Multi-Band Emission of Pr3+-Doped Ca3Al2O6 and the Effects of Charge Compensator Ions on Luminescence Properties

Multi-band emission luminescence materials are of great significance owing to their extensive application in diverse fields. In this research, we successfully prepared a series of Pr3+-doped Ca3Al2O6 multi-band emission phosphors via a high-temperature solid-state method. The phase structure, morphology, luminescence spectra and decay curves were investigated in detail. The Ca3Al2O6:Pr3+ phosphors can absorb blue lights and emit lights in the 450–750 nm region, and typical emission bands are located at 488 nm (blue), 525–550 nm (green), 611–614 nm (red), 648 nm (red) and 733 nm (deep red). The influence of the Pr3+ doping concentration was discussed, and the optimal Pr3+ doping concentration was determined. The impacts of charge compensator ions (Li+, Na+, and K+) on the luminescence of Pr3+ were also investigated, and it was found that all the charge compensator ions contributed positively to the emission intensity. More importantly, the emission intensity of the as-prepared phosphors at 423 K can still maintain 65–70% of that at room temperature, and the potential application for pc-LED was investigated. The interesting results indicate that the prepared phosphors may serve multifunctional and advanced applications.


Introduction
In recent years, luminescence materials have attracted great attention owing to their extensive applications, such as daily lighting, backlight displays, biological imaging and medical diagnosis [1][2][3][4][5].Nowadays, the demand for high-performance materials is growing rapidly with the development of technology.Thus, an investigation on novel luminescence materials is of great significance for both application and basic research.Luminescence center ions play a vital role for phosphors, which determine the luminescence performance of a phosphor to a large extent.Over the past decades, rare earth ions have been widely investigated, as luminescence centers in phosphors, such as Ce 3+ , Eu 3+ , Tb 3+ , Pr 3+ , and Er 3+ [6][7][8][9][10].Pr 3+ is one of the important luminescence activators, which is rich in energy levels.More importantly, the emission spectra of Pr 3+ in different host lattices can cover ultraviolet to infrared regions [11,12].
The Ca 3 Al 2 O 6 compound may be a good host matrix due to its excellent physical and chemical stability [24].To our knowledge, the luminescence of Ce 3+ , Eu 2+ , Dy 3+ , Bi 3+ , Sm 3+ , Tb 3+ , and Mn 2+ in Ca 3 Al 2 O 6 has hitherto been investigated [24][25][26][27][28][29].In this research, considering the advantages of the Ca 3 Al 2 O 6 host and the Pr 3+ ion mentioned above, we performed a study on the luminescence properties of Pr 3+ in the Ca 3 Al 2 O 6 host.As reported, charge compensator ions may play a vital role to achieve a balance of charges, which could further have great impacts on the luminescence properties of a phosphor [30,31].Herein, considering the charge imbalance between Ca 2+ and Pr 3+ , alkali metal R + (R = Li, Na, K) ions were selected as charge compensator ions.The effects of charge compensator ions (Li + , Na + , and K + ) were also discussed in detail.

Materials and Methods
In this research, a series of Pr 3+ -activated multi-band emission phosphors, Ca 3−x Pr x Al 2 O 6 and Ca 2.97−x Pr 0.03 R x Al 2 O 6 (R + = Li + , Na + , K + ), were synthesized through a solid-state reaction technique.The raw starting reactants CaCO 3 (99.99%,Aladdin, Wallingford, CT, USA), Al 2 O 3 (99.99%,Aladdin), Pr 2 O 3 (99.9%,Aladdin), Li 2 CO 3 (99.9%,Aladdin), Na 2 CO 3 (99.5%,Aladdin) and K 2 CO 3 (99.5%,Aladdin) were used for chemical reactions.As is typical for the process of synthesis, the raw reactants were accurately weighed according to the stoichiometric ratio, and then the mixed reactants were ground for about 30 min in an agate mortar.Afterwards, the well-mixed reactants were fully transferred into alumina crucibles and sintered at 1623 K for 4 h in air.Lastly, the samples were cooled down to room temperature naturally and thoroughly ground to obtain fine white powders.
The X-ray diffraction (XRD) patterns were measured using a D8 Advance diffractometer (Cu Kα, λ = 1.5406Å) for phase analysis.Structure refinements were performed using the TOPAS 5.0-Academic software.Scanning electron micrographs (SEM) were conducted on a field emission scanning electron microscopy (Hitachi SU5000, Tokyo, Japan) for micro-morphology analysis.Diffuse reflectance spectra were measured by a UV3600 spectrofluorometer (Shimadzu, Kyoto, Japan).Luminescence spectra and decay curves at different temperatures were all collected on an Edinburgh FLS1000 spectrofluorometer, and the excitation sources were a 450 W xenon lamp and a µF900 lamp, respectively.The electroluminescence spectra of a pc-LED device were measured on an OHSP-350M LED fast-scan spectrophotometer (Hangzhou Hopoo Light and Color Technology Co., Ltd., Hangzhou, China).

Phase Structure and Morphology
To confirm the phase structure of the as-prepared phosphors, X-ray Rietveld refinements were performed for two typical samples, Ca 3 Al 2 O 6 and Ca 2.97 Pr 0.03 Al 2 O 6 .Figure 1a,b depict the refinement results.All the calculated diffraction patterns accord well with the observed ones, which indicates that the as-prepared samples are of a single pure phase.The detailed cell parameters for the X-ray Rietveld refinements are illustrated in Table 1.The undoped and Pr 3+ -doped Ca 3 Al 2 O 6 phosphors crystallize in a cubic system with a Pa3 space group.Due to the similar ionic radii for Ca 2+ and Pr 3+ (e.g., r(Ca 2+ ) = 1.00 Å, CN = 6; r(Pr 3+ ) = 0.99 Å, CN = 6), the cell parameters a, b, c, and cell volumes remain nearly unchanged for Ca 3 Al 2 O 6 and Ca 2.97 Pr 0.03 Al 2 O 6 .Herein, a small expansion is most probably ascribed to the experimental errors.The crystal structure of the Ca 3 Al 2 O 6 host is displayed in Figure 1c.The frame structure consists of [AlO 4 ] tetrahedrons and [CaO 6 /CaO 7 /CaO 8 /CaO 9 ] polyhedrons.There are six different Ca 2+ sites in this structure.Ca 2+ (1), Ca 2+ (2) and Ca 2+ (3) are coordinated with six oxygen atoms, and the average Ca 2+ -O 2− bond lengths are 2.338 Å, 2.391 Å and 2.354 Å, respectively.Ca 2+ (4), Ca 2+ (5), Ca 2+ (6) are coordinated with nine, eight and seven oxygen atoms, and the average Ca 2+ -O 2− bond lengths are 2.693 Å, 2.625 Å and 2.525 Å, respectively.The coordination environments of Ca 2+ are also shown in Figure 1c.In the present case, Pr 3+ ions may enter all six Ca 2+ sites, and the luminescence properties we observed should be the whole contribution of Pr 3+ in Ca 2+ sites.
nearly unchanged for Ca3Al2O6 and Ca2.97Pr0.03Al2O6.Herein, a small expansion is most probably ascribed to the experimental errors.The crystal structure of the Ca3Al2O6 host is displayed in Figure 1c.The frame structure consists of [AlO4] tetrahedrons and [CaO6/CaO7/CaO8/CaO9] polyhedrons.There are six different Ca 2+ sites in this structure.Ca 2+ (1), Ca 2+ (2) and Ca 2+ (3) are coordinated with six oxygen atoms, and the average Ca 2+ -O 2− bond lengths are 2.338 Å, 2.391 Å and 2.354 Å, respectively.Ca 2+ (4), Ca 2+ (5), Ca 2+ (6) are coordinated with nine, eight and seven oxygen atoms, and the average Ca 2+ -O 2− bond lengths are 2.693 Å, 2.625 Å and 2.525 Å, respectively.The coordination environments of Ca 2+ are also shown in Figure 1c.In the present case, Pr 3+ ions may enter all six Ca 2+ sites, and the luminescence properties we observed should be the whole contribution of Pr 3+ in Ca 2+ sites.Figure 2a shows the XRD patterns of Pr 3+ -doped phosphors Ca3−xPrxAl2O6.The observed diffraction peaks are similar in the investigated doping concentration range, and all the diffractions are consistent with the standard card PDF 38-1429[Ca3Al2O6], demonstrating that Pr 3+ ions were successfully introduced into the Ca3Al2O6 host.The doping of Pr 3+ did not have a significant impact on the host structure.In addition, a series of  Figure 2a shows the XRD patterns of Pr 3+ -doped phosphors Ca 3−x Pr x Al 2 O 6 .The observed diffraction peaks are similar in the investigated doping concentration range, and all the diffractions are consistent with the standard card PDF 38-1429[Ca 3 Al 2 O 6 ], demonstrating that Pr 3+ ions were successfully introduced into the Ca 3 Al 2 O 6 host.The doping of Pr 3+ did not have a significant impact on the host structure.In addition, a series of Ca 2.97−x Pr 0.03 R x Al 2 O 6 (R = Li + , Na + , K + ) samples were also prepared, and the XRD results are shown in Figure 2b.As can be seen, the diffraction patterns also accord well with the standard card.All the samples are of a single pure phase.Figure 2c displays the representative SEM image of the Ca 2.97 Pr 0.03 Al 2 O 6 sample, and the as-prepared sample shows irregular morphology with several micrometers in size.The EDS (energy-dispersive spectroscopy) images were obtained from one particle [marked by a red square] selected from the SEM image in Figure 2c.The elements Ca, Al, Pr and O were successfully detected, as shown in Figure 2d.The elemental mapping results indicate all the elements Ca, Al, O, Pr have been uniformly distributed over the whole particle, and there is no obvious element aggregation in the particles.
shown in Figure 2b.As can be seen, the diffraction patterns also accord well with the standard card.All the samples are of a single pure phase.Figure 2c displays the representative SEM image of the Ca2.97Pr0.03Al2O6sample, and the as-prepared sample shows irregular morphology with several micrometers in size.The EDS (energy-dispersive spectroscopy) images were obtained from one particle [marked by a red square] selected from the SEM image in Figure 2c.The elements Ca, Al, Pr and O were successfully detected, as shown in Figure 2d.The elemental mapping results indicate all the elements Ca, Al, O, Pr have been uniformly distributed over the whole particle, and there is no obvious element aggregation in the particles.

Luminescence Properties of Pr 3+ in Ca3Al2O6
Figure 3a illustrates the diffuse reflectance spectra of Ca3Al2O6 and Ca2.97Pr0.03Al2O6.A very weak absorption band before 400 nm can be observed, which was ascribed to host-related absorption.The diffuse reflectance spectrum of the Ca3Al2O6 host was in good accordance with the reported one [25].For the Ca2.97Pr0.03Al2O6sample, a series of sharp absorption lines in the 400-650 nm range also can be observed.In comparison with the pure Ca3Al2O6 host, the absorptions in 400-650 nm are assigned to the 4f-4f transition absorptions of Pr 3+ in the host.To further characterize the luminescence properties of Ca2.97Pr0.03Al2O6, the excitation spectrum and the corresponding emission spectrum are shown in Figure 3b,c, respectively.After monitoring the emission at 612 nm, a series of excitation bands were detected.The sharp excitation bands in the 425-500 nm wave-  A very weak absorption band before 400 nm can be observed, which was ascribed to host-related absorption.The diffuse reflectance spectrum of the Ca 3 Al 2 O 6 host was in good accordance with the reported one [25].For the Ca 2.97 Pr 0.03 Al 2 O 6 sample, a series of sharp absorption lines in the 400-650 nm range also can be observed.In comparison with the pure Ca 3 Al 2 O 6 host, the absorptions in 400-650 nm are assigned to the 4f-4f transition absorptions of Pr 3+ in the host.To further characterize the luminescence properties of Ca 2.97 Pr 0.03 Al 2 O 6 , the excitation spectrum and the corresponding emission spectrum are shown in Figure 3b,c, respectively.After monitoring the emission at 612 nm, a series of excitation bands were detected.The sharp excitation bands in the 425-500 nm wavelength range are ascribed to the 3 H 4 → 3 P 0,1,2 transition absorptions of Pr 3+ .A very weak band at around 300 nm may relate to the essential absorption of the Ca 3 Al 2 O 6 host.The excitation spectrum agrees with the diffuse reflectance spectra in Figure 3a.This result indicates that a weak energy transfer from the host lattice to Pr 3+ may occur.Based on the excitation spectrum, the corresponding emission spectrum was measured, as shown in Figure 3c.Upon 446 nm excitation, a series of sharp emission bands from the blue to deep red region were achieved, which are mainly related to the 3 P 0 → 3 H 4 , 3 P 0 → 3 H 5 , 3 P 0 → 3 H 6 / 1 D 2 → 3 H 4 , 3 P 0 → 3 F 2 , 3 P 0 → 3 F 4 transitions of Pr 3+ [32,33].In order to the reveal luminescence process, the energy levels of Pr 3+ in the 0-25,000 cm −1 range are shown in the inset of Figure 3c.Upon 446 nm excitation, the exaction energy was absorbed by Pr 3+ through 3 H 4 → 3 P 2 transitions.Then, electrons returned to 3 P 0 and 1 D 2 levels via non-radiative relaxation processes, and the emission bands in 450-750 nm were finally observed.Herein, a multiband emission can be obtained under blue light (446 nm) excitation for samples singly doped with Pr 3+ , which indicates that the phosphor may have potential applications, such as LED application.
length range are ascribed to the 3 H4→ 3 P0,1,2 transition absorptions of Pr 3+ .A very weak band at around 300 nm may relate to the essential absorption of the Ca3Al2O6 host.The excitation spectrum agrees with the diffuse reflectance spectra in Figure 3a.This result indicates that a weak energy transfer from the host lattice to Pr 3+ may occur.Based on the excitation spectrum, the corresponding emission spectrum was measured, as shown in Figure 3c.Upon 446 nm excitation, a series of sharp emission bands from the blue to deep red region were achieved, which are mainly related to the 3 P0→ 3 H4, 3 P0→ 3 H5, 3 P0→ 3 H6/ 1 D2→ 3 H4, 3 P0→ 3 F2, 3 P0→ 3 F4 transitions of Pr 3+ [32,33].In order to the reveal luminescence process, the energy levels of Pr 3+ in the 0-25,000 cm −1 range are shown in the inset of Figure 3c.Upon 446 nm excitation, the exaction energy was absorbed by Pr 3+ through 3 H4→ 3 P2 transitions.Then, electrons returned to 3 P0 and 1 D2 levels via non-radiative relaxation processes, and the emission bands in 450-750 nm were finally observed.Herein, a multi-band emission can be obtained under blue light (446 nm) excitation for samples singly doped with Pr 3+ , which indicates that the phosphor may have potential applications, such as LED application.When the Pr 3+ doping concentration increases from 0.002 to 0.10, all the emission spectra are similar [see Figure 3d], demonstrating that the Pr 3+ doping concentration has little effect on the spectral shape of Ca 3−x Pr x Al 2 O 6 .However, the integrated emission intensity changes greatly with the doping concentration.As displayed in Figure 3e, the emission intensity greatly increases with increasing Pr 3+ concentrations at first and reaches a maximum at x = 0.03, then it decreases gradually with x value owing to the concentration quenching and the non-radiative energy transferring to quenching centers.Normally, the optimal doping concentration is associated with the crucial energy transfer distance (R c ). Herein, the R c value between Pr 3+ (activator ions) in the Ca 3 Al 2 O 6 host could be estimated through the following equation [3]: where V represents the unit cell volume, N is the number of Ca 2+ ions in a unit cell, and x c refers to the optimal doping concentration.For Pr 3+ -doped Ca 3 Al 2 O 6 , V = 3558.45Å 3 and N = 24.As a consequence, the R c value is estimated to be 26.63Å.In general, exchange interaction should be responsible for forbidden transitions with an R c value less than 5 Å.Clearly, the R c value is much larger than 5 Å in the present case.Therefore, multipolar interactions should be the dominant factor for the concentration quenching of Pr 3+ .
Figure 3f depicts the luminescence decay curves of 3 P 0 → 3 H 4 transition emission (488 nm).The decay processes nearly follow a first-order exponential form at a low Pr 3+ concentration, and then exhibit certain deviations for a high Pr 3+ concentration.First, some defects will exist in the phosphors due to the charge imbalance between Ca 2+ and Pr 3+ , and the high temperature sintering process may also generate some defects as well.The complex defects could affect the excited state relaxation process of Pr 3+ in the host.Second, the inner energy transfer or interactions between adjacent Pr 3+ ions increase gradually with the Pr 3+ doping concentration.Third, the multi-site luminescence of Pr 3+ exists in Ca 3 Al 2 O 6 , and the luminescence decay for Pr 3+ in each Ca 2+ site may also show some differences.Therefore, the decay curves of Pr 3+ exhibit bi-exponential or even multi-exponential decay behaviors with increasing the Pr 3+ doping concentration.Because of the deviations, the average decay constants can be estimated using Equation (2) [34]: The estimated decay constants are also shown in Figure 3f.The decay constants shortened from 127.84 µs (x = 0.002) to 118.22 µs (x = 0.10).For luminescence materials singly doped with Pr 3+ , the average lifetime τ is the reciprocal sum of all the radiative transition (W R ) and non-radiative transition (W NR ), as can be described by Equation (3) [35]: Herein, the decrease in τ confirms the increasing non-radiative energy transfer with x value.The influence of temperature is a key factor for further applications.Figure 4a shows the emission spectra of Ca 2.97 Pr 0.03 Al 2 O 6 at various temperatures.The 3 P 0 → 3 H 4 , 3 P 0 → 3 H 5 , 3 P 0 → 3 H 6 / 1 D 2 → 3 H 4 , 3 P 0 → 3 F 2 , 3 P 0 → 3 F 4 transition emission lines of Pr 3+ can be observed, All the emission spectra are similar, but the emission intensity changes remarkably.Figure 4b displays the emission intensity dependent on temperature.The relative emission intensity decreases gradually with increasing temperature owing to the temperature-involved thermal quenching.The emission intensity at 423 K maintains about 66.8% of that at 298 K (room temperature).In general, the ∆E a value (activation energy) can be used to evaluate the thermal quenching properties of a phosphor, and the relevant equation is described as following [36]: where I 0 and I T refer to the initial emission intensity and the intensity at a given temperature T, respectively.k represents the Boltzmann constant, and A can be treated as constant in specific cases.The Equation (4) could also be expressed as [37] ln where I0 and IT refer to the initial emission intensity and the intensity at a given temperature T, respectively.k represents the Boltzmann constant, and A can be treated as constant in specific cases.The Equation ( 4) could also be expressed as [37] ln (  0  − 1) = ln − ∆ As a consequence, the activation energy ΔEa can be obtained according to the relationship between ln[(I0/I) − 1] and 1/(kT).As depicted in the inset of Figure 4b, a well-fitted straight line with a slope of −0.149 was achieved.Thus, the ΔEa value is 0.149 eV for Ca2.97Pr0.03Al2O6.As reported, the ΔEa value for the Ca2ZnSi2O7:0.005Pr3+ phosphor is 0.2255 eV [38], and the values are 0.22 eV, 0.18 eV for Pr 3+ -doped SrLaMgTaO6:Pr 3+ and BaLaMgTaO6:Pr 3+ , respectively [35].The ΔEa value in this research is similar to that of Ca9MgLi(PO4)7:Pr 3+ (ΔEa = 0.15 eV), which is slightly larger than that of CaLaB7O13:Pr 3+ (ΔEa = 0.116 eV) [39,40].As a consequence, the activation energy ∆E a can be obtained according to the relationship between ln[(I 0 /I) − 1] and 1/(kT).As depicted in the inset of Figure 4b, a well-fitted straight line with a slope of −0.149 was achieved.Thus, the ∆E a value is 0.149 eV for Ca 2.97 Pr 0.03 Al 2 O 6 .As reported, the ∆E a value for the Ca 2 ZnSi 2 O 7 :0.005Pr3+ phosphor is 0.2255 eV [38], and the values are 0.22 eV, 0.18 eV for Pr 3+ -doped SrLaMgTaO 6 :Pr 3+ and BaLaMgTaO 6 :Pr 3+ , respectively [35].The ∆E a value in this research is similar to that of Ca 9 MgLi(PO 4 ) 7 :Pr 3+ (∆E a = 0.15 eV), which is slightly larger than that of CaLaB 7 O 13 :Pr 3+ (∆E a = 0.116 eV) [39,40].
Figure 4c shows the CIE (Commission International de I ′ Eclairage 1931) chromaticity coordinates for the emission of Ca 2.97 Pr 0.03 Al 2 O 6 at various temperatures.Although all the emission lines can be observed in Figure 4a, the chromaticity coordinates also show some differences, which move from (0.431, 0.379) at 298 K to (0.416, 0.417) at 573 K due to the thermal population of electrons between the 3 P 0 and 1 D 2 levels.The emission colors are located at the orange-yellow region in all the temperature ranges.
Upon 446 nm excitation and detecting the emission at 488 nm, temperature-dependent luminescence decay curves were collected and illustrated in Figure 4d.The luminescence decay times become shorter and shorter with increasing temperature, which also demonstrates the increasing non-radiative energy transfer processes.These results are consistent with the temperature-dependent emission spectra in Figure 4a.

The Influences of Charge Compensator Ions
In the above section, the phosphors were designed by nonequivalent substitution, that is, one Pr 3+ substitutes one Ca 2+ in the Ca 3 Al 2 O 6 host.Therefore, charge defects will exist due to the nonequivalent substitution, which may have impacts on the luminescence of Pr 3+ .Figure 5a shows the emission spectra of Ca 2.97−x Pr 0.03 Li x Al 2 O 6 (x = 0, 0.01, 0.02, 0.03) samples.The introduction of compensator ions Li + does not significantly influence the emission spectral shape.The inset depicts the integrated emission intensity at various Li + concentrations.The co-doping of Li + contributes positively to the emission intensity of Pr 3+ .The emission intensity is about 3.  5b,c.The incorporation of Na + and K + can also improve the emission intensity of Pr 3+ .The relative emission intensity is about 2.8 (0.01 Na + ), 2.3 (0.02 Na + ), 2.7 (0.03 Na + ), 1.2 (0.01 K + ), 1.6 (0.02 K + ), 1.3 (0.03 K + ) times that of Ca 2.97 Pr 0.03 Al 2 O 6 , respectively.Among all the samples, the optimal emission intensity can be achieved for Ca 2.96 Pr 0.03 Li 0.01 Al 2 O 6 .For the Ca 2.97 Pr 0.03 Al 2 O 6 sample, defects may be caused via several paths [31,41,42]: (1) Three Ca 2+ replaced by two Pr 3+ ions generates a Ca 2+ vacancy at the same time, which can be described by 3Ca Ca →2Pr • Ca +V ′′ Ca .
(2) Two Ca 2+ ions replaced by two Pr 3+ ions may cause an interstitial O ′′ i defect.
(3) Two Ca 2+ ions replaced by two Pr 3+ ions may cause an oxygen vacancy V ′′ O according to the possible process 2Ca Ca →2Pr • Ca +V ′′ O .In fact, some defects could act as killers of luminescence centers, resulting in the quenching of luminescence intensity.For Ca 2.97−x Pr 0.03 R x Al 2 O 6 samples, two Ca 2+ ions would be substituted by one Pr 3+ ion and one R + ion according to 2Ca Ca →Pr • Ca +R ′ Ca .Some vacancies or defects were reduced.Therefore, the observed luminescence intensity can be improved.Furthermore, the ionic radii of K + and Na + are larger than that of Li + .Li + ions more easily fill the vacancy defects, which may also further promote the effective entrance of the Pr 3+ into Ca 2+ sites in the host [31,43,44].As a consequence, the emission intensity can be significantly enhanced by the introduction of Li + ions into the host lattice.The influence of charge compensator ions on some phosphors have been reported, such as BaZrGe 3 O 9 :Cr 3+ , Ca 2 GdTaO 6 :Mn 4+ , M (M = Li + , Na + , K + , and Mg 2+ ), Ca 2 ZnSi 2 O 7 :Pr 3+ and α-Sr 2 P 2 O 7 :Dy 3+ [38,42,45,46].
Luminescence decay curves of Ca When charge compensator ions were introduced into the host lattice, the defects and interactions between adjacent Pr 3+ ions were be changed.Luminescence decay curves further demonstrate that non-radiative energy transfer processes have been reduced, which leads to the increases of emission intensity in Figure 5a-c.Luminescence decay curves of Ca2.97−xPr0.03LixAl2O6,Ca2.97−xPr0.03NaxAl2O6and Ca2.97−xPr0.03KxAl2O6were collected at room temperature to confirm the influence of charge compensator ions, as illustrated in Figure 5d-f.As can be observed, several luminescence decay curves show notable increase in comparison with Ca2.97Pr0.03Al2O6,especially for Ca2.96Pr0.03Li0.01Al2O6and Ca2.96Pr0.03Na0.01Al2O6.When charge compensator ions were introduced into the host lattice, the defects and interactions between adjacent Pr 3+ ions were be changed.Luminescence decay curves further demonstrate that non-radiative energy transfer processes have been reduced, which leads to the increases of emission intensity in Figure 5a-c.
To evaluate the influence of charge compensator ions on thermal quenching properties, temperature dependent emission spectra and luminescence decay curves were measured for Ca2.96Pr0.03Li0.01Al2O6,Ca2.96Pr0.03Na0.01Al2O6and Ca2.95Pr0.03K0.02Al2O6.Emission intensity declines with increasing temperature for Li + , Na + and K + co-doped samples, and all the emission profiles are similar as shown in Figure 6a-c.Normalized integrated emission intensity dependent on different temperatures are listed in Figure 6d.The observed emission intensities at 423 K are all about 65-70% of those at 298 K, which are similar to that of Ca2.97Pr0.03Al2O6.To evaluate the influence of charge compensator ions on thermal quenching properties, temperature dependent emission spectra and luminescence decay curves were measured for Ca  7a-c.The variation tendencies are the same and accord with the Ca 2.97 Pr 0.03 Al 2 O 6 sample in Figure 4c.Luminescence decay curves and decay times at various temperatures are illustrated in Figure 7d-i.Luminescence decay processes become faster and gradually deviate from the first-order exponential, owing to the heat-involved energy transfer.The average decay times were also estimated by Equation (2), and the results are shown in Figure 7g-i.The decay times decrease from 122.37 µs (298 K) to 120.16 µs (573 K) for Ca 2.96 Pr 0.03 Li 0.01 Al 2 O 6 .The values are 122.33 µs (298 K, Na + doped), 119.81 µs (573 K, Na + doped), 122.16 us (298 K, K + doped), 119.93 µs (573 K, K + doped).The decreases in decay times are also similar for the three samples, which are very consistent with the observed temperaturedependent emission spectra.Based on the above discussions, it is can be found that certain amounts of compensator ions will enhance the emission intensity of Pr 3+ , especially for Li + co-doped ones.The CIE chromaticity coordinates of Ca2.96Pr0.03Li0.01Al2O6,Ca2.96Pr0.03Na0.01Al2OCa2.95Pr0.03K0.02Al2O6 at different temperatures are shown in Figure 7a-c.The vari tendencies are the same and accord with the Ca2.97Pr0.03Al2O6sample in Figure 4c.L nescence decay curves and decay times at various temperatures are illustrated in F 7d-i.Luminescence decay processes become faster and gradually deviate from first-order exponential, owing to the heat-involved energy transfer.The average d times were also estimated by Equation (2), and the results are shown in Figure 7gdecay times decrease from 122.37 μs (298 K) to 120.16 μs (573 K) for Ca2.96Pr0.03Li0.01AThe values are 122.33 μs (298 K, Na + doped), 119.81 μs (573 K, Na + doped), 122.16 us K, K + doped), 119.93 μs (573 K, K + doped).The decreases in decay times are also si for the three samples, which are very consistent with the observed tem ture-dependent emission spectra.Based on the above discussions, it is can be found certain amounts of compensator ions will enhance the emission intensity of Pr 3+ , cially for Li + co-doped ones.

Potential Applications
A phosphor-converted light-emitting diode (pc-LED) was fabricated with a blue LED chip, Y 3 Al 5−x Ga x O 12 :Ce 3+ (YAGG:Ce, yellow-green component) and Ca 2.96 Pr 0.03 Li 0.01 Al 2 O 6 .Multi-emission bands from ~425 nm to 750 nm were detected at 20-320 mA driven currents, as shown in Figure 8a.Herein, differences appear in comparisons with the above emission spectra.Several factors may contribute to this: 1  ⃝ The responses of the relative emission intensity some exhibit are different for different spectrophotometers. 2  ⃝ The filters used in measurement setup. 3 ⃝ Most importantly, the absorption of YAGG:Ce phosphor in blue region.The emission intensity of the pc-LED device increases gradually with driven currents.We did not observe light saturation in the 20-320 mA current range.The inset of Figure 8a displays the photographs of the pc-LED, and bright white light can be observed clearly with the driven power on.CIE chromaticity coordinates of the working pc-LED are (0.3682, 0.3598), and the CRI (color rendering index) and CCT (correlated color temperature) are 81.9 and 4236 K driven by 160 mA current, respectively.The output optical power also increases with a driven current, as shown in Figure 8b.The luminous efficiency in this situation is around 8.99 lm/W, and the maximum photoelectric efficiency is about 3.5%.

Potential Applications
A phosphor-converted light-emitting diode (pc-LED) was fabricated with a blue LED chip, Y3Al5−xGaxO12:Ce 3+ (YAGG:Ce, yellow-green component) and Ca2.96Pr0.03Li0.01Al2O6.Multi-emission bands from ~425 nm to 750 nm were detected at 20-320 mA driven currents, as shown in Figure 8a.Herein, differences appear in comparisons with the above emission spectra.Several factors may contribute to this: ① The responses of the relative emission intensity some exhibit are different for different spectrophotometers.② The filters used in measurement setup.③ Most importantly, the absorption of YAGG:Ce phosphor in blue region.The emission intensity of the pc-LED device increases gradually with driven currents.We did not observe light saturation in the 20-320 mA current range.The inset of Figure 8a displays the photographs of the pc-LED, and bright white light can be observed clearly with the driven power on.CIE chromaticity coordinates of the working pc-LED are (0.3682, 0.3598), and the CRI (color 160 mA current, respectively.The output optical power also increases with a driven current, as shown in Figure 8b.The luminous efficiency in this situation is around 8.99 lm/W, and the maximum photoelectric efficiency is about 3.5%.

Conclusions
In summary, a series of Pr 3+ -doped Ca3Al2O6 with multi-band emission were successfully designed and prepared.All the samples crystallize in cubic system, but the emission intensity is strongly dependent on the Pr 3+ doping concentration.The optimal Pr 3+ concentration is 0.03, and the crucial energy transfer distance Rc was determined to be 26.63Å. Pr 3+ -doped phosphors exhibit good thermal quenching properties.The emission intensity at 423 K can maintain 65-70% of that at room temperature, and the estimated activation energy ΔEa is 0.149 eV for Ca2.97Pr0.03Al2O6.The introduction of charge compensator ions can greatly enhance the emission intensity of Pr 3+ due to a possible decrease in charge defects, especially for the Li + co-doped ones.The luminescence intensity of Ca2.96Pr0.03Li0.01Al2O6can be increased by 340% in comparison to that of Ca2.97Pr0.03Al2O6.A white light emission pc-LED was created using Y3Al5−xGaxO12:Ce 3+ and Ca2.96Pr0.03Li0.01Al2O6as color converters.The CIE coordinates of the working pc-LED are (0.3682, 0.3598), and the CRI and CCT are 81.9 and 4236 K under 160 mA current.Thanks to good multi-band emission properties, the designed phosphors may have potential applications.

Figure 1 .
Figure 1.(a) X-ray Rietveld refinements of Ca 3 Al 2 O 6 host; (b) X-ray Rietveld refinements of Ca 2.97 Pr 0.03 Al 2 O 6 phosphor; (c) crystal structure of Ca 3 Al 2 O 6 and the coordination environments of Ca 2+ in the host.

3. 2 . 6 Figure
Figure 3a illustrates the diffuse reflectance spectra of Ca 3 Al 2 O 6 and Ca 2.97 Pr 0.03 Al 2 O 6 .A very weak absorption band before 400 nm can be observed, which was ascribed to host-related absorption.The diffuse reflectance spectrum of the Ca 3 Al 2 O 6 host was in good accordance with the reported one[25].For the Ca 2.97 Pr 0.03 Al 2 O 6 sample, a series of sharp absorption lines in the 400-650 nm range also can be observed.In comparison with the pure Ca 3 Al 2 O 6 host, the absorptions in 400-650 nm are assigned to the 4f-4f transition absorptions of Pr 3+ in the host.To further characterize the luminescence properties of Ca 2.97 Pr 0.03 Al 2 O 6 , the excitation spectrum and the corresponding emission spectrum are shown in Figure3b,c, respectively.After monitoring the emission at 612 nm, a series of excitation bands were detected.The sharp excitation bands in the 425-500 nm wavelength range are ascribed to the 3 H 4 → 3 P 0,1,2 transition absorptions of Pr 3+ .A very weak band at around 300 nm may relate to the essential absorption of the Ca 3 Al 2 O 6 host.The excitation spectrum agrees with the diffuse reflectance spectra in Figure3a.This result indicates that a weak energy transfer from the host lattice to Pr 3+ may occur.Based on the excitation spectrum, the corresponding emission spectrum was measured, as shown in Figure3c.Upon 446 nm excitation, a series of sharp emission bands from the blue to deep red region

Figure 4 .
Figure 4. (a) Emission spectra of Ca 2.97 Pr 0.03 Al 2 O 6 at various temperatures; (b) normalized emission intensity dependent on temperature, and inset displays the relationship between ln[(I 0 /I) − 1] and 1/(kT); (c) CIE chromaticity coordinates of Ca 2.97 Pr 0.03 Al 2 O 6 at different temperatures; (d) luminescence decay curves of Ca 2.97 Pr 0.03 Al 2 O 6 at different temperatures.

Figure 7 .
Figure 7. (a) CIE chromaticity coordinates of Ca 2.96 Pr 0.03 Li 0.01 Al 2 O 6 at different temperatures; (b) CIE chromaticity coordinates of Ca 2.96 Pr 0.03 Na 0.01 Al 2 O 6 at different temperatures; (c) CIE chromaticity coordinates of Ca 2.95 Pr 0.03 K 0.02 Al 2 O 6 at different temperatures; (d) luminescence decay curves of Ca 2.96 Pr 0.03 Li 0.01 Al 2 O 6 at different temperatures; (e) luminescence decay curves of Ca 2.96 Pr 0.03 Na 0.01 Al 2 O 6 at different temperatures; (f) luminescence decay curves of Ca 2.95 Pr 0.03 K 0.02 Al 2 O 6 at different temperatures; (g) luminescence decay times of Ca 2.96 Pr 0.03 Li 0.01 Al 2 O 6 dependent on temperature; (h) luminescence decay times of Ca 2.96 Pr 0.03 Na 0.01 Al 2 O 6 dependent on temperature; (i) luminescence decay times of Ca 2.95 Pr 0.03 K 0.02 Al 2 O 6 dependent on temperature.

Figure 8 .
Figure 8.(a) Emission spectra of pc-LED device driven at 20-320 mA, and inset shows the pictures of pc-LED prototype and working LED; (b) output optical power on dependent of driven current.

Table 1 . Refined cell parameters of Ca 3 Al 2 O 6 and Ca 2.97 Pr 0.03 Al 2 O 6 . Samples Ca 3 Al 2 O 6 Host Ca 2.97 Pr 0.03 Al 2 O 6
4 (0.01 Li + ), 2.3 (0.02 Li + ), 2.6 (0.03 Li + ) times that of Ca 2.97 Pr 0.03 Al 2 O 6 , respectively.The emission spectra of Ca 2.97−x Pr 0.03 Na x Al 2 O 6 and Ca 2.97−x Pr 0.03 K x Al 2 O 6 samples are displayed in Figure 2.97−x Pr 0.03 Li x Al 2 O 6 , Ca 2.97−x Pr 0.03 Na x Al 2 O 6 and Ca 2.97−x Pr 0.03 K x Al 2 O 6 were collected at room temperature to confirm the influence of charge compensator ions, as illustrated in Figure 5d-f.As can be observed, several luminescence decay curves show notable increase in comparison with Ca 2.97 Pr 0.03 Al 2 O 6 , especially for Ca 2.96 Pr 0.03 Li 0.01 Al 2 O 6 and Ca 2.96 Pr 0.03 Na 0.01 Al 2 O 6 .
2.96 Pr 0.03 Li 0.01 Al 2 O 6 , Ca 2.96 Pr 0.03 Na 0.01 Al 2 O 6 and Ca 2.95 Pr 0.03 K 0.02 Al 2 O 6 .Emission intensity declines with increasing temperature for Li + , Na + and K + co-doped samples, and all the emission profiles are similar as shown in Figure 6a-c.Normalized integrated emission intensity dependent on different temperatures are listed in Figure 6d.The observed emission intensities at 423 K are all about 65-70% of those at 298 K, which are similar to that of Ca 2.97 Pr 0.03 Al 2 O 6 .The CIE chromaticity coordinates of Ca 2.96 Pr 0.03 Li 0.01 Al 2 O 6 , Ca 2.96 Pr 0.03 Na 0.01 Al 2 O 6 and Ca 2.95 Pr 0.03 K 0.02 Al 2 O 6 at different temperatures are shown in Figure