1. Introduction

This article is the second part of an ongoing study on the phosphor series Ba_1-xSr_xAl₂O₄, Sr_1-xCa_xAl₂O₄ and Ba_1-xCa_xAl₂O₄ doped with Eu²⁺. In [1] we have described the crystal structures and luminescence of phosphors in the Sr_1-xCa_xAl₂O₄:Eu²⁺ series of materials. Herein we shall describe the results for Ba_1-xSr_xAl₂O₄:Eu²⁺, while the results for Ba_1-xCa_xAl₂O₄ will be published separately. We have presented a general introduction to the structure and luminescence of alkaline earth aluminates doped with Eu²⁺ in reference [1] and we refer to this for the motivation of this research.

SrAl₂O₄ has a monoclinic crystal structure at ambient temperature and pressure [1], the hexagonal structure is the stable phase at a temperature > 675°C, then it is again monoclinic at even higher temperatures [2,3]. BaAl₂O₄ has a hexagonal structure at 25°C [4]; it attracted special attention due to its phase change from the ferroelectric (space group P6₃) to the paraelectric state (space group P6₃22), which takes place at 400 K-450 K [5,6].

Fig. 1 presents the composition diagram of the ternary system BaO-SrO-Al₂O₃, which is largely based on the data presented in Shuklás thesis [7], the literature mentioned therein and the work of Ptáček [8] and Ropp [9]. In Fig. 1 the notation of the cement chemistry has been adopted, in which A stands for Al₂O₃, B stands for BaO and S stands for SrO. These abbreviations will also be used in this paper. The red line BA-SA indicates the compositions that were investigated and are described herein. The compounds in Fig. 1 emphasized with a red mark that are not positioned on the red line could be present as a byproduct of the all-solid state reactions, carried out in the this investigation.

 

Fig. 1. Composition diagram of BaO-SrO-Al₂O₃. The cement chemistry notation has been adopted to denote the compounds in this ternary system. The red line indicates the compositions that were studied.

Download Full Size | PDF

The stable compounds in the vicinity of the line BA-SA in Fig. 1 at 1350°C are: B₂S₂A (di-barium di-strontium aluminate), B₃A (tri-barium aluminate), S₃A (tri-strontium aluminate), S₂A₃ (di-strontium tri-aluminate), S₄A₇ (tetra-strontium hepta-aluminate) and SA₂ (strontium di-aluminate). We shall focus on only a limited part of the ternary system of B-S-A, namely on the compounds formed on the line BA-SA (Sr_1-xBa_xAl₂O₄, abbreviated BSA) in B-S-A. The crystal phases in BSA were studied by Rodehorst et al. [10] using electron diffraction and infrared spectroscopy: they found ferroelectric hexagonal BSA at the Ba-rich side and monoclinic BSA at the Sr-rich side. Kawaguchi et al. [11] and Tanaka et al. [12] found that partially substituting Sr²⁺ for Ba²⁺ in BaAl₂O₄ suppressed the ferroelectric phase with space group P6₃ in Ba_0.9Sr_0.1Al₂O₄. These results indicated that the temperature of the phase transition decreased appreciably from about 450 K for pure BaAl₂O₄ to around 300 K by introducing only a small quantity of Sr. Rezende et al. [13] observed Eu²⁺ and Eu³⁺ peaks in the X-ray excited spectra of BSA doped with Eu²⁺. However, their samples were annealed in air and not in a reducing atmosphere. Blasse and Bril [14] mentioned that in Sr_1-xBa_xAl₂O₄:Eu²⁺ the emission band showed a blue shift upon increasing the Ba content. More details were published by Poort et al. [15], Ryu et al. [16] on Sr_1-xBa_xAl₂O₄:Eu²⁺ and Xie et al. [17] on the PL spectra of Sr_1-xBa_xAl₂O₄:Eu²⁺,Dy³⁺. The latter authors measured minimum PL at x_Ba=0.45, which is the boundary between the hexagonal and monoclinic phases in the phase diagram of Sr_1-xBa_xAl₂O₄ [10].

As part of the study on (Ba, Ca, Sr)Al₂O₄:Eu aluminates we describe the results of the XRD and spectroscopic investigations of the Sr_1-xBa_xAl₂O₄:Eu series in this article. Detailed results for BaAl₂O₄:Eu²⁺ will be described in the forthcoming articles on Ba_1-xCa_xAl₂O₄:Eu²⁺ and BaAl₂O₄:Eu²⁺.

2. Experimental

2.1. Materials

Starting materials were: strontium carbonate (Sigma Aldrich, UK, 99.9%), barium carbonate (Alfa Aesar, UK, 99%), aluminum oxide (SASOL Inc., USA), europium oxide (Ampere Industrie, France, 99.99%), and concentrated hydrochloric acid (Sigma Aldrich, UK, 37%). All materials were used as supplied without further purification. The final annealing of the powders was made in Al₂O₃ crucibles at high temperatures in H₂ (10%)/N₂ (90%) gas.

2.2. Synthesis

Solid state synthesis methods were used to prepare Sr_1-y-xEu_yBa_xAl₂O₄ with x varying between 0 and 0.99 or 0.97 in steps of 0.1 and y = 0.01 and 0.03. The samples were prepared by calcining mixtures of an appropriate molar ratio of SrCO₃, BaCO₃, γ-Al₂O₃ and EuCl₃ powders in a flow of 90% N₂–10% H₂. After calcination the powders were carefully ground by ball milling (Al₂O₃) for 3 hours. The final annealing of the Sr_1-y-xEu_yBa_xAl₂O₄ samples was made at 1350°C. PL spectra and X-ray diffraction (XRD) spectra were recorded a few days after synthesizing the phosphors. CL spectra of some samples were recorded at 200 keV electron beam energy after 4 years of shelf life at ambient conditions.

2.3. Characterization

The characterization equipment, X-ray diffractometer (XRD) and transmission electron microscope (TEM), has been described in detail in part 1 [1]. Crystalline phases in the prepared samples were identified from the XRD-patterns by peak-search matching using the ICCD PDF-2 data files. The identifiable phases were refined according to the Rietveld procedures using the Topas package. The fundamental parameters approach was used. For the morphology and particle size assessment of the phosphor powders we also used a field emission scanning electron microscope (FESEM): Supra 35 VP, Carl Zeiss, Germany. PL excitation and emission spectra of the samples were recorded using a Bentham spectrometer system (Bentham Instruments Ltd., Reading, UK.), configured with M300 excitation and emission monochromators, which were equipped with 0.2 mm slits. The wavelength scale of the monochromators had a maximum error of ∼0.4 nm.

3. Results and discussion

3.1. Electron microscope

The particle size of the alkaline earth aluminates after the high temperature annealing process was in all cases rather large and varied from about 1 to 6 µm. Figs. 2(a) and 2(b) are SEM images of samples after the final annealing step.

 

Fig. 2. (a) SEM image at 7 keV of BaAl₂O₄:3%Eu²⁺ annealed at 1350°C. (b) SEM image at 7 keV of Sr_0.37Ba _0.6Al₂O₄:3%Eu²⁺.

Download Full Size | PDF

Figs. 2(a) and 2(b) manifest that crystallites sinter and form agglomerates. At temperatures >1400°C there is more sintering and the agglomerates grow in size. For this reason we have limited the annealing temperature to 1350°C.

3.2. X-ray diffraction and crystal structure

Figs. 3(a) and (b) present powder XRD patterns of Sr_0.99-xBa_xEu_0.01Al₂O₄ (0 ≤ x ≤ 0.99).

In the Ba_0.97-xSr_xEu_0.03Al₂O₄ (and Ba_0.99-xSr_xEu_0.01Al₂O₄) series we found only two phases, viz. the hexagonal BaAl₂O₄ phase (BA) with space group P6₃ and the monoclinic SrAl₂O₄ phase (MCSr) with space group P2₁. The lattice parameters of these two phases are indicated in Tables 1 and 2 respectively in the appendix. These phosphor series behave rather ideally: from 0 < x ≤ 0.6 we have 100% solid solution of hexagonal BA and from 0.6 < x < 1 we have a 100% solid solution of MCSr. This behaviour is illustrated in Fig. 4, which is a Vegard plot of the volumes of the hexagonal and monoclinic cells of this phosphor series. This result agrees with the findings of Rodehorst et al. [10]. The latter scientists and Kawaguchi et al. [11] found a phase transition in hexagonal BSA at the Ba-rich side (x > 0.1) from P6₃(2A)→P6₃(√3A) [10] or P6₃(2A)→P6₃22(A) [12]. It was impossible to confirm this result from the XRD pattern in Fig. 3(a); however, we found evidence for this transition by analysing the PL spectra, to be described in the next section.

 

Fig. 3. Powder XRD patterns of Sr_0.99-xBa_xEu_0.01Al₂O₄ for 0 ≤ x ≤ 0.99. (a) 5° ≤ 2θ ≤ 80°. (b) Enlarged pattern for the range 28° ≤ 2θ ≤ 36°.

Download Full Size | PDF

 

Fig. 4. Vegard plot of the cell volumes of the hexagonal BA phase and MCSr in Ba_0.97-xSr_xEu_0.03Al₂O₄ and Ba_0.99-xSr_xEu_0.01Al₂O₄. The straight lines have been fitted to the experimental data points.

Download Full Size | PDF

The straight lines that are fitted to the data points presented in Fig. 4 coincide nicely for the series with 1 and 3 mol % Eu²⁺.

3.3. PL and CL spectra

In Fig. 5 the PL spectra of the phosphor series Sr_0.97-xBa_xAl₂O₄:3%Eu²⁺ have been plotted for various values of the molar fraction of Ba²⁺. The PL spectra illustrated in Figs. 5(a) and 5(b) agree nicely with the spectra published by Xie et al. [17] for Sr_0.9-xBa_xEu_0.05Dy_0.05Al₂O₄. The maximum spectral radiance has the lowest value at x_Ba=0.37: the spectra of the single alkaline earth aluminates have higher spectral radiances than aluminates with two alkaline earth ions. A common feature of the emission spectra in Fig. 5(a) is the asymmetric shape of the main emission bands, viz. the bands show a tail at the long wavelength side. This is even the case in a wavenumber or energy representation of the spectra, which reduces the asymmetry slightly. The PL spectra of Sr_1-xCa_xAl₂O₄:Eu²⁺ (CSA) published in [1] feature broadening of the emission band at mole fractions between 0.4 and 0.6, whereas Fig. 5(a), does not show broadening of the band at 0.2 < x < 0.8. In the BSA system solid solutions are formed, which imply that at a certain molar fraction of Ba only one phase, either having the monoclinic or the hexagonal structure, can be present in the sample. This is different in the other phosphor systems: in CSA we have at 0.2 < x_Ca<0.5 three different phases in sufficiently high concentrations, as indicated in [1]. These different phases have different crystal structures and create different electrostatic environments for the Eu²⁺ ions, which leads to band broadening. In the PL spectra of Sr_0.99-xCa_xEu_0.01Al₂O₄ published in part 1 [1] we found an emission band at 440 nm when a small quantity of Ca was added to SrAl₂O₄:Eu²⁺. Due to the presence of a small quantity of the monoclinic P2₁/n phase in Sr_1-xCa_xAl₂O₄:Eu²⁺ with a small molar fraction of Ca this band was assigned to the 440 nm emission band of CaAl₂O₄ at room temperature. In the PL spectra of Sr_1-xBa_xAl₂O₄:Eu²⁺ no sign of an additional emission band was found when (small amounts of) Ba was added to SrAl₂O₄:Eu²⁺.

 

Fig. 5. Photoluminescence spectra of Sr_0.97-xBa_xAl₂O₄:3%Eu²⁺ at various values of x_Ba. (a) Emission spectra. (b) Excitation spectra. For clarity reasons only a limited number of spectra are shown. The kink at 400 nm in the excitation spectra is due to a filter change of the spectrometer.

Download Full Size | PDF

From the temperature dependence of the PL spectra Poort et al. [15] determined the quenching temperatures T_q of some Sr_1-xBa_xAl₂O₄:Eu²⁺ compounds. Fig. 6 is a comparison of results.

 

Fig. 6. Quenching temperature T_q versus mole fraction of Ba in Sr_0.97-xBa_xEu_0.03Al₂O₄. Curves have been fitted to the experimental data.

Download Full Size | PDF

For SrAl₂O₄:Eu we found a much lower quenching temperature than Poort et al. [15]. From the data published by Botterman et al. [18] and Ueda et al. [19] T_q of SrAl₂O₄:Eu,Dy can be estimated to be about 500 K.

Fig. 7 is a collection of diagrams that show deconvolutions of PL spectra of the BSA-series at the Ba-side recorded at room temperature. The fitting of the deconvoluted spectrum to the experimental spectrum was carried out using Gaussian profiles and a least squares algorithm in a wavenumber (cm⁻¹) representation as described previously [1,20,21]. In the deconvolutions we have taken the minimum number of profiles that gave a good fit with the experimental spectra. The radiance R of a Gaussian profile is the area under the curve. λ₀ is the wavelength at maximum spectral radiance for a profile and ν₀ is the corresponding wavenumber.

 

Fig. 7. Deconvolution of PL spectra of Sr_0.97-xBa_xAl₂O₄:3%Eu²⁺ with two (or one) Gaussian profiles. (a) Ba_0.97 Eu_0.03Al₂O₄, exc.: 341 nm. (b) Sr_0.07Ba_0.9Eu_0.03Al₂O₄, exc.: 345 nm. (c) Sr_0.07Ba_0.9Eu_0.03Al₂O₄, exc.: 345 nm, deconvoluted with only one Gaussian profile. (d) Sr_0.17Ba_0.8Eu_0.03Al₂O₄, exc.: 355 nm.

Download Full Size | PDF

The emission bands in Fig. 7 are asymmetric and can be well rendered by two Gaussian profiles. However, the emission band of Sr_0.07Ba_0.90 Eu_0.03Al₂O₄ in Fig. 7(b) is less asymmetric and can adequately be described by only one Gaussian profile; this is shown in Fig. 7(c). The spectra of the other samples of this series cannot accurately be represented with one Gaussian profile. In Figs. 8(a) and  8(b) we have plotted ν₀ and the full width at half maximum (FWHM) respectively of the two profiles p1 and p2 profiles versus x_Ba.

The data shown in Figs. 7 and 8 indicate that the spectrum of Sr_0.07Ba_0.9Eu_0.03Al₂O₄ deviates from the others, since the asymmetry of its emission band, expressed as the difference ν₀(p2) - ν₀(p1) in Fig. 8(a), is much smaller than it is for the other spectra. Although ν₀(p2) - ν₀(p1) does not go to 0 cm⁻¹ for the sample at x_Ba=0.9, it is apparent that the residual asymmetry, which could be caused by the presence of two phases or electron-phonon coupling, is small. This leads to the assumption that the emission band of the Sr_0.07Ba_0.9Eu_0.03Al₂O₄ sample may be rendered by one Gaussian profile, as shown in Figs. 7(c) and 8(b). The FWHM of the single profile for this sample is equal to the FWHM of the p1 profile. If the luminescence of a phosphor doped with Eu²⁺ can be represented by one Gaussian profile, then two possibilities need to be considered: (1) there may be more than one cation site, but only one is occupied by Eu²⁺; (2) there is only one cation site present in the lattice. When ferroelectric BaAl₂O₄ with space group P6₃ changes into the paraelectric phase with space group P6₃22, the volume of the unit cell becomes four times smaller and the number of different alkaline earth cation sites reduces from 2 to 1. In BaAl₂O₄:Eu²⁺ without any Sr addition (see Fig. 7(a)) it is apparent that two Gaussian profiles are needed to represent the spectrum: these profiles refer to the Eu²⁺ ions at the two Ba sites. By introducing a small quantity of Sr there is no reason why the Eu²⁺ ions should migrate to only one Ba site. Hence, it is assumed here that Sr_0.07Ba_0.90 Eu_0.03Al₂O₄ has the high symmetry space group P6₃22 with only one unique Ba site. For the other mole fractions of Ba the hexagonal phase of BSA has the ferroelectric P6₃ phase. It is therefore concluded that the data in Fig. 7 are an indication that the hexagonal phase of BSA changes from the ferroelectric P6₃ structure at x_Ba=1 to the paraelectric P6₃22 phase at x_Ba≈0.9 and at x_Ba≈0.8 it goes back to P6₃.

 

Fig. 8. (a) ν₀ of the two profiles p1 and p2 versus x_Ba of the BSA series. The values at x_Ba=0.9 cannot be considered to be outliers, since all deconvolutions were performed with more than 200 data points. (b) FWHM of p1 and p2 versus x_Ba. The data indicate clearly the phase transition at x_Ba≈0.4.

Download Full Size | PDF

In Fig. 9 some CL spectra of Sr_0.07Ba_0.9Eu_0.03Al₂O₄ and Sr_0.27Ba_0.7Eu_0.03Al₂O₄ are presented. It is apparent that under Cl excitation that there is clear evidence of Eu³⁺ in the spectra in Fig. 9. As mentioned in section 2.2, these spectra were recorded after 4 years of shelf life at ambient conditions. Compared to the amount of the Eu²⁺ emission present in the spectra, the Eu³⁺ emission is small and is not further covered herein as it is treated as an oxidation contaminant. It should be noted however that the Eu³⁺ emission bands differ in the two samples possibly providing evidence that the lattice sites it occupies in the two materials have different symmetries. Indeed this finding does fit with the findings for the Eu²⁺ emission bands which are discussed in detail below.

 

Fig. 9. (a) CL spectra of Sr_0.27Ba_0.7Eu_0.03Al₂O₄ recorded at 200 keV and various temperatures. The insert is an Arrhenius plot of the maximum spectral radiance. (b) Deconvolution with one Gaussian profile of the CL spectrum of Sr_0.27Ba_0.7Eu_0.03Al₂O₄ recorded at 13°C. (c) CL spectra of Sr_0.07Ba_0.9Eu_0.03Al₂O₄ recorded at 200 keV and various temperatures. (d) Deconvolution with one Gaussian profile of the CL spectrum of Sr_0.07Ba₀₉Eu_0.03Al₂O₄ recorded at 25°C.

Download Full Size | PDF

Figs. 9(a) and 9(c) indicate that Sr_0.27Ba_0.7Eu_0.03Al₂O₄ and Sr_0.07Ba_0.9Eu_0.03Al₂O₄ yield completely different CL spectra upon lowering the temperature. The low temperature CL spectrum of Sr_0.07Ba_0.9Eu_0.03Al₂O₄ is similar to that of BaAl₂O₄:Eu without any Sr. The peak at about 410 nm is ascribed herein to an F-centre; this interesting behaviour will be described in detail in a forthcoming publication. Figs. 9(b) and 9(d) present deconvolutions of the room temperature spectra shown in Figs. 9(a) and  9(c) respectively. Unlike the PL spectra of Sr_0.97-xBa_xEu_0.03Al₂O₄ at x < 0.9, the CL spectra can be represented by one Gaussian profile, which means that as soon as Sr has been added to BA, the hexagonal paraelectric phase is more stable than the ferroelectric phase. The insert of Fig. 9(a) is an Arrhenius plot of the maximum spectral radiance. The curve is a Fermi-Dirac type fitting to the experimental data [22]. The single-barrier height is 0.06 eV.

The suppression of the ferroelectric P6₃ phase of BaAl₂O₄ by introducing a small quantity of Sr has been reported by Kawaguchi et al. [11] and Tanaka et al. [12]. They based their findings on XRD-investigations at low (and high) temperatures. They also found that the paraelectric P6₃22 phase returns to the ferroelectric P6₃ phase upon increasing the mole fraction of Sr²⁺ above 0.1. The analysis of the PL spectrum with 7 mol% Sr presented herein matches favourably with their findings. Rodehorst et al. [10] also found that the P6₃→P6₃22 transition in BaAl₂O₄ takes place at temperatures much lower than 450 K by adding a small amount of Sr. The different behaviour of Sr_0.97-xBa_xEu_0.03Al₂O₄ upon excitation with UV or electron beam may be caused by the excitation energy. In the case of CL the crystals are bombarded with 200 keV electrons, which are supposed to facilitate the transition from the ferroelectric phase to the paraelectric phase at room temperature.

Unlike the excitation spectrum of Sr_0.99-xCa_xAl₂O₄:1%Eu²⁺ described in [1], the excitation spectra of Sr_0.97-xBa_xAl₂O₄:3%Eu²⁺ presented in Fig. 5(b) do not present much structure. Moreover the kink at 400 nm makes the spectra less inappropriate for detailed analyses.

4. Conclusion

Apart from a confirmation of forming solid solutions of hexagonal BSA and monoclinic BSA in the series Sr_0.97-xBa_xEu_0.03Al₂O₄, we have presented evidence for phase transitions in hexagonal BSA as a function of the Ba molar fraction. At x_Ba=0.97 we have the ferroelectric P6₃ phase, which changes to the paraelectric P6₃22 phase at x_Ba≈0.9 and at x_Ba≈0.8 it goes back to P6₃. These phase transitions take place at room temperature. This conclusion is based on the deconvolution of the PL spectra. Upon excitation with high energy electrons we only detected the paraelectric phase of hexagonal Sr_0.97-xBa_xEu_0.03Al₂O₄.

Appendix

Table 1. Hexagonal BA in Ba_0.97-xSr_xEu_0.03Al₂O₄

View Table | View all tables in this article

Table 2. Monoclinic SA in Ba_0.97-xSr_xEu_0.03Al₂O₄

View Table | View all tables in this article

Funding

Engineering and Physical Sciences Research Council (EPSRC) (TS/1003053/1, FAB3D, PRISM-EP/N508974/1, TP11/MFE/6/1/AA129F;EP-SRC TS/G000271/1); Technology Strategy Board (TSB) (CONVERT).

Acknowledgments

We are grateful to the EPSRC and Technology Strategy Board (TSB) for funding the PURPOSE (TP11/MFE/6/1/AA129F; EP-SRC TS/G000271/1) and CONVERTED (JeS no. TS/1003053/1), PRISM (EP/N508974/1) and FAB3D programs. We are finally grateful to the TSB for funding the CONVERT program.

References

1. L. Yu, D. den Engelsen, J. Gorobez, G. R. Fern, T. G. Ireland, C. Frampton, and J. Silver, “Crystal structure, photoluminescence and cathodoluminescence of Sr_1-xCa_xAl₂O₄ doped with Eu²⁺,” Opt. Mater. Express 9(5), 2175–2195 (2019). [CrossRef]  

2. A.-R. Schulze and H. Müller-Buschbaum, “Compound Formation MeO:M₂O₃. IV. Structure of Monoclinic SrAl₂O₄,” Z. Anorg. Allg. Chem. 475(4), 205–210 (1981). [CrossRef]  

3. M. Avdeev, S. Yakovlev, A. A. Yaremchenko, and V. V. Kharton, “Transitions between P2₁, P6₃($\sqrt 3 A$), and P6₃2 modifications of SrAl₂O₄ by in situ high-temperature X-ray and neutron diffraction,” J. Solid State Chem. 180(12), 3535–3544 (2007). [CrossRef]  

4. W. Hörkner and H. Müller-Buschbaum, “About the crystal structure of BaAl₂O₄,” Z. Anorg. Allg. Chem. 451(1), 40–44 (1979). [CrossRef]  

5. S. Y. Huang, R. Von Der Muehll, J. Ravez, and M. Couzi, “Phase transition and symmetry in BaAl₂O₄,” Ferroelectrics 159(1), 127–132 (1994). [CrossRef]  

6. A. M. Abakumov, O. I. Lebedev, L. Nistor, G. Van Tendeloo, and S. Amelinkx, “The ferroelectric phase transition in tridymite type BaAL₂O₄ studied by electron microscopy,” Phase Transitions 71(2), 143–160 (2000). [CrossRef]  

7. A. Shukla, “Development of a critically evaluated thermodynamic database for the systems containing alkaline-earth oxides,” Thesis, University of Montreal, (2012).

8. P. Ptáček, Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Application, (InTech, 2014), Chapter 1.

9. R. C. Ropp, Encyclopedia of the Alkaline Earth Compounds (Elsevier, 2013), Chapter 6.

10. U. Rodehorst, M. A. Carpenter, S. Marion, and C. M. Henderson, “Structural phase transitions and mixing behaviour of the Ba-aluminate (BaAl₂O₄)-Sr-aluminate (SrAl₂O₄) solid solution,” Mineral. Mag. 67(5), 989–1013 (2003). [CrossRef]  

11. S. Kawaguchi, Y. Ishii, E. Tanaka, H. Tsukasaki, Y. Kubota, and S. Mori, “Giant thermal vibrations in the framework compounds Ba_1−xSr_xAl₂O₄,” Phys. Rev. B 94(5), 054117 (2016). [CrossRef]  

12. E. Tanaka, Y. Ishii, H. Tsukasaki, S. Mori, M. Osada, H. Taniguchi, Y. Sato, and Y. Kubota, “Structural changes and microstructures of Ba_1−xSr_xAl₂O₄ for 0 < x < 0.4,” J. Korean Phys. Soc. 66(9), 1355–1358 (2015). [CrossRef]  

13. M. V. d. S. Rezende, A. B. Andrade, M. E. G. Valerio, and P. J. R. Montes, “The effect of the host composition on the lifetime decay properties of barium/strontium aluminates compounds,” J. Appl. Phys. 115(10), 103510 (2014). [CrossRef]  

14. G. Blasse and A. Bril, “Fluorescence of Eu²⁺-activated alkaline-earth aluminates,” Philips Res. Rep. 23, 201–206 (1968).

15. S. H. M. Poort, W. P. Blokpoel, and G. Blasse, “Luminescence of Eu²⁺ in Barium and Strontium Aluminate and Gallate,” Chem. Mater. 7(8), 1547–1551 (1995). [CrossRef]  

16. H. Ryu, B. K. Singh, and K. S. Bartwal, “Effect of Sr substitution on photoluminescent properties of BaAl₂O₄:Eu²⁺, Dy³⁺,” Phys. B 403(1), 126–130 (2008). [CrossRef]  

17. Q. Xie, B. Li, X. He, M. Zhang, Y. Chen, and Q. Zeng, “Correlation of structure, Tunable Colors, and Lifetimes of (Sr, Ca, Ba)Al₂O₄:Eu²⁺, Dy³⁺ Phosphors,” Materials 10(10), 1198–1210 (2017). [CrossRef]  

18. J. Botterman, J. Joos, and P. F. Smet, “Trapping and detrapping in SrAl₂O₄:Eu,Dy persistent phosphors: Influence of excitation wavelength and temperature,” Phys. Rev. B: Condens. Matter Mater. Phys. 90(8), 085147 (2014). [CrossRef]  

19. J. Ueda, T. Nakanishi, Y. Katayama, and S. Tanabe, “Optical and optoelectronic analysis of persistent luminescence in Eu²⁺-Dy³⁺ codoped SrAl₂O₄ ceramic phosphor,” Phys. Status Solidi C 9(12), 2322–2325 (2012). [CrossRef]  

20. D. den Engelsen, P. G. Harris, T. G. Ireland, G. Fern, and J. Silver, “Symmetry-related transitions in the photoluminescence and cathodoluminescence spectra of nanosized cubic Y₂O₃:Tb³⁺,” ECS J. Solid State Sci. Technol. 4(12), R145–R152 (2015). [CrossRef]  

21. D. den Engelsen, P. G. Harris, T. G. Ireland, G. Fern, and J. Silver, “Symmetry-Related Transitions in the Spectrum of Nanosized Cubic Y₂O₃:Tb³⁺,” ECS J. Solid State Sci. Technol. 4(7), R105–R113 (2015). [CrossRef]  

22. D. den Engelsen, G. R. Fern, T. G. Ireland, and J. Silver, “Reassignment of electronic transitions in the laser-activated spectrum of nanocrystalline Y₂O₃:Er³⁺,” J. Lumin. 196, 337–346 (2018). [CrossRef]