The crystal structure, photoluminescence and some cathodoluminescent spectra of Sr0.99-xCaxEu0.01Al2O4 (Eu2+) are described. Five different phases have been found: three different monoclinic phases, one hexagonal and one cubic phase. Based on the cathodoluminescence of SrAl2O4:Eu2+ at low temperature and photoluminescence of Sr1-xCaxAl2O4:Eu2+ at 0 ≤ x ≤ 0.1, we consider an alternative explanation for the origin of the 440 nm peak in the low temperature spectrum of SrAl2O4:Eu2+, namely that it can be attributed to the emission from Eu2+ ions situated on the alkaline earth sites of the monoclinic P21/n structure that generate the 440 nm emission of CaAl2O4. However, this alternative hypothesis has been eliminated by XRD analyses of SrAl2O4 at low temperature.
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As part of an ongoing study on metal aluminate phosphors activated with Eu2+ we have investigated the luminescence of the phosphor series Ba1-xSrxAl2O4, Sr1-xCaxAl2O4 and Ba1-xCaxAl2O4 doped with Eu2+. Herein we shall describe the results for Sr1-xCaxAl2O4, the results for Ba1-xSrxAl2O4 and Ba1-xCaxAl2O4 will be published separately. In consequence of their rather high photoluminescence (PL) efficiency the alkaline earth (Ca, Sr or Ba) aluminates doped with Eu2+ have been extensively studied . When excited in the near UV, the fluorescence emission colour of SrAl2O4:Eu2+ is green, that of CaAl2O4:Eu2+ is blue and that of BaAl2O4:Eu2+ is cyan [1–7]. This colour variation is due to the sensitivity of the crystal field components of the 4f65d excited state configuration of the Eu2+ ion in the three crystal structures.
The synthesis and luminescence properties of MAl2O4:Eu2+ (M = Ba, Ca, Mg and Sr) phosphor powders were almost simultaneously reported in 1968 by Blasse et al. [2,3] and Palilla et al. . Blasse and Bril reported that the PL efficiency of these phosphors is rather high, whereas the cathodoluminescence (CL) efficiency is only 1-3% . In a study on the optical and electrical properties of SrAl2O4:Eu2+ in 1971 by Abbruscato  it was concluded that the method of excitation of Eu2+ under UV- and electron beam excitation is predominantly of the charge transfer type. Abbruscato also found that the luminescence of SrAl2O4:Eu2+ increased substantially by changing the SrO:Al2O3 ratio from the stoichiometric value to 5 mol% excess Al2O3. As well as having short decay times, the luminescence decay of the Eu2+ doped alkaline earth aluminates, MAl2O4::Eu2+ (M = Ca, Sr, Ba), can also be adapted using co-activators (particularly Dy3+) to manifest luminescence with very long lifetimes and the same characteristic luminescence in the visible spectrum [7–12] These investigations have led to the widespread use of especially SrAl2O4:Eu2+,Dy3+ as a persistent luminescence material.
From the first decade of the last century onwards, alkaline earth aluminates have been intensively studied for their use in cements. These are refractory materials and were traditionally prepared by solid state reactions at high temperature (1200-1600°C) [13,14]. For phosphor applications with Eu2+ as dopant, the reactions need to be carried out in reducing atmospheres so that the europium activator is present as the divalent cation [1–7]. BaAl2O4, CaAl2O4 and SrAl2O4 have stuffed tridymite crystal structures, which can be represented as layers of rings of vertex sharing AlO4 tetrahedra, in which Ba, Ca or Sr respectively occupy tricapped trigonal anti-prismatic cavities. The ideal undistorted structures of MAl2O4 (M = Ca, Sr or Ba) at high temperature for SrAl2O4 and BaAl2O4 are represented by the P6322 space group [15–18]. At room temperature this ideal structure is distorted, either to monoclinic structures as in the case of CaAl2O4 and SrAl2O4, or to a lower symmetry hexagonal structure (P63) for BaAl2O4. SrAl2O4 has a monoclinic crystal structure at ambient temperature and pressure , the hexagonal structure is the stable phase at a temperature > 675°C, then it is again monoclinic at even higher temperatures . BaAl2O4 has 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 about 400 K [19,20]. Aitasalo et al.  reported the existence of hexagonal CaAl2O4:Eu2+, which can only be prepared by a sol-gel method.
As introduced above, the efficient PL of the alkaline earth aluminates doped with Eu2+ has stimulated much research, especially for aluminates that contain Sr [2–12,22–27]. The primary reason for the interest in SrAl2O4:Eu2+ is its rather intense green luminescence compared to the less bright luminescence of CaAl2O4:Eu2+ and BaAl2O4:Eu2+. The PL spectrum of SrAl2O4:Eu2+ at low temperature has attracted much attention since the assignment of the high energy band at 440 nm of the emission spectrum by Poort et al. . There are two suggested explanations to account for the origin of this high energy band; the first is that the high energy peak (at 440 nm) can be assigned to an Eu2+ ion at one of the two Sr sites in monoclinic SrAl2O4 [6,24,26], while the main emission band at 520 nm is attributed to Eu2+ at the other Sr site. This explanation ignores the asymmetric character of this emission band. The second explanation acknowledges the asymmetric character of the main emission band and concludes that it consists of two components, which are attributed to the emission of Eu2+ ions at the two Sr sites [25,28]. Clabau et al.  assumed that the high energy peak at 440 nm arises from charge transfer from oxygen to residual Eu3+ that takes place upon UV irradiation and is associated with hole trapping at Sr2+ vacancies.
Figure 1 presents the composition diagram of the ternary systems CaO-SrO-Al2O3, which is largely based on the data presented in Shuklás thesis , the literature mentioned therein and the work of Ptáček  and Ropp . In Fig. 1 the notation of the cement chemistry has been adopted, in which A stands for Al2O3, B stands for BaO, C stands for CaO and S stands for SrO. These abbreviations will also be used in this paper. The red lines SA-CA and BA-SA indicate the compositions that were investigated and are described herein. The compounds in Fig. 1 emphasized with a red mark that are not positioned on one of the red lines could be present as a byproduct of the all-solid state reactions, carried out in the this investigation.
The stable compounds in the vicinity of the line CA-SA in Fig. 1 are: CA2 (calcium di-aluminate or grossite), C12A7 (mayenite, C2SA (di-calcium strontium aluminate), S3A (tri-strontium aluminate), S2A3 (di-strontium tri-aluminate), S4A7 (tetra-strontium hepta-aluminate) and SA2 (strontium di-aluminate), which has the similar structure as grossite (CA2).
The nature and concentration of the crystallographic phases present in the ternary systems shown in Fig. 1 depend critically on factors such as firing temperatures, use of fluxes and firing times, calcination procedures, etc.[13,14,29]. Furthermore, phases may change as a function of temperature as in the case of SrAl2O4 (monoclinic-hexagonal). Literature referring to phosphor performance in relation to preparation conditions has largely concentrated on the Sr-containing aluminates [30–32]. Cordoncillo et al.  reported that the hexagonal and monoclinic phases were confirmed in SrAl2O4:Eu2+; after firing at 1000°C they obtained a hexagonal phase, which transformed into the monoclinic phase at lower temperatures, in agreement with the findings of Avdeev et al. . Since the publications of Blasse and his co-authors [2,6] on BaAl2O4:Eu2+, it is known that the measuring conditions, especially the temperature, have a substantial effect on the spectra. The influence of the processing conditions on the luminescence of SrAl2O4:Eu2+ nanoparticles that were produced by combustion methods were also reported .
As mentioned above, herein we shall focus on only a limited part of the ternary system of C-S-A, namely on the compounds formed on the line CA-SA (Sr1-xCaxAl2O4, abbreviated CSA) in S-C-A. Ju et al.  recorded the CL spectra of CSA doped with Eu2+. They measured a blue shift in going from x = 0 (pure Sr) to x = 0.1 and x = 0.3. The system CSA was also studied by Prodjosantoso and Kennedy  using X-ray diffraction; they found monoclinic CSA at the Ca-rich side and another monoclinic phase at the Sr-rich side. At 0.2 < x < 0.5 they also found a hexagonal phase based on the structure of BaAl2O4. Pöllmann and Kaden  also found 3 phases in CSA upon changing the ratio between Ca and Sr; their results confirmed largely the work of Prodjosantoso and Kennedy. Recently, the PL spectra of the system CSA doped with 5 mol% Eu2+ and 5 mol% Dy3+ have been published by Xie et al. . The phases they found largely confirmed the results of Prodjosantoso and Kennedy and Pöllmann and Kaden.
In view of the scarcity of literature on the luminescent properties of CSA doped with only Eu2+ and the scientific debate over the origin of the two luminescence bands of SrAl2O4:Eu2+, we decided to investigate the mixed aluminates doped with Eu2+ in more detail.
Starting materials were: strontium carbonate (Sigma Aldrich, UK, 99.9%), calcium carbonate (Sigma Aldrich, UK, 99.9%), 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.
Solid state synthesis methods were used to prepare Sr0.99-xEu0.01CaxAl2O4 with x varying between 0 and 0.99 in steps of 0.1. The samples were prepared by calcining mixtures of an appropriate molar ratio of SrCO3, CaCO3, 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 Sr0.99-xEu0.01CaxAl2O4 samples was at 1400°C under H2/N2 for 2 hours. In the wake of the work of Abbruscato  we also varied the molar ratio between SrO and Al2O3 in SrAl2O4 and found maximum PL at 5% excess Al2O3. For that reason it was decided to prepare all samples of Sr0.99-xEu0.01CaxAl2O4 with 5% excess Al2O3 with respect to the sum of alkaline earth moles. The molar ratio xCa in this article refers to the molar ratio between Ca and Sr in the compounds.
The crystalline phases of the products were determined by X-ray powder diffraction at room temperature using Bruke’s D8 Advance X-ray diffraction (XRD) equipment fitted with a nickel-filtered copper source, CuKα at λ=1.5406 Å, and a LynxEye silicon strip detector. Diffraction peaks were recorded at 5° < 2θ < 100° and 25°C. The diffractometer was calibrated using an Al2O3 line position standard from Bruker and a LaB6 NIST SRM 660a line profile standard. Diffractograms were collected using the fired powders in a conventional holder. The emission of the X-ray source and hence the instrumental line broadening was determined by fitting the NIST standard using Bruke’s Topas (version 5) software package. 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. Fits were made taking into consideration the easily identified phases and taking into account how the phase ratios vary according to the observed data across the calcium / strontium range studied. Hence, phases were included in the Rietveld refinements even at low concentrations according to this procedure. R-Bragg values were fairly constant for each phase identified and Rwp values were in the range of 8 to 16.
Variable temperature X-ray powder diffraction measurements were performed on a Rigaku Oxford Diffraction (ROD), SuperNova, Dualflex, AtlasS2 X-ray diffractometer equipped with an Oxford Cryosystems Cobra plus variable temperature device. The X-ray source was a fine-focus, CuKα, Enhance microsource, (λ = 1.54184 Å) and the detector an AtlasS2 CCD. A small amount of powder was distributed on the tip of a 50 µm MiTeGen mount and X-ray powder diffraction patterns were accumulated using the “Powder Power Tool” application from the CrysAlis PRO 220.127.116.11a operating software (Rigaku OD, 2015), which controls both the data collection and data reduction process. Data were accumulated using four θ settings for the detector, two positive and two negative, with different exposure times for the high and low angle segments and merged to give a final powder pattern. Data were collected at 290 K and 100 K for undoped SrAl2O4.
Morphology and particle size assessment of the phosphor powders were conducted in a scanning field emission electron microscope (FESEM), Supra 35 VP (Carl Zeiss, Germany). Some samples were also investigated in a transmission electron microscope (TEM), (2100F, JEOL, Japan) equipped with a Schottky-type field emission gun. The TEM was equipped with a Vulcan CL detector, Gatan, USA, for imaging and spectroscopic purposes. This system used a Czerny–Turner spectrometer with back-illuminated CCD and gratings with 1200 or 2400 grooves per mm for collection of CL emission spectra. A small cryostat connected to the sample holder enabled cooling of the samples in the TEM down to 103 K (-170°C); adjustment of the sample temperature anywhere between 103 K and 303 K could be made.
PL excitation and emission spectra of the samples were recorded using a Bentham phosphor spectrometer system (Bentham Instruments Ltd., Reading, UK.), configured with M300 excitation and emission monochromators, which were equipped with 0.2 mm slits. The absolute wavelength calibration of this emission monochromator had maximal error of 0.4 nm; however, relative wavelength values were accurate within 0.05 nm.
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. Figures 2a-2d are electron microscope images of samples after the final annealing step.
Figure 2(a) shows 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 temperatures to 1400°C. Figs. 2(c) and 2(d) are STEM images of the particle shown in Fig. 2(b). Figure 2(d) is a panchromatic image; it indicates that the CL from phosphor particles depends on the thickness. At the edge the particle is rather thin and the CL is much weaker.
3.2. X-ray diffraction and crystal structure
Figures 3(a) and 3(b) present powder XRD-diffractograms of Sr0.99-xCaxEu0.01Al2O4 (0 ≤ x ≤ 0.99). By comparing the XRD-diffraction patterns of the Sr0.99-xCaxEu0.01Al2O4 series in Fig. 3(a) with powder diffraction files (PDF) of the compounds that are located in the vicinity of the SA-CA line in Fig. 1, we have identified five different phases in this phosphor series. These are listed in Table 1. Atomic coordinates and isotropic thermal parameters were not refined. Figure 11 in the appendix shows a typical example of the quality of the fit between the experimental a calculated XRD pattern for Ca0.99Eu0.01Al2O4. The difference between these two patterns is also indicated.
Cell parameters of compounds that are present in concentrations >10 mol % are listed in the appendix. Unlike monoclinic CA and monoclinic CA2, which are present over a wide concentration range of Ca, the monoclinic SA and hexagonal phases can only exist at a limited Ca concentration range.
The compounds CA2 (3) and C12A7 (5) are also known under the mineralogy names grossite and mayenite respectively. Cubic mayenite is present at high Ca content in small concentration only and is therefore not listed in the appendix. The cell constant found for this material agreed with the published value . The comparison with literature data on the cell parameters in non-mixed alkaline earth aluminates is also indicated in the appendix tables (Tables 2–5). The phases 1 and 2 in Table 1, monoclinic SrAl2O4 and monoclinic CaAl2O4 respectively, are abbreviated as MCSr and MCCa in this work.
Figure 4(a) illustrates the compositions of the phases that we found in the CSA system, while Fig. 4(b) depicts the phases and compositions found by Prodjosantoso and Kennedy . The eye-catching difference between Figs. 4(a) and 4(b) is the presence of large quantities of grossite, (CA2+SA2), in Fig. 4(a), whereas Prodjosantoso and Kennedy  and Pöllmann and Kade  did not find any grossite. As mentioned in section 2.2 we made our samples with 5% excess of Al2O3. We assumed that this was the main cause for the presence of grossite. This hypothesis was checked by repeating the synthesis of Sr0.49Eu0.01Ca0.5Al2O4 without excess of Al2O3. In this case we did not find grossite either. From Fig. 4(a) it can be derived that the rather large quantity of Al2O3 stored in grossite at xCa=0.49 needs to be compensated by CaO + SrO in other phases to account for the mass balance. These phases were not present in crystalline form, but likely in the amorphous state. We tried to fit the back ground curve of the XRD-patterns at low 2θ-values to the quantity of amorphous material, but this procedure was unsuccessful. Nevertheless, it is assumed that CaO and SrO in amorphous state make up the mass balance.
MCSr has two sites for the alkaline earth cations, Sr(1) and Sr(2), which are located in two different channels formed by the AlO4 tetrahedra along the c-axis [15,16]. Both cation sites have nine nearest neighbour O2- ions, which belong to six different AlO4 tetrahedra. Schulze and Müller-Buschbaum  commented on the curious distribution of the Sr-O distances for both Sr-sites: three of these distances are > 3 Å and cannot be considered to create a strong Sr-O bond. This conclusion is based on the radii of Eu2+, Ca2+, Sr2+, Ba2+ and O2- ions in six coordination arrangement, which are 1.31 Å, 1.14 Å, 1.32 Å, 1.49 Å and 1.26 respectively . The other six Sr-O bonds have an average distance of about 2.62 Å, which is only slightly more than the sum of the O2- and Sr2+ radii. Schulze and Müller-Buschbaum  evaluated the effective coordination numbers (N), which are 6.07 and 6.23 for Sr(1) and Sr(2) respectively. Only Schulze and Müller-Buschbaum  published a list with all nine Sr-O bond distances for both the Sr(1) and the Sr(2) sites in SrAl2O4. When comparing their results with our data, we found a perfect match for the Sr-O bond distances of Sr(1), but for Sr(2)-O distances we observed a difference that is much larger than the error bar. We used PDF number 76-7488 in the refinements, which is based on the work of Avdeev et al. . We found also differences between the average Sr(2)-O distance of Dutczak et al.  and Clabau et al.  and the average Sr(2)-O distance determined in this work, namely 2.72 Å for N = 9 and 2.60 Å for N = 7, whereas our result agrees with the average Sr(2)-O distance for N = 9 published by Botterman et al. . The issue for this determination of the crystal structure is (1): the structures with two Sr2+ cation sites and 6 independent oxygen atom positions were calculated from a small number of XRD-patterns, and (2): most of the electron density is arising from the heavier Sr2+ cations, so the calculated positions of the oxygens show a rather large spread. After analysis of the various crystallographic information files (CIF) in the ICSD database for monoclinic SrAl2O4 (P21), we conclude that the refinement result depends on the choice for the CIF, which leads to slightly different atomic coordinates for the oxygen atoms: in other words, this choice causes the disagreement.
The next phase at xCa ≥ 0.2 in Fig. 4(a) is the monoclinic phase of calcium di-aluminate: the grossite structure. This structure has one cation site, which is seven-coordinated. Although this structure has only one cation site, it is important considering where the Sr2+ is located, since the Eu2+, responsible for PL, is closer in size to Sr2+ than to Ca2+. Since the Eu2+ ion is slightly smaller in size than Sr2+, it will predominantly occupy Sr2+-sites and can only be forced into the smaller cation sites in the presence of an excess of Sr2+. From the Rietveld refinements it was concluded that the Sr2+ cation occupancy in grossite is constant between x = 0.3 and 0.6, while the composition in the samples did not change significantly either. It is thus to be expected that this structure will not strongly affect the luminescence in this concentration range, apart from an effect on the band broadening. Figure 4(a) indicates that MCCa is the dominant phase in Sr0.99-xCaxEu0.01Al2O4. Its unit cell contains 12 CaAl2O4 formula units and has three alkaline earth cation sites, which are labelled Ca(1), Ca(2) and Ca(3), having equal multiplicity. The first two sites are six-coordinated, in which the O2- anions belong to four different AlO4 tetrahedra . These cationic sites are located in one of the channels formed by the rings made by the AlO4 tetrahedra. The cation site Ca(3) is different, since it is located in a second set of channels and it has nine O2- anion nearest neighbours belonging to five different AlO4 tetrahedra. Six of these nine O2- anions are shared by two Ca(3) cations forming an elongated octahedron (thereby forming a continuous chain of the type Ca(3)-O-Ca(3)). The other three O2- anions are in a plane perpendicular to this chain. We found that the Sr2+ cations in Sr1-xCaxAl2O4:Eu2+ prefer the Ca(3) sites at 0.4 < x < 0.9 and it is only around xCa = 0.2 that the three sites become equally populated with Sr2+. Both MCCa and MCSr in Sr0.99-xCax Eu0.01Al2O4 show a monotonous decrease of their cell volumes upon increasing xCa.
Figure 4(a) furthermore indicates that a hexagonal phase exists in the Sr0.99-xCaxEu0.01Al2O4 series at 0.1 < x < 0.6. Prodjosantoso and Kennedy  found that the hexagonal phase in Sr1-xCaxAl2O4 was present at 0.35 < x < 0.65 with its maximum occurring at x = 0.4 as shown in Fig. 4(b). The hexagonal phase in Sr0.99-xCaxEu0.01Al2O4 is isostructural with BaAl2O4: both having space group P63. It should be realized that BaAl2O4 shows a phase transition from the paraelectric state (space group P6322) at high temperature to the ferroelectric state (space group P63) by cooling down below ≈450 K [19,20]. At this phase transition the cell parameter “a” doubles from 5.22 Å to 10.44 Å  while the positions of the Ba and Al ions are slightly displaced [18,19]. In the case of hexagonal BA in the Ba0.97-xSrxEu0.03Al2O4 series we actually find that a = 10.4452 Å, while in the hexagonal phase of Sr0.99-xCaxEu0.01Al2O4 it is 10.1943 Å at xCa=0.27, which is about 2% smaller, “apparently” reflecting the decrease of cation size in going from Ba2+ to Sr2+. We use the adverb apparently as the change in cell parameter “a” size is in fact not as large as would be expected from the change in ionic radius from Sr2+ to Ba2+ and this must indicate that the network of connected AlO4 tetrahedra is the dominating building block of the lattice and the cations are sitting in spaces that are not too constrictive. The assignment of very weak reflections in the XRD-pattern, from which the space group for the hexagonal phase of pure BaAl2O4 can be identified [18–20], is particularly difficult in the Sr0.99-xCaxEu0.01Al2O4 samples with contaminating compounds such as mayenite and grossite present.
3.3. PL and CL spectra
In Fig. 5 the PL spectra of the phosphor series Sr0.99-xCaxAl2O4:1%Eu2+ have been plotted for various values of the molar fraction of one of the alkaline earth ions.
The PL spectra illustrated in Figs 5(a) and 5(b) agree nicely with the spectra published by Xie et al. . A feature of the emission spectra in Fig. 5(a) is the asymmetric shape of the main emission band, viz. the band show a tail at the long wavelength side. This is even the case in a wavenumber representation of the spectra, which reduces the asymmetry slightly. This kind of asymmetry is usually observed in the fluorescence spectra of Eu2+ doped phosphors [1,24]. The standard explanation of this phenomenon is the presence of more than one site for the Eu2+ ion in the lattice: different sites create different electrostatic fields, so the presence of two or more sites would explain the asymmetry. However, the presence of more than one site for the Eu2+ does not explain that the tail is at the long wavelength side in the vast majority of Eu2+ doped phosphors. Another explanation of the asymmetry may be due to electron-phonon coupling, which can be described in terms of the Huang-Rhys parameter, the lattice vibrational energy and the line width of vibronic transitions . From Figs. 5(a) and 5(b), using also the deconvolution of the excitation spectrum presented in Fig. 10(a) hereafter, the Stokes shift for the 520 nm band of SrAl2O4:Eu2+ can be estimated as 4200 cm-1, which is close to the estimate (4000 cm-1) made by Poort et al.  and larger than the value (3500 cm-1) reported by Botterman et al. . The Huang-Rhys parameter S is calculated to be 5.1 (assuming an average lattice vibration of 450 cm-1) for SrAl2O4:Eu2+; then, according to the calculations presented by de Jong et al. , asymmetric broadening of a rare earth electronic transition will be unlikely. For this reason we decided to analyse the asymmetry of the fluorescence bands by deconvolution and to assign the profiles to Eu2+ ions positioned at alkaline earth sites in the lattice (whenever possible). This will be discussed in the next sections.
The emission spectra presented in Fig. 5(a) show that single alkaline earth aluminates have higher spectral radiances than aluminates with two alkaline earth ions. The spectra in Fig. 5(a) feature broadening of the emission band at mole fractions between 0.3 and 0.7. The spectra also indicate that the spectral radiance at xCa>0.47 is largely determined by the CaAl2O4 content in the samples. λ0 in these spectra is close to 445 nm. Figure 5(a) manifests furthermore the large difference between the maximum spectral radiance at xCa=0 (Sr0.99Eu0.01Al2O4) and xCa=0.1 (Sr0.89 Ca0.1Eu0.01Al2O4). This substantial decrease in spectral radiance occurred largely between xCa is 0 and 0.04 and it indicates that the luminescence of monoclinic SrAl2O4 is very sensitive to relatively small changes of its structure as found by Abbruscato in 1971 . As mentioned above, we have confirmed Abbruscatós result  by synthesizing SrAl2O4:Eu2+ phosphors with various SrO/Al2O3 molar ratios, close to unity, and investigating their PL spectra and XRD-patterns. We found that the spectral radiance at 515 nm increased by 40% by lowering the SrO/Al2O3 molar ratio to 0.95, while the structure remained essentially MCSr. The spectra presented in Fig. 5(a) also show the largest colour change: from green at the Sr-rich side to deep blue at the Ca-rich side; however, the rather low spectral radiance at 0.1 < xCa<0.5 makes this phosphor system not particularly attractive for colour tuning in display or lighting devices. We shall describe the analyses the PL and CL spectra of the system CSA in the next two sections. The first is focussing on SrAl2O4:Eu2+ and the MCSr phases with a small quantity of Ca, the second is focussing on CaAl2O4:Eu2+ and MCCa with a small amount of Sr.
In Fig. 6 we present deconvolutions of PL-spectra of the MCSr phases of the Sr0.99-xCaxAl2O4:1%Eu2+ series that are presented in Fig. 5(a), viz. the spectra at xCa=0 and 0.1. The fitting of the deconvoluted spectrum to the experimental spectrum was carried out using a least squares algorithm in a wavenumber (cm-1) representation as described previously [43,44]. 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, often indicated in the literature as integrated intensity.
The PL spectrum of SrAl2O4:Eu2+ at room temperature as shown in Fig. 6(a) can be decomposed into two Gaussian profiles, p1 and p2. From the PL spectra of SrAl2O4:Eu2+,Dy3+ recorded between 100 K and 500 K by Ueda et al. , it can be concluded that their main emission band also consisted of two profiles. The wavelengths of p1 and p2 in Fig. 6(a) are 545 nm and 513 nm respectively; the radiance R of p1 is smaller: Rp2/Rp1=1.54. Ngaruiya et al.  published the deconvoluted PL spectrum of SrAl2O4:Eu2+,Dy3+. They also found that the emission band can be described by two Gaussian profiles at λ0 = 528 nm and 567 nm respectively, which deviates from our result presented in the table inserted in Fig. 6(a). This difference is likely caused by another excitation wavelength (325 nm) and their choice for a wavelength base in executing the deconvolution. When a small quantity of Ca is introduced in the MCSr phase, a third profile emerges at the high wavenumber side as shown in Fig. 6(b). The radiance of this peak increases when increasing the quantity of Ca, however, the radiance of the p3 profile is small compared to the radiances of the p1 and p2 profiles. Figure 7 illustrates the deconvolution of a CL spectrum recorded at -169.5°C of Sr0.99Eu0.01Al2O4. The radiance (CL) of p3 is almost equal to the radiances measured by Clabau et al.  and Botterman et al.  at 100 K and excitation with UV radiation at 310 nm and 370 nm respectively.
The normalized radiance of the p3 profile has been plotted versus temperature in Fig. 7(b): at room temperature this profile has virtually disappeared, which agrees with the data of Botterman et al. .
In Fig. 8 the normalised radiance of p3 has been plotted versus the mole fraction of Ca in Sr0.99-xCaxEu0.01Al2O4. The normalized radiance ρpi of profile pi is:
Figure 8 provides the evidence that the p3 profile can be assigned to the emission of the MCCa phase with space group P21/n. This assignment is obvious at xCa>0.5, where the concentration of the MCCa phase is increasing, while the concentration of CA2 (grossite) is strongly decreasing (compare Fig. 4(a)). At xCa<0.2 the radiance of p3 and the concentration of MCCa behave identically. Although the grossite phase (CA2) also increases at 0 < xCa<0.2, we may exclude this latter phase as a candidate for the p3 profile because of the behaviour of ρp3 at xCa>0.5.
Clabau et al.  and Ngaruiya et al.  assigned the two bands p1 and p2 in Figs. 6 and 7 to Eu2+ ions that are positioned at the Sr(1) and Sr(2) cation sites in monoclinic SrAl2O4. According to Poort et al. , Botterman et al.  and Dutczak et al.  this is not the case: p3 and the combination p1-p2 should be assigned to Eu2+ at Sr(2) and Sr(1) respectively. The work of Botterman et al. presents an extensive study of the PL from SrAl2O4:Eu2+ and supports this conclusion. The conclusion has recently been confirmed in a theoretical study by Ning et al. . They found that the p3 profile of SrAl2O4:Eu2+ at 440 nm is due to the 5d-4f emission of Eu2+ situated at the Sr(2) site. Unfortunately, these recent studies do no address Clabaús point : by combining p1 and p2 profiles to one emission band, the band gets asymmetric. This asymmetry was not considered in the studies mentioned above. Ueda et al.  and Nazarov et al.  explained the disappearance of p3 at high temperature by energy transfer (down conversion) from the Eu2+ at the Sr(1) to Eu2+ at Sr(2). These Eu2+ ions differ in preferential orientation of the 5d orbitals of Eu2+ at Sr(1) and Sr(2). Nevertheless, we agree with the conclusion of Nazarov et al. that there is still no satisfactory explanation in the literature to account for the origin of the p3 profile.
Based on the similarity of the PL and CL measurements presented in Figs. 6(b) and 7(b) respectively, here we would like to discuss an alternative mechanism, namely a phase transition of the MCSr structure to MCCa at low temperature. The three alkaline earth sites in MCCa, viz. Ca(1), Ca(2) and Ca(3), when occupied by Eu2+ ions, explain the occurrence of p1, p2 and p3 at low temperature in SrAl2O4:Eu2+ and at small Ca2+ mole fractions in Sr1-xCaxAl2O4:Eu2+. Hence, what occurs in SrAl2O4 at high temperature, a transition from the monoclinic to the hexagonal phase, takes place in reversed sense at low temperature: transition from the MCSr phase (space group P21) to the slightly more distorted MCCa phase (space group P21/n) with three cation sites. This phase transition does not occur at one temperature but takes place gradually: upon lowering the temperature the MCCa quantity will increase, which explains the increase of the p3 signal at about 440 nm, being the emission maximum of CaAl2O4:Eu2+ (Fig. 5(a)). As mentioned above, XRD-data on SrAl2O4 at cryogenic temperatures have not been found in the literature. Nevertheless, we found support for the alternative hypothesis presented above by comparing the excitation spectra of CaAl2O4:Eu2+,Dy3+ and SrAl2O4:Eu2+,Dy3+ at cryogenic temperatures by Ueda et al.  and Botterman et al.  respectively. The excitation spectrum of the latter compound, when monitored at λ0 of the p3 profile (440 nm), is almost similar to the excitation spectrum of CaAl2O4:Eu2+,Dy3+ monitored at 440 nm. A second consideration refers to the measurement of the decay time of the green and blue bands of SrAl2O4:Eu published by Botterman et al. . These authors found that the decay curve of the green emission band can be described with two exponentials. This does not contradict with the alternative presented here: their result could also be explained in terms of a double emission centre with Eu2+ ions at the Sr(1) and Sr(2) sites. Botterman et al.  and Bierwagen et al.  determined the 1/e-value of the decay curve of the blue band at 0.1-0.2 µs at room temperature, which is (almost) equal to the decay constant of the emission band of CaAl2O4:Eu2+ . Finally, Bierwagen et al. presented an energy transfer model from Eu2+ positioned at Sr(2) (blue emission) to Eu2+ at the Sr(1) site (green emission). This model does not easily comply with our alternative hypothesis; however, on the other hand it is not obvious either to explain the increase of the p3 profile by doping SrAl2O4:Eu with a small quantity of Ca in terms of an energy transfer model. It is worth commenting on the fact that the increase of the p3 profile by doping SrAl2O4:Eu with a small quantity of Ca as reported herein is contrary to what would have been expected by an energy transfer model. Such a model predicts better energy transfer if the lattice decreased in size as the Eu2+ cations would be closer together, which is the case here, so it would not be expected that the p3 band would be observed at room temperature. The fact that the p3 profile is seen (Fig. 6(b)) we take as evidence that a smaller lattice stabilizes the small proportion of the low temperature phase (MCCa phase) so that it does not completely convert back at higher temperature. This explanation would be in keeping with the fact that the phase at room temperature switches over to the MCCa phase at just a little higher Ca concentration. The results on the 440 nm luminescence
A second issue is the discrepancy between the alternative hypothesis, presented above, and the assignment of the 440 nm band to Eu2+ at Sr(2) by Ning et al.  based on a multi-configurational ab initio study. It would be interesting to extend these calculations to Eu2+ positioned at the three alkaline earth sites in the P21/n lattice (CaAl2O4 lattice) for SrAl2O4. Although the alternative hypothesis put forward here cannot (elegantly) explain the results of Bierwagen et al. , we suggest that XRD-analyses of SrAl2O4 at low temperatures should be done to resolve the issues discussed above.
It is clear that the hypothesis about the nature of the low temperature p3 band in SrAl2O4 presented above can easily be tested by XRD at low temperature. We have recorded XRD-spectra of undoped SrAl2O4 annealed at 1350°C in H2/N2 at 100 K and 290 K. The result was that there was hardly any difference between these patterns and the conclusion must be that the hypothesis regarding SrAl2O4 without Ca has been shown to be false. This conclusion does not affect the result presented in Fig. 8, which presents a fair correspondence between the MCCa content and the radiance of the p3 profile in the Sr0.99-xCaxEu0.01Al2O4 compounds. In other words, although the similarity between the p3 profiles in Figs. 6(b) and 7(a) is striking, the nature of the two bands at 440 nm is different.
In a recent article about the luminescence of undoped SrAl2O4 Vitola et al.  attribute the 440 nm low temperature band to an F-centre as present in α-Al2O3. We believe that this is a more reasonable explanation and that as the temperature increases the energy in the F-centres transfers to the Eu2+ cations, where it is emitted as part of the emission bands. We will present this in more detail in another paper. We have previously studied energy transfer from intrinsic emission bands in Y2O3 caused by cathodoluminescence to Eu3+ cations depending on the concentration of the rare earth cation and believe a similar phenomenon is taking place here [51,52]. From this previous work it is perfectly reasonable to see emission from two different phenomena in the same material depending on temperature. In fact, the quantitative analysis of Bierwagen et al.  on the energy transfer in SrAl2O4:Eu2+ is equally valid for the transference of energy from an F-centre (at 440 nm) to the Eu2+ cations (at 520 nm). Moreover we note that the lack of an isosbestic point in the temperature dependence of the spectra in the paper of Bierwagen et al.  is evidence that more than two species are present and this would fit with our interpretation of an F centre and two Eu2+ cation sites. An analysis of the occurrence of isosbestic points in the emission spectra of solids was published by Greger et al. . We note that the absence of an isosbestic point was not addressed . An examination of the data in Table 1 in the Bierwagen et al.  paper shows that the lifetime of the blue band decreases as the Eu2+ concentration increases; a fact that was not commented on either in the paper of Bierwagen et al. This is exactly what would be expected if energy were transferring from an F-centre to Eu2+ cations. The F-centre itself would have a slow decay , but as the energy is transferred the decay would be measured faster and would depend on the number of Eu2+ cations present to which the energy could be transferred.
Further evidence for our assignment of the optical emission arises by considering the implications of the paper written by Denault et al.  for our results. A reviewer kindly suggested we should consider this paper. This paper considers “Average and Local Structure, Debye Temperature, and Structural Rigidity in Some Oxide Compounds Related to Phosphor Hosts”; one of the structures they studied was SrAl2O4. They refer to average Sr-0 bond lengths for Sr(1) of 2.680 Å assuming the site is 7 co-ordinate and for Sr(2) the average is 2.749 Å assuming this site is 8 co-ordinate. These values are larger than those we discussed in section 3.2 of this paper but are based on different coordination numbers and so are in reasonable agreement with our earlier discussion. More significantly Denault et al. analysed the Debye temperature of the lattice from a number of different methods and considered it relative to both local structure and longer range structure. They found that SrAl2O4 has a high Debye temperature and commented that the higher the value the more rigid the lattice. This is in keeping with the fact that this structure has all its AlO4 tetrahedra fully three-dimensionally connected by the oxygen atoms. The authors stated that the higher degree of connectivity in SrAl2O4 leads to the higher Debye temperature observed. The results of this study provide evidence that the two Sr sites in the SrAl2O4 would not be expected to have very different thermal behaviour, and thus do not lend support to suggestions that the thermal dependence of emission from these two sites should be very different. In other words, the substantial differences between the behaviour of the 440 nm band and the main band at 520 nm in SrAl2O4 might be explained in terms of originating from different independent mechanisms for these bands.
In Fig. 9 we present deconvolutions of PL and CL spectra of CaAl2O4:Eu2+ and two samples of Sr1-xCaxAl2O4:Eu2+.
The main emission band shown in Figs. 9(a), 9(b) and 9(d) is asymmetric and cannot be represented by a single Gaussian profile. Two Gaussian profiles can be fitted rather well to the main emission bands: these have about equal radiance and are close (0.1 ± 0.02 eV) to each other. However, the main band of Sr0.49Ca0.5Al2O4:Eu0.01 in Fig. 9(c) can be well described by a single Gaussian profile. This latter sample consists predominantly of two phases, grossite and MCCa as indicated in Fig. 4(a): this could be one of the reasons that the asymmetry in the MCCa band has disappeared. The other reason, which seems more likely, is that the occupancy of Sr for the Ca(3) site at this mole fraction of Ca is much larger than the occupancies for the other two sites. Since Eu2+ has about the same ion size as Sr2+, we assume that this symmetric emission band indicates the emission from Eu2+ ions situated at Ca(3) in MCCa. The spectra illustrated in Figs. 9(a) and 9(b), which refer to CaAl2O4:Eu2+ without Sr, have been fitted with three Gaussian profiles respectively. In analysing the CL spectrum at -169°C of Fig. 9(b), we could not accurately determine the profile of q1: the insert of Fig. 9(b) indicates that there must be an emission signal at about 18500 cm-1. The profile q0 in the CL spectra gradually disappeared upon increasing the temperature to 25°C. The q2 and q3 profiles are assigned to the Eu2+ ions at the Ca(3) site in CaAl2O4. As discussed in section 3.2, the Ca(3) site in the P21/n lattice of CaAl2O4 is roomy and can easily lodge the Eu2+ ion. From the asymmetry of the 440 nm band it must be concluded that there are two positions for the Eu2+ ions at the Ca(3) lattice site. As the radiances of q2 and q3 are almost equal, it is likely that these two Ca(3) sites have equal probabilities of being occupied. This conclusion includes that CaAl2O4 would be ferroelectric as in the case of BaAl2O4.
The q1 profile is attributed to Eu2+ situated at the Ca(2) and Ca(1) sites, while q0 is assigned to europium trapped exciton (ETE) emission , which explains that this emission band disappears at room temperature. Ueda et al.  studied the PL from CaAl2O4:Eu2+ at various temperatures and found the profiles q0 and q1 at 650 nm and 540 nm respectively. These values are somewhat larger than our results. A second difference is that their measurements indicated that the luminescence of q1 (540 nm) at room temperature is virtually zero. They also assigned the main emission at 440 nm to Eu2+ ions on the Ca(3) sites, but they did not consider the asymmetry of this emission peak. In contrast to the assignment of the 540 nm band by Ueda et al.  (who assigned it solely to Ca(2)); here it is assigned to both the Ca(1) and Ca(2) sites as they are both six-coordinated and only a very small amount of Eu2+ ions occupy these lattice sites. Finally it should be pointed out that the ETE emission does not need to be connected to one particular Eu2+ ion, but may rather be generated at the various sites, where Eu2+ ions are sitting, again in contrast to the suggestion of Ueda et al. .
3.4. Excitation spectra
We shall now briefly describe an analysis of the excitation spectra shown in Fig. 5(b) of the Sr0.99-xCaxEu0.01Al2O4 series: Fig. 10 manifests deconvolutions for a selection of excitation spectra of this system. These deconvolutions are necessary to obtain accurate values for the Stokes shift of electronic transitions, as described in the beginning of section 3.3. Nazarov et al.  have assigned the p4 profile in Fig. 10(a) to a charge transfer transition. Since this band persists in Figs. 10(b), 10(c) and 10(d), we assign p4 in these figures also to charge transfer. Figs. 10 show that the largest changes in the spectra occur between 10(a), 10(b) and 10(c). These changes refer largely to the p1 and p2 bands, while the p3 and p4 bands show only moderate changes. The disappearance of p1 in Fig. 10(c) may be caused by changing the monitoring wavelength from 520 nm to 440 nm, because in the excitation spectrum at xCa=0.3 monitored at 520 nm the p1 band is still present (not shown). According to Fig. 4(a) we have only monoclinic SrAl2O4:Eu2+ at xCa=0, and at xCa=0.2 and xCa=0.4 we have three phases: grossite, hexagonal Sr0.99-xCaxAl2O4:Eu2+ and MCCa. We assume that p1 in Fig. 10(a) is attributed to the 4f→5d transition of Eu2+ in monoclinic SrAl2O4:Eu2+, whereas in Fig. 10(b) it is attributed to the 4f→5d transition of Eu2+ in the hexagonal phase.
This assumption is also based on the fact that monoclinic SrAl2O4 transfers to the hexagonal phase at 670°C . In Fig. 10(d) the p2 and p3 profiles are assigned to the 4f→5d transition of Eu2+ in MCCa; this assignment refers also to Fig. 10(c) because the monitoring wavelength for this spectrum is at 440 nm, the emission maximum of CaAl2O4:Eu2+. Possible contributions from grossite and hexagonal Sr0.99-xCaxAl2O4:Eu2+ are likely too small to be distinguished in the spectrum of Fig. 10(c). The profiles p2 and p3 in Fig. 10(b) are attributed to the grossite and hexagonal phases in Sr0.99-xCaxAl2O4:Eu2+. Because of the complex character of excitation spectra due to the overlap between CT-bands and 4f-5d transitions, the above consideration is necessarily tentative.
Studies on the phases present and their luminescence in the solid state syntheses of the aluminate series CSA doped with Eu2+ yielded some expected and unexpected results. In the category of expected results we found and assigned a large number of phases, whereas amongst the unexpected was the finding of a large quantity of grossite. As discussed above, it is assumed herein that the rather large concentration of grossite found in the CSA samples is caused by the presence of the 5% excess of Al2O3 with respect to SrO + CaO. Evidence for this arises from the fact that when we did not use excess Al2O3 we did not find the presence of grossite.
The analyses of the PL spectra were largely based on deconvolutions of the emission bands. From the results reported here it can be concluded that this analysis technique is rather powerful. All deconvolutions of the spectra have been executed using a wavenumber base, because line broadening mechanisms are largely energy-related. Many deconvolutions of Eu2+ emission bands published in the literature are based on wavelength, which yield deviating parameters, particularly for ν0 and FWHM, for profiles with FWHM > 1000 cm-1 (>0.12 eV) in the visible range. Hence, in citing and comparing data on deconvoluted spectra from the literature, attention must be paid to the deconvolution algorithm.
From the analyses of the spectra of SrAl2O4:Eu2+ at low temperature and with small Ca additions, herein we have discussed an alternative mechanism for the origin of the 440 nm emission band observed in SrAl2O4:Eu2+ at low temperatures, namely that it is due to a phase transition that goes from monoclinic SrAl2O4:Eu2+ with space group P21 to monoclinic SrAl2O4:Eu2+ with space group P21/n, which is the space group of CaAl2O4. This phase transition was assumed to take place in SrAl2O4:Eu2+ upon lowering the temperature. From XRD-measurements of SrAl2O4 at low temperature it was concluded that this alternative mechanism cannot be true: apart from the explanations in the literature, which do not account for the asymmetry of the 515 nm band of SrAl2O4, we believe that the assignment of the low temperature 440 nm band to an alumina F-centre is the most viable explanation. We intend to investigate this latter hypothesis in more detail and report this work in a future paper.
We also propose herein a new assignment of the bands in the spectrum of CaAl2O4:Eu2+ based on the asymmetry of the main emission band that was ignored by previous researchers. From this latter asymmetry it also concluded that CaAl2O4 is ferroelectric, which has not been tested in this work any further.
Engineering and Physical Sciences Research Council (EPSRC) (CONVERTED (JeS no. TS/1003053/1), FAB3D, PRISM (EP/N508974/1), PURPOSE (TP11/MFE/6/1/AA129F; EP-SRC TS/G000271/1)); TECHNOLOGY STRATEGY BOARD, UK (CONVERT).
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