7) and Sr²⁺ sites (n= 6) when , but occupy bigger Sr²⁺ sites (n = 7) when , because the effective ionic radii for six-coordinated Ca²⁺, Sr²⁺, and Eu²⁺ are 1 Å, 1.13 Å, and 1.17 Å, and seven-coordinated Ca²⁺ Sr²⁺, and Eu²⁺ are 1.06 Å, 1.13 Å, and 1.20 Å, respectively.^[20–22]

Fig. 1.

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	Fig. 1. (color online) (a) XRD patterns and (b) the detailed XRD patterns ranging from 35° to 36.4° of as-prepared samples SCAS: xEu2+ (x = 0, 0.1%, 1%, 3%, 5%, 7%, 8%, and 10%). The standard data for SrAl2Si2O8 (JCPDS 38-1454) is shown as a reference.

The Rietveld structural refinements of the XRD data profiles for SCAS: 3%Eu²⁺ are presented in Fig. 2. The results show that Sr_0.8Ca_0.2Al₂Si₂O₈: 3%Eu^{2 +} has a triclinic unit cell with cell parameters a = 8.35 Å, b = 12.976 Å, c = 14.227 Å, and V = 1392.16 ų. The refinements finally converge to the goodness of fit coefficient with , , and , which indicates that the crystal structures of these phosphors match well with the starting model (Sr_0.8Ca_0.2Al₂Si₂O₈) after the refinement.

Fig. 2.

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	Fig. 2. (color online) Rietveld refinements of powder XRD profiles of the representative SCAS: 3%Eu2+ phosphor.

Figure 3 presents the crystal structure of Sr_0.8Ca_0.2Al₂Si₂O₈. The structure consists of a layered framework made from Si–O as well as Al–O tetrahedron.^[23] The Si–O and Al–O tetrahedrons link to each other by sharing corners, forming an unusual eight-member ring. The Ca²⁺ and Sr²⁺ ions surrounded with oxygen neighbors in sheets compensate for the charge difference between Al³⁺ and Si⁴⁺.^[24,25] The Sr_0.8Ca_0.2Al₂Si₂O₈ host contains two kinds of cation sites denoted as Ca1/Sr1 and Ca2/Sr2. Ca1/Sr1 are coordinated by 7 oxygen atoms, and Ca2/Sr2 are coordinated by 6 oxygen atoms. Figure 4 show the SEM images of microstructures and shapes for the representative SCAS host and SCAS: 3%Eu²⁺ samples. It is observed that the particles have irregular blocky shape with a size of about .

Fig. 3.

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	Fig. 3. The crystal structure of Sr0.8Ca0.2Al2Si2O8.

Fig. 4.

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	Fig. 4. SEM images of (a) SCAS host and (b) SCAS: 3%Eu2+.

The diffuse reflectance spectra (DRS) of SCAS and Eu²⁺ doped SCAS phosphors are displayed in Fig. 5. It is obvious that the profiles of Eu²⁺-doped SCAS are very different from that of the SCAS host, which illustrates that the Eu²⁺ ions can be an effective activator and exhibit their characteristic excitation and emission spectral properties in this host. As can be seen in the DRS of the SCAS host, the absorption in the ultraviolet can be attributed to the valenceto-conduction band. The band gap of the SCAS host is shown in the inset of Fig. 5. The band gap of the SCAS host can be estimated using the following formula:^[26] 1 where h is Planck’s constant, υ is the frequency, is the band gap, A is the proportional constant, and exponent n is related to the nature of electron transition: denotes the direct allowed transition and n = 0.5 for an indirect allowed transition. is a Kubelka–Munk function defined as^[27] 2 where K represents the absorption coefficient, and S and R are the absorption coefficient and the scattering coefficient, respectively.

Fig. 5.

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	Fig. 5. (color online) UV–visible DRS of SCAS host. Inset shows the plot of versus photoenergy for the SCAS host.

From the reflection spectrum, the band gap is estimated to be about 3.683 eV. As the Eu²⁺ ions are introduced into the SCAS host, the DRS shows a broad deep valley from 280 nm to 650 nm, which is attributed to the 4f–5d absorption of the Eu²⁺ ions.^[28–30] In addition, it can also be found that the absorption extent increases continuously with Eu²⁺ concentration increasing, which further demonstrates the absorption bands deriving from the Eu²⁺ ions.

The excitation spectra of SCAS: xEu²⁺ ( ) phosphors are shown in Fig. 6. It can be seen that the intensity increases firstly and then decreases with Eu²⁺ concentration increasing. The quenching concentration is 3%. Under monitoring at 417 nm, an intense and broad excitation band (200 nm to 400 nm) with a peak at 353 nm can be observed, which is ascribed to the transition from the 4f⁷5d⁰ ground state to the 4f⁶5d¹ excited state of the Eu²⁺ ion. The broad excitation spectra with high absorption ability from 350 nm to 380 nm can match well with n-UV LED chips.

Fig. 6.

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	Fig. 6. (color online) Excitation (PLE) spectra of SCAS: xEu2+ ( ) phosphors ( ).

Figure 7 shows the emission spectra of SCAS: xEu²⁺ ( ) phosphors under the excitation of 353 nm. It is clear to see that the emission spectra present an asymmetric broad band (400–650 nm), which indicates that the Eu²⁺ ions have more than one luminescent center in the lattice. In addition, photos of SCAS: xEu²⁺ ( ) under 365 nm excitation are also shown in Fig. 7. As we can see, the emission changes from deep blue to light blue, which indicates that a small red-shift appears with the increase of Eu²⁺ concentration. To clearly show the red-shift behavior, the normalized emission spectra of SCAS: xEu²⁺ ( ) are shown in Fig. 8.

Fig. 7.

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	Fig. 7. (color online) Emission (PL) spectra of SCAS: xEu2+ ( ) phosphors ( ). Insets show the relative intensity of peak and digital images ( ) of SCAS: Eu2+ phosphor with various concentrations of Eu2+.

Fig. 8.

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	Fig. 8. (color online) Normalized emission spectra of SCAS: xEu2+ phosphors ( ), and the inset shows the corresponding peak wavelength.

It can be seen that the peak of the emission band shifts to longer wavelength from 413 nm to 435 nm together with increasing full width at half maximum (FWHM) as the Eu²⁺ concentration increases. In order to find out the origin of the red shift and increasing FWHM, we carry out the following study. Firstly, as is mentioned in the inset of Fig. 3, there are four cation sites in the SCAS host (Ca1/Sr1 coordinated by seven oxygen atoms and Ca2/Sr2 coordinated by six oxygen atoms). To study the occupation of Eu²⁺ in the four cations, we conduct Gauss fitting on the emission spectra, as shown in Fig. 9.

Fig. 9.

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	Fig. 9. (color online) Emission spectra of SCAS: xEu2+ phosphors excited at 353 nm with Gaussian peaks.

The emission spectra are deconvoluted into four Gaussian components peaking at , , , and , respectively. In order to clarify the origin of the four emission bands, the well-known experiential equation given by Van Uitert is used as follows:^[31] 3 where E represents the energy position of the d-band edge for the rare-earth ion (cm⁻¹), Q is the energy position for the lower d-band edge for the free ion and is 34000 cm⁻¹ for Eu²⁺, V is the valence of the activator (for Eu²⁺ ion, V = 2), n is the number of anions in the immediate shell around the Eu²⁺ ion, r is the radius of the host cation replaced by Eu²⁺ ion, and is the electron affinity of the atoms that form anions relying on the anion type. Although is very complex and difficult to determine, it is a fixed value in a specific host. Therefore the value of E depends on the parameters n and r directly, and the value of E increases with the increase of . In order to make clear which sites the four emission bands come from, the values of are presented in Table 1. It is clear to see that the values of for Sr1 and Ca1 are 8.47 and 7.42, respectively. The values of for Sr2 and Ca2 are 6.78 and 6, respectively. Since E is proportional to , it can be deduced that the four Gaussian components from right to left belong to Eu²⁺ centers at Sr1, Ca1, Sr2, and Ca2, respectively. In addition, it is well known that the shift of the spectrum is closely related to the splitting of the crystal field. So we study the crystal field splitting caused by the substitution of Eu²⁺ for different cations. According to the reports of Robertson et al.^[32] and Jang et al,^[33] crystal field splitting can be determined by the following equation:^[34,35] 4 where is a measure of the energy level separation, Z is the anion charge, e is the electron charge, r is the radius of the d wavefunction, and R is the bond length. When the Eu²⁺ ion substitutes a smaller ion, the distance between Eu²⁺ and O²⁻ becomes short. Since the crystal field splitting is proportional to , this shorter Eu²⁺–O²⁻ distance also leads to the enhancement of the crystal field strength surrounding the Eu²⁺ ion and further results in a larger crystal field splitting of the Eu²⁺ 5d energy levels, which makes the lowest 5d state of Eu²⁺ closer to its ground state and finally gives a red shift of the emission peak of Eu²⁺. Oppositely, when the Eu²⁺ ion replaces a bigger ion, the emission spectrum shows a blue shift.^[36] However, in this work, although the host contains both Ca²⁺ ( and Sr²⁺ ( , the obtained emission spectra just show the red shift, which may attribute to the combination effect of the following two cases.

Table 1.

Table 1.

Table 1. Values of at different sites. . Site Sr1 Ca1 Sr2 Ca2 n 7 7 6 6 r 1.21 1.06 1.13 1 8.47 7.42 6.78 6

Table 1. Values of at different sites. .

In order to find out the cause of the spectrum red shift, figuring out Eu²⁺ substitution priority is a primary work. As is known to us, the thermal quenching behavior is associated with the crystal structure of the phosphor. So the substitution of Eu²⁺ ions for different cations will result in different thermal quenching behaviors. Based on this, the emission spectra at different temperatures (25–300 °C) excited by 353 nm for SCAS: xEu²⁺ (x = 0, 0.1%, 3%, 5%, 8%, and 10%) are measured and plotted in Fig. 10. At the same time, as a comparison, the temperature properties of Eu²⁺ doped single-cation host phosphors SrAl₂Si₂O₈: 3%Eu²⁺ and CaAl₂Si₂O₈: 3%Eu²⁺ are also measured under the same conditions and presented at the top of Fig. 10. It can be seen that the emission intensity of SrAl₂Si₂O₈: 3%Eu²⁺ decreases much faster than that of CaAl₂Si₂O₈: 3%Eu²⁺ with temperature increasing, which indicates that Eu²⁺ replacing Ca²⁺ is better for the thermal stability of the phosphor. The temperature quenching behaviors of SCAS: xEu²⁺ phosphors show an interesting phenomenon. For x = 0.1%–3%, the emission intensities decline weakly with increasing temperature, which are close to the behavior of the CaAl₂Si₂O₈: 3%Eu²⁺. Meanwhile, the emission intensity for x = 10% declines relatively quickly with the increase of temperature, which is similar to the thermal quenching behavior of SrAl₂Si₂O₈: 3%Eu²⁺. For the intermediate contents, x = 5% and 8%, the samples present the transition state of the two. The results indicate that the Ca²⁺ ions are preferentially substituted by Eu²⁺ at lower doping. With the Eu²⁺ concentration increasing, the probability of substitution for Sr²⁺ is greater than that of replacing Ca²⁺, which is basically consistent with the above analysis of the XRD. In addition, it can be observed that the emission intensities show an abnormal increase with the increasing temperature for x = 3%, 5%, and 8% in Fig. 10. To investigate the cause of the phenomenon, the thermoluminescence spectra of SCAS: xEu²⁺ (x = 0.1%, 3%, 5%, 8% and 10%) are measured and shown in Fig. 11.

Fig. 10.

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	Fig. 10. (color online) Emission spectra of MAl2Si2O8: 3% Eu2+ (M = Sr and Ca) and SCAS: xEu2+ (x = 0.1%, 3%, 5%, 8%, 10%) at different temperatures.

Fig. 11.

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	Fig. 11. (color online) TL glow curves of SCAS: x Eu2+ (x = 0.1%, 3%, 5%, 8%, and 10%).

It is obvious that there is defect luminescence in all the samples, and moreover the TL intensities decrease with Eu²⁺ concentration increasing. So, for x = 10%, the TL intensity is so weak that it makes no obvious effect on the photoluminescence intensity. Additionally, as shown in Fig. 10, an obvious blue shift of the emission peak is observed with increasing temperature for SCAS: xEu²⁺, which can be explained by the thermally active phonon-assisted tunneling from lower energy sublevel to high-energy sublevel of the excited state of Eu²⁺.^[37–39] Two emissions (Eu(I) (occupying Ca(1) and Sr(1)) and Eu (II) (occupying Ca(2) and Sr(2)) arise from different minima on the potential energy surface of the relaxed excited state as given in Fig. 12. Since Eu(I) and Eu(II) ions are in different crystal fields, the 4f⁶5d¹ excited states of the two are located at different energy levels. When SCAS: Eu²⁺ phosphor is excited by the UV light, the electrons release different energies from different excited states to ground state. At low temperature, the thermal transfer ( ) over barrier can be overcome and the low-energy emission Eu (2) is dominant. At high temperature, the thermal back-transfer ( ) over the barrier is possible, and consequently the high energy emission Eu (I) is dominant. Thus, the blue shift behavior is observed with increasing temperature.

Fig. 12.

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	Fig. 12. (color online) Schematic illustration of a configurational coordinate diagram of the ground state (Eu2+: 4f75d0), two split excited states (Eu(I) and Eu(II), and showing the energy barrier of the transition Eu(I) state to Eu(II), and showing the energy barrier of the transition Eu(II) state to Eu(I).

Through the discussion of temperature properties, the substitution process is that the Ca²⁺ ions are preferentially substituted by Eu²⁺at lower doping, and with the Eu²⁺ concentration increasing, the probability of substitution for Sr²⁺ is greater than that of replacing Ca²⁺. Considering the red-shift of the emission spectra, it is far more likely that there is energy transfer from Eu²⁺ ions at higher energy to Eu²⁺ ions at lower energy. In order to prove this speculation, a series of fluorescent decay curves are measured by monitoring two different luminescence centers at Eu(I) and Eu (II) for SCAS: xEu²⁺ (x = 0.1%, 1%, 3%, 5%, 7%, 8%, and 10%) phosphors. The decay curves are obtained by monitoring the emission peaks at around 405 nm, 419 nm, 439 nm, and 451 nm, corresponding to the different emission centers, and plotted in Fig. 13.

Fig. 13.

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	Fig. 13. (color online) Decay curves of different luminescence centers (Eu(I) of Ca1 (a), Sr1 (b) and Eu (II) of Ca(2) (c), Sr (2) (d)).

The decay behaviors can be fitted with a second-order exponential decay mode because of the two luminescence centers (Eu(I) and Eu(II) as follows:^[40] 5 where I refers to the luminous intensity, A₁ and A₂ are constants, t is time, and τ₁ and τ₂ are the rapid and slow lifetimes for the exponential components, respectively. According to Eq. (6), A₁, A₂, τ₁, and τ₂ can be calculated based on the fitting of the decay curves. Therefore the effective lifetime constant τ can be calculated by^[41] 6

The variations of τ for Eu(I) and Eu (II) with increasing Eu²⁺ concentration are depicted in Fig. 14. For x = 0.1%, is close to and is close to . As we discussed above, Eu²⁺ preferentially replaces the Ca²⁺ ion at low Eu²⁺ concentration. Considering the similar lifetimes and smaller 2θ shift of XRD, we deduce that the calculated and should be and in SCAS: 0.1%Eu²⁺. With the increase of the Eu²⁺ concentration, decreases while increases firstly and then decreases. Furthermore, decreases while increases. The above results can strongly prove the existence of energy transfer from Eu (I) to Eu (II). In summary, the size mismatch of Eu²⁺, Sr²⁺, and Ca²⁺ and the energy transfer between the Eu²⁺ ions together lead to the red shift of the emission spectra. The energy transfer from Eu (I) to Eu (II) also contributes to the increasing FWHM of the emission spectra. A schematic diagram of the red shift is shown in Fig. 15. In order to clarify the energy transfer mechanism from Eu (I) to Eu (II), it is necessary to know the critical distance ( between the activators. With the increasing Eu²⁺ content, the distance between the Eu²⁺ ions becomes shorter, thus the probability of energy transfer increases. When the distance becomes small enough, the concentration quenching^[42] occurs and the energy migration is hindered. The calculation of has been given by Blasse^[43] 7 where V corresponds to the volume of the unit cell, N is the number of host cations in the unit cell, and is the critical concentration of doping ions. For the SCAS host, N = 8, V = 1400 ų, and is 3% for Eu²⁺. Accordingly, the critical distance ( is estimated to be about 22.33 Å. In general, there are three mechanisms for non-radiant energy transfer, including exchange interaction, radiation reabsorption, and electric multipolar interaction.

Fig. 14.

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	Fig. 14. (color online) Lifetimes of Eu2+ ions in different luminescence centers for different Eu2+ contents.

Fig. 15.

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	Fig. 15. (color online) Schematic diagram of red-shift for the emission spectra.

The result obtained above indicates little possibility of exchange interaction since the exchange interaction is predominant only for about 5 Å which is far less than 22.33 Å. Consequently, we can conclude that the energy transfer mechanism between the Eu²⁺ ions is electric multipolar interaction. According to the formula proposed by Dexter and Van Uitert, the emission intensity (I) per activator ion follows the equation^[44,45] 8 where I represents the emission intensity, x is the activator ion concentration, K and β are constants for the given matrix under the identical excitation conditions. The type of energy transfer mechanism of electric multipolar interaction can be estimated by analyzing the constant θ from this formula. The value of θ is 6, 8, 10, corresponding to electric dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The curve of log (I/x) versus log(x) for SCAS: Eu²⁺ phosphors beyond the quenching content of Eu²⁺ is plotted in Fig. 16. The θ value is obtained by the fitted straight line with a slope equal to . The θ is determined to be 4.41, which is close to 6. The result indicates that the concentration quenching mechanism of the Eu²⁺ emission in SCAS host is dominated by the dipole–dipole interaction.

Fig. 16.

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	Fig. 16. (color online) Fitting line of vs. in SACA: xEu2+ ( ) phosphors beyond the quenching concentration.

As important parameters, CIE chromaticity coordinates and internal quantum yields (IQYs) of SCAS phosphors are also measured and presented in Table 2. The CIE chromaticity coordinates vary from (0.1575, 0.0792) to (0.1599, 0.1935) corresponding to x = 1%–10%, and the color shifts from blue to cyan with Eu²⁺ concentration increasing. The maximum IQY in as-prepared samples is 66.53%. To demonstrate the advantages of this blue-emitting phosphor, a series of samples are used to fabricate warm white LEDs. In order to obtain white light, commercial red phosphor (Ca Sr)SiAlN₃: Eu²⁺ and green phosphor (Y Lu)₃Al₅O₁₂: Ce³⁺ are employed to adjust the luminescence. The white LED devices of various Eu²⁺ concentrations under a driven current of 350 mA are given in Fig. 17. The relevant parameters are presented in

Fig. 17.

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	Fig. 17. (color online) Photographs of the fabricated warm white-LEDs driven by 350 mA current (x = 3%, 5%, 7%, 8%, and 10%).

Table 2.

Table 2.

Table 2. CIE chromaticity coordinates and IQYs of SCAS: xEu2+ phosphors excited at 365 nm UV radiation. . SCAS: xEu2+ (X,Y) IQY/% x = 1% (0.1579, 0.0792) 66.53 x = 3% (0.1574,0.0928) 60.22 x = 7% (0.158, 0.1097) 53.24 x = 8% (0.1595, 0.1235) 54.36 x = 10% (0.1600, 0.1236) 58.79

Table 2. CIE chromaticity coordinates and IQYs of SCAS: xEu2+ phosphors excited at 365 nm UV radiation. .

Table 3. The CIE chromaticity coordinates of white light turn from (0.3582, 0.3591) to (0.4346, 0.4063). The color rendering indexes range from 86.6 to 89.6. The low correlated color temperatures vary from 3000 K to 4500 K, which meets the indoor lighting requirements. Moreover, the luminescent efficiency of x= 3% reaches 45 lm/W. The above results demonstrate that the white LED device of x= 3% may be an excellent candidate for warm white-LEDs.

Table 3.

Table 3.

Table 3. Main parameters of the selected white LED devices. . Sample /K CIE color coordinate 3%Eu2+ 3058 87.8 (0.4346, 0.4063) 5%Eu2+ 3049 86.8 (0.4323, 0.4003) 7%Eu2+ 4419 89.6 (0.3609, 0.3679) 8%Eu2+ 4459 86.6 (0.3582, 0.3591) 10%Eu2+ 3114 89.6 (0.4342, 0.4126)

Table 3. Main parameters of the selected white LED devices. .