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Abstract
The synthesis, crystal structure, photoluminescence (PL) and cathodoluminescence (CL) spectra of Ba1-xCaxAl2O4 doped with 3 mol% Eu2+ between x = 0 and x = 1 are described. The molar fractions of the alkaline earth elements were varied in steps of 0.1. The materials have been synthesized by all solid state reactions at 1300°C in mixed gas (H2/N2). The identification of the crystal phases in the samples was based on analyses of the X-ray diffraction patterns. The Ba1-xCaxAl2O4 system contains one dominant monoclinic phase, one dominant hexagonal phase and two different cubic phases that were present in low concentrations. The main characteristic of the PL spectra was that the intensity of the Eu2+ photoluminescence decreased upon adding a second alkaline earth ion in the aluminate lattice. The hexagonal and monoclinic phases in the Ba1-xCaxAl2O4 samples showed an unexpected behaviour, namely increasing their unit cell volumes upon decreasing the mole fraction of Ba2+. For the hexagonal phase this behaviour has been explained qualitatively in terms of enhanced spontaneous polarization of the uncompensated anti-ferroelectric state.
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1. Introduction
This article is the third part of a study of alkaline earth (Ca, Sr or Ba) aluminates doped with Eu2+. In the first parts [1,2] we have described the crystal structure, photoluminescence (PL) and cathodoluminescence (CL) spectra of Sr1-xCaxAl2O4 and Sr1-xBaxAl2O4 respectively; herein we shall describe the phosphor system Ba1-xCaxAl2O4 doped with 3 mol% Eu2+. For the introduction on the luminescence of alkaline earth aluminates doped with Eu2+ we refer the reader to [1] and the literature mentioned therein.
Figure 1 presents the composition diagram of the ternary system BaO-CaO-Al2O3, based on the data presented in Shuklás thesis [3], the literature mentioned therein and the work of Ptáček [4] and Ropp [5]. In Fig. 1 the notation of the cement chemistry has been adopted, in which A stands for Al2O3, B stands for BaO, and C stands for CaO. These abbreviations will also be used in this paper. The red line BA-CA 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. The stable compounds in the vicinity of the line BA-CA in Fig. 1 at 1300°C are: CA2 (calcium di-aluminate or grossite), C12A7 (mayenite) and BC2A4 (barium di-calcium tetra-aluminate) and slightly further from the line we have B3CA (tri-barium calcium aluminate), B3A (tri-barium aluminate) and C3A (tri-calcium aluminate). The stability areas of the various phases in the ternary system of BCA depend critically on the firing temperature. BA has the hexagonal crystal structure (space group P63); it converts into the more symmetrical hexagonal phase (space group P6322) between 130°C and 170°C. This latter phase is paraelectric while the room temperature phase is ferroelectric [6–8].
Fig. 1. Composition diagram of BaO-CaO-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.
Ju et al. [9] studied Ba1-xCaxAl2O4:Eu2+ using cathodoluminescence (CL) and X-ray diffraction (XRD). For 0 < x < 0.15 they observed two broad bands in the CL spectra, one at about 430 nm and the second at about 500 nm. They also found that the lattice constant for the hexagonal phase at 0 < x < 0.4 decreased upon increasing the Ca concentration and increased at x > 0.4. They assumed that this behaviour was due to the limited solubility of CaAl2O4 in the BaAl2O4 host material.
Due to the scarcity of literature on the structure and luminescent properties of BCA doped with a rare earth element, we decided to investigate BCA doped with Eu2+ and we shall report the results of this study herein.
2. Experimental
Starting materials for the syntheses were: barium carbonate (Alfa Aesar, UK, 99%), 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. Standard solid state synthesis methods were used to prepare Ba0.97-xEu0.03CaxAl2O4 with x varying between 0 and 0.97 in steps of 0.1. The samples were prepared by calcining mixtures of an appropriate molar ratio of CaCO3, BaCO3, γ-Al2O3 and EuCl3 powders in a flow of 90% N2–10% H2. The concentration of Eu2+ is indicated in mole percent relative to the alkaline earth ion(s). After calcination the powders were carefully ground by ball milling (Al2O3) for 3 hours. The samples were annealed at 1300°C for 3 hours in H2/N2 mixed gas; this rather low temperature was necessary to reduce sintering. The reported synthesis conditions were the result of various optimization experiments. The decision on the firing temperature was made after evaluating electron microscope images and photo luminescence (PL) spectra of samples. The electron microscopes including the Gatan spectrometer for recording CL spectra with the transmission electron microscope (TEM), the XRD-equipment and the spectrometer for PL have been described in [1].
3. Results and discussion
3.1. Electron microscope
The particle size of the alkaline earth aluminates after the high temperature annealing process was rather large and varied from about 0.5 to 6 µm. Figure 2 presents scanning electron microscope (SEM) images of samples after the final annealing step.
Figures 2(a) and 2(b) manifest that crystallites sinter and form agglomerates. At temperatures >1350°C there was more sintering and the agglomerates grew in size. For this reason we have limited the annealing temperatures to 1300°C.
3.2. X-ray diffraction and crystal structure
Figure 3 presents the XRD patterns of Ba0.97-xCaxEu0.03Al2O4 (0 ≤ xCa ≤ 0.97).
Fig. 3. XRD patterns of Ba0.97-xCaxEu0.03Al2O4 for 0 ≤ x ≤ 0.97. (a) 5° ≤ 2θ ≤ 85°. (b) Enlarged pattern at 28° ≤ 2θ ≤ 36°. The CaAl2O4 and BaAl2O4 patterns in Fig. 3(a) have been taken from the standard cards ICDD PDF number 01-077-3822 and ICDD PDF number 01-077-4274 respectively.
In BCA doped with Eu2+ we have identified 4 different phases as presented in Table 1. The hexagonal BA and monoclinic CA phases occur in large concentrations, while C3A (tri-calcium aluminate) and C12A7 (mayenite) have much lower concentrations. Fitting was carried out using Bruker's AXS Topas Version 5 using the fundamental parameters fitting method. The phase ratios presented represent the ratio of the crystalline phases present. The parameters of the initial structure for the Rietveld refinement were taken from the various ICDD PDF number standards. The cell parameters of the phases after the refinements have been listed in the Tables 3–6 of the appendix as a function of the Ca mole fraction.
Table 1. Observed phases in Ba0.97-xCaxEu0.03Al2O4 phosphor series.
When Table 1 is compared with Fig. 1, it can be seen that of the seven candidate byproducts indicated in Fig. 1 as red dots, we found only C3A and C12A7. In the BCA series without an Al2O3 excess we did not find C2A (grossite), which was found as a relatively large phase in the system CSA, described in [1]. The cell dimensions of the aluminates with one alkaline earth ion agree well with the data published in the literature [6,10–12].
Figure 4 illustrates the composition of the phases found in BCA doped with Eu2+.
Fig. 4. Composition diagram of the phases in Ba0.97-xCaxEu0.03Al2O4. C3A is a short notation for (Ba0.97-xCaxEu0.03)Al2O6.
Figure 4 shows that monoclinic BCA based on monoclinic CaAl2O4 and hexagonal BCA, based on hexagonal BaAl2O4 are the dominant phases in Ba0.97-xCaxEu0.03Al2O4. In the range 0.2 < xCa<0.9 these phases do not form a solid solution. Ju et al. [9] found that upon annealing BA and CA for 3 days at 1200°C in a H2S a mixture of hexagonal BCA and monoclinic BCA phases at 0.2 < xCa < ≈0.7 was obtained. They did not report the presence of C3A and C12A7 in their samples. It should be noticed that the C3A and C12A7 phases found in our samples have a deviating stoichiometric ratio (B1-x + Cx)/A, which should be 1 after weighing the correct quantities of starting materials. The excess of alkaline earth material in C3A must be compensated by1-6% Al2O3. Since we have not detected alumina lines in the XRD-spectra, we assume that some alumina is present as amorphous material in the samples. This of course means that the presence of C3A and C12A7 phases leads to Fig. 4 being slightly inaccurate due to the fact that the excess Al2O3 (that has not been compensated) has not been included in the phase diagram.
We have described in detail the three sites for the alkaline earth cations in the monoclinic BCA phase (i.e. monoclinic CaAl2O4 with space group P21/n) in [1] and also the two sites in hexagonal BSA (P63) [2]. The paraelectric BA phase (P6322) has only one Ba site; we have used this well-known fact in analyzing the PL spectra, to be discussed in the next section. Figure 5 is a plot of the cell volumes and cell dimensions of the hexagonal and monoclinic phases in the Ba0.97-xCaxEu0.03Al2O4 series. In Fig. 5(b) the cell dimension of the cubic C3A phase has also been depicted.
Fig. 5. (a) Cell volumes of hexagonal and monoclinic phases in Ba0.97-xCaxEu0.03Al2O4. (b) Lattice parameter C of hexagonal tridymite structure for the hexagonal, monoclinic and cubic (C3A) phases in Ba0.97-xCaxEu0.03Al2O4.
The behaviour of the cell volumes of the two largest phases in the Ba0.97-xCaxEu0.03Al2O4 series is surprising and unexpected, because in the range 0.27 < xCa<0.77 the cell volumes increase (Fig. 5(a)) while the Ba-content decreases. Figure 5(b) presents the behaviour of the lattice parameter C of the hexagonal tridymite structure (P6322), which may be considered as the simplest building block of the alkaline earth aluminates [1,6,8,10,13]. For hexagonal BCA with space group P63 the parameter C is equal to the c-axis as indicated in Table 3, for monoclinic BCA (P21/n) the lattice parameter C is equal to the a-axis in Table 4 and for the cubic C3A-B3A this lattice parameter is equal to the a-axis in Table 5 divided by √3. The odd behaviour of the cell volume and lattice parameter C in the range 0.27 < xCa<0.77 has been highlighted by the yellow areas in Figs. 5(a) and 5(b) respectively.
The c-axis of the monoclinic structure jumps at xCa= ∼0.4 in the reverse direction as the a-axis, shown in Fig. 5(b), which leads to the result that the volume of the monoclinic cell does not change significantly. As mentioned in the introduction, Ju et al. [9] reported a similar effect for the hexagonal phase of BCA; however, they did not provide information on the monoclinic phase. In Fig. 6 the cell volumes of the hexagonal phases in the series Ba0.97-xCaxEu0.03Al2O4, Sr0.97-xBaxEu0.03Al2O4 and Sr0.99-xCaxEu0.01Al2O4 (data taken from [1] and [2]) have been plotted. Even though the space group (or ferroelectric state) of the hexagonal phase in Sr0.99-xCaxEu0.01Al2O4 has not been identified herein, it is assumed that the actual space group does not affect this comparison.
Fig. 6. Volumes of the hexagonal unit cell in Ba0.97-xCaxEu0.03Al2O4 (BCA), Sr0.97-xBaxEu0.03Al2O4 (BSA) and Sr0.99-xCaxEu0.01Al2O4 (CSA).
The volume of the hexagonal structure in the BCA system shows deviating character as presented in Fig. 5(a) and described above, whereas the volumes of the hexagonal cells in the other aluminate systems (presented in Fig. 6) have a conventional behaviour, viz. decreasing in size upon decreasing the content of the large Ba2+ or Sr2+ ions. This surprising result is discussed in more detail in section 4 of this article.
3.3. PL and CL spectra
In Figs. 7(a) and 7(b) the PL emission and excitation spectra respectively of the phosphor series BCA have been plotted for various values of the molar fraction of Ca.
Fig. 7. PL spectra of Ba0.97-xCaxAl2O4:3%Eu2+ at various values of xCa. (a) Emission spectra; for 0.2 < x < 0.4 the excitation wavelength was 360 nm. (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.
The spectra in Fig. 7(a) feature broadening of the emission band at 0.3 < xCa < 0.8. This range corresponds well with the range of xCa where two different phases were found in sufficiently high concentrations, as indicated in Fig. 4. These two phases have different crystal structures and create different electrostatic environments for the Eu2+ ions, which leads to band broadening. Although these two phases correspond with the phases of pure BA and CA, the spectra in Fig. 7a are not identical with the corresponding linear combinations of the spectra of the mother compounds; this can easily be verified from the spectrum of Ba0.5Ca0.47Eu0.03 Al2O4, which is not the 50/50 linear combination of the spectra of Ba0.97Eu0.03Al2O4 and Ca0.97Eu0.03Al2O4.
Ju et al. [9] published the CL-spectra (10 kV excitation) of the BCA series doped with Eu2+. They found deviating spectra for the samples with a high Ba content: instead of only one emission band with λ0 ≈500 nm, they found a second band at about 430 nm. This second emission band seems to be related to the hexagonal structure, because it disappeared between xCa=0.15 and xCa=0.4. This 430 nm emission band was not detected in our PL spectra of Ba1-xCaxAl2O4:Eu2+ illustrated in Fig. 7(a), nor was it present in the CL spectrum of BaAl2O4:Eu2+ recorded at 200 keV and room temperature, as indicated in Fig. 9(a) hereafter. The emission spectrum of CaAl2O4:Eu2+ (xCa=0.97) in Fig. 7(a) is similar to the PL spectrum measured at 300 K by Ueda et al. [14]. By comparing the PL spectrum of BA (xCa=0) in Fig. 7(a) with literature data some surprising differences are apparent regarding the presence or absence of a second emission band at 430 nm [9,15–22]. These differences are represented in Table 2 as the ratio R, which is the spectral radiance at 430 nm divided by the spectral radiance at 500 nm. Although this table does not present the results of all publications relating to the emission band of BaAl2O4:Eu2+, we assume that the key information is displayed.
Table 2. Ratio between spectral radiances at 430 nm and 500 nm of BaAl2O4:Eu2+
It is impossible to indicate one factor that is responsible for the different values of R in Table 2; however, it seems that the combination of adding a co-dopant and annealing at low temperature is maximizing R. It is tempting to compare the 430 nm band observed by various workers in the PL spectrum of BaAl2O4 with the low temperature 440 nm emission band in SrAl2O4, because the similarity of the ∼435 nm bands in the spectra of BaAl2O4 and SrAl2O4 is striking [1]. In our study on Sr1-xCaxAl2O4:Eu2+ we have proposed to assign the 440 nm band in SrAl2O4 to an F-centre [1]. Support for this assignment was obtained in our study of the BSA series [2], where the 430 nm band in the low temperature CL spectrum of Ba0.9Sr0.07Eu0.03Al2O4 was found to be much stronger than the 500 nm band. It was outside the scope of the present work to investigate the cause of the differences shown in Table 2. As already mentioned by Peng and Hong [21], more investigations are necessary to clarify this problem. Currently we are investigating the PL and CL of BaAl2O4, doped with Eu and without any dopant.
In Fig. 8 the deconvolutions of various PL-spectra of the Ba0.97-xCax Eu0.03Al2O4 series are presented.
Fig. 8. Deconvolutions of PL spectra of BCA doped with Eu2+. (a) Ca0.97Eu0.03Al2O4.; the inset shows a part of the spectrum at an enlarged scale (taken from [1]). (b) Ba0.97Eu0.03Al2O4. (c) Ba0.2Ca0.77Eu0.03Al2O4. (d) Ba0.7Ca0.27Eu0.03Al2O4. The p-profiles refer to the hexagonal (BaAl2O4) phase, whereas the q-profiles refer largely to the monoclinic (CaAl2O4) phase.
The fitting of the deconvoluted spectrum to the experimental spectrum was carried out with a minimum set of Gaussian profiles and using a least squares algorithm in a wavenumber (cm−1) representation as described previously [1,23,24]. The spectra of Figs. 8(b), 8(c) and 8(d) can be well represented by two Gaussian profiles whereas the spectrum for Ca0.97Eu0.03Al2O4 needs three Gaussian profiles. Figure 8(a) has been taken from [1].
Before discussing the spectra presented in Fig. 8, a short description of the monoclinic BCA phase is helpful. The unit cell of this phase contains 12 Ca/BaAl2O4 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 [10]. These 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 the second channel and it has nine O2− anion nearest neighbours belonging to five different AlO4 tetrahedra. In our previous work [1] we have assigned the q3 and q2 profiles in Fig. 8a to emission from Eu2+ situated at the Ca(3) site. This site is roomy and can easily lodge the Eu2+ ion. From the asymmetry of the 440 nm band in Fig. 8a we have concluded that there are two positions for the Eu2+ ions at the Ca(3) lattice site [1]. 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.
The q1 profile in Fig. 8a is attributed to Eu2+ situated at the Ca(2) and Ca(1) sites. By adding 20% Ba, the q1 band grows substantially and experiences a blue shift, while the main band may be represented by a single Gaussian (Fig. 8(c)). We assume that this may be explained by the presence of the hexagonal (BaAl2O4) phase which has a concentration of 10.9% in the Ba0.2Ca0.77Eu0.03Al2O4 sample of Fig. 8c
Figures 8(b) and 8(d) refer to the Ba-rich, hexagonal side of the BCA-series, viz. Ba0.97Eu0.03Al2O4 and Ba0.7Ca0.27Eu0.03Al2O4. Kawaguchi et al. [25] found that adding a small amount of Sr to BaAl2O4 gave rise to a phase transition from ferroelectric to paraelectric BaAl2O4. In the study on the BSA series that we have published recently [2], this could be verified from an analysis of the PL spectra: the spectrum of the Sr0.07Ba0.9Eu0.03Al2O4 sample could be represented by one single Gaussian profile. This is evidence for a hexagonal phase with only one alkaline earth cation site: the more symmetrical paraelectric structure. Herein the spectra of the BCA series with low xCa were analysed in the same way. It was found that for an appropriate deconvolution, two Gaussian profiles were needed, as indicated in Fig. 8(d). Hence, by adding Ca to BaAl2O4:Eu2+ the ferroelectric structure of BaAl2O4 is maintained and does not convert to the paraelectric phase at 0 < xCa < 0.3. This is an interesting difference with the BSA series.
Figure 9(a) manifests the CL spectra of a BaAl2O4:3%Eu2+ sample that had been stored for 3 years in air at room temperature. Spectrum 1 refers to a specimen that was not hit by electrons before recording. This CL-spectrum compares favourably with the PL spectrum illustrated in Fig. 8(b) and that of Blasse and Bril [15], while the PL spectrum of BaAl2O4:Eu2+ recorded at 4.2 K by Poort et al. [16] has a narrower band width and is somewhat red-shifted. After reannealing this three-years-old BaAl2O4:3%Eu2+ sample at 1350°C in H2/N2, a CL spectrum was recorded that was identical with spectrum 1 in Fig. 9(a). After about five minutes of electron bombarding the BaAl2O4:Eu2+ specimen (shelf life sample) at 200 keV in the TEM, the spectrum was recorded again: it had changed drastically (2). It can be seen that there is a lot of Eu3+ in the sample: the electron bombardment apparently catalysed the oxidation of Eu2+ to Eu3+. Spectra that were recorded of this BaAl2O4:Eu2+ sample at intermediate times manifested lower intensity Eu3+ peaks (with their intensities roughly proportional to the time of electron bombardment) than spectrum 2 in Fig. 9(a). Samples of SrAl2O4:Eu2+ and CaAl2O4:Eu2+ that had a shelf life of 3 years in air at room temperature were also examined in these studies; however, these samples were stable upon prolonged electron bombardment and did not show any sign of Eu3+. From these results it is concluded that H2O and O2 have diffused into the bulk of BaAl2O4:Eu2+, because an adsorbed layer of H2O, O2 and CO2 at the surface of the crystals would be insufficient to oxidize sufficient Eu2+ to Eu3+ to observe the strong Eu3+ luminescence in Fig. 9(a). It may thus be concluded that this absorption of oxidizing gases into SrAl2O4:Eu2+ and CaAl2O4:Eu2+ crystals did not occur during the shelf life. Rezende et al. [26] found that the spectrum of Sr1-xBaxAl2O4 (BSA) doped with Eu2+ manifested Eu3+ characteristics upon irradiation with high energetic X-rays. Since their samples were annealed in air, we assume that some oxygen was absorbed by the crystals and that the same oxidation mechanism played a role in BSA doped with Eu2+.
Fig. 9. CL spectra recorded at 200 keV and room temperature of a BaAl2O4:3%Eu2+ sample that has been stored for three years in air at room temperature. (a) The spectral radiance of the spectra has been normalized at λ=500 nm. First spectrum 1 was recorded (without e-beam exposure before recording), spectrum 2 was recorded after about 5 minutes electron bombardment of the same specimen in the TEM. (b) Deconvolution of spectrum 1 (from (a)) with two Gaussian profiles.
Figure 9(b) is the deconvolution of spectrum 1 in Fig. 9(a). The result deviates slightly from the deconvolution of the PL spectrum illustrated in Fig. 8(b) and deviates also from the result published by Poort et al. [16]. These authors found p1 and p2 at 540 nm and 510 nm, while here they are found at 523 nm and 493 nm respectively; the band p1 refers to Eu(1) ions (Eu2+ ions at Ba site 1) and p2 to Eu(2) ions (Eu2+ ions at Ba site 2). This assignment is based on the ratio of the radiances of the two bands: ideally it should be 3 (p2/p1), here it is found to be 2.1, while Poort et al. found that it was about 3. This factor reflects the multiplicity ratio between the numbers of Ba2+ ions in the two cation sites of ferroelectric BaAl2O4. The conclusion of Poort et al. was largely confirmed by Stefani et al. [19], although these latter authors measured the two bands in the PL spectrum of BaAl2O4:Eu2+,Dy3+ at λ0=435 nm and 500 nm Finally, He et al. [20] measured a band at 419 nm in BaAl2O4:Eu2+. In the PL spectra reported herein bands at 435 nm and 419 nm were not observed. As mentioned above, this scientific debate has not yet been resolved and this issue needs more research.
4. Ferroelectricity of Ba0.97-xCaxAl2O4:3%Eu2+
In this section the surprising results presented in Figs. 5 and 6 are discussed. The surprise is the increase of the cell volumes of both the hexagonal and monoclinic phases in Ba0.97-xCaxAl2O4:3%Eu2+ upon increasing the content of Ca, which has a much smaller ion diameter than Ba. The hexagonal structure of BaAl2O4 in the distorted structure (space group P63) is illustrated in Fig. 10: the view is parallel to the hexagonal c-axis.
Fig. 10. View of the hexagonal unit cell (space group P63) of BaAl2O4 along the c-axis. Ba2+ ions at site 1 are indicated by Ba1 (green), Ba2+ ions at site 2 are indicated by Ba2 (blue). Red balls are O2− ions and the Al3+ ions are indicated inside the tetrahedra.
The question is: what happens to the structure depicted in Fig. 10 when the large Ba2+ ions are replaced by the smaller Ca2+ ions? Up to xCa=0.27 the Ca ions can be inserted without giving rise to critical mechanical tension in the lattice, which results in a gradual decrease of the cell volume, as illustrated in Figs. 5(a) and 5(b). The decrease of the cell dimensions when the Ca molar fraction is increased from 0 to 0.27 is 0.9% for the a-axis and 0.35% for the c-axis, while the decrease of ionic radius in going from Ba2+ to Ca2+ is 23%. The Ca2+ ions are too small for the Ba-cavities: they are “rattling” in these cavities. As described by Huang et al. [7,27], BaAl2O4 is not a classical ferroelectric material such as BaTiO3, but rather a ferrielectric material with spontaneous antiparallel polarizations in two sub-lattices that do not compensate each other. This explains the rather low values for the spontaneous polarization and the dielectric constant. The displacement of the Ca ion in the Ba-cavity can be much larger in the [001] direction, which may create a much larger polarization. However this much larger displacement of the Ca2+ ions can only be stabilized if there are enough Ca2+ ions to interact and consequently to lower the free energy of the system [27,28]. It is assumed that Ca2+ ions at the Ba(1) site move in one direction and those at the Ba(2) site move in the opposite direction from xCa=0.27 onwards, then although the resulting polarizations will be antiparallel, they do not compensate each other because of the difference between the numbers of Ba(1) and Ba(2) sites; thus, the total spontaneous polarization is expected to increase. This polarization effect is the driving force to resist further shrinking of the structure as indicated in Figs. 5 and 6. The channels of the AlO4 tetrahedra in aluminates like BCA determine largely the dimension of the unit cell. This refers mainly to the widths of the channels, which are measured in the a-b plane, perpendicular to the hexagonal axis. What happens in this ferroelectric phase is that the displacements of Ca2+ ions in the [001] direction causes strain in the rings of tetrahedra preventing them from contracting upon the introduction more Ca2+ ions. This strain is strong enough to slightly inflate the cell dimension from xCa=0.27 to 0.77. The large anisotropic displacement ellipsoid of the O1 anion in the a-b plane of the BaAl2O4 lattice reported by Kawaguchi et al. [25] is an important piece of information that supports this hypothesis. Herein this hypothesis on ferroelectricity in hexagonal Ba0.97-xCaxEu0.03Al2O4 has not been tested by measuring the polarization and dielectric constant, as these measurements were outside the scope of the present research: we are planning to investigate the effect(s) of ferroelectricity in these materials in the near future.
The correspondence between the behaviour of the hexagonal and monoclinic phases in Fig. 5 suggests that the mechanisms underlying the odd behaviour is or could be similar for both structures. Figs. 5(a) and 5(b) show that the reverse behaviour in the monoclinic phase is less evident than in the hexagonal phase. Nevertheless, at xCa<0.77 both volume and lattice parameter C change clearly. Monoclinic Ba0.97-xCaxAl2O4:3%Eu2+ has the structure of CaAl2O4, which does not show ferroelectricity. It is therefore less obvious that the behaviour of the cell volume and cell dimension of monoclinic CA in Fig. 5 can be explained in terms of ferroelectricity. For the monoclinic structure the a-axis is perpendicular to the rings of six AlO4 tetrahedra that form the skeleton of the crystal structure. In going from Ca0.97Eu0.03Al2O4 to Ba0.77Ca0.2Eu0.03Al2O4 the cell volume increases by 2.1%, while the total volume of the alkaline earth ions (VCa + VBa) increases by 24% for the monoclinic structure. This shows that the monoclinic aluminate structure is rather roomy, because it is able to absorb the bulky Ba ions without large dimensional changes. The Rietveld refinements of the XRD-patterns of the Ba0.97-xCaxAl2O4:3%Eu2+ samples showed that at xCa<0.77 the hexagonal phase has a larger Ba occupancy than the monoclinic phase, which enables the monoclinic phase to largely maintain its dimensions upon further increasing the molar fraction of Ba. In spite of the absence of ferroelectric phases in CaAl2O4, we cannot exclude ferroelectricity in monoclinic Ba0.97-xCaxEu0.03Al2O4 in the range 0.27 < xCa<0.77, because the built-in Ba ions have inflated the channels formed by the AlO4 tetrahedra slightly which creates more space for the Ca2+ ions to move and eventually leads to spontaneous polarization. Finally, Fig. 5(b) also shows that the cubic phase C3A has an unexpected variation of the lattice parameter C as a function of xCa. Since the concentration of the C3A in the material is small, the relative error in the structural data is larger than for the hexagonal and monoclinic phases. Moreover, we dońt know if there is some interference from the stoichiometry problem mentioned in section 3.2. For these reasons we do not feel we have enough information to explain the result for C3A presented in Fig. 5(b) at the present time.
Finally, it is necessary to comment on the instability of the BaAl2O4 lattice under electron bombardment after prolonged shelf life in air at room temperature as presented in Fig, 9(a). It is perhaps fair to assume that the rather wide channels formed by the AlO4 tetrahedra in BaAl2O4 allow faster diffusion of O2, CO2 and H2O into the crystal than the channels in the sister materials CaAl2O4 and SrAl2O4. Since the shelf life behaviour of the mixed alkaline earth aluminates was not investigated in detail, a more quantitative conclusion cannot be made. It should be noted that only the BaAl2O4 and BaMgAl10O17 (BAM) lattices show oxidation of Eu2+ to Eu3+ [29,30]. The BAM lattice consists of spinel blocks sandwiched between layers, where the Ba2+ (and Eu2+) ions are located. This is in keeping with the large Ba2+ cations making the lattices more porous. This also means that the Eu2+ cations that are on these sites can easily move off special positions. Indeed the Ba2+ cations have excess space on their sites as there is evidence that they can move of their special positions in either hexagonal space group.
Dawson et al. [31] and Boolchand et al. [32] found further evidence for possible movement of the Eu2+ cations on cooling in BAM. Dawson et al. based their conclusion on spectroscopy studies, while Boolchand et al. studied BAM by Mössbauer spectroscopy. With this latter technique they found evidence for five different Eu sites: three were for Eu2+, one was for Eu3+ and the final one was a mixed valence site [32]. The mixed valence site implies two sites (one an Eu2+ and the other an Eu3+) close enough to each other for rapid electron exchange. Heating the BAM to oxidise it led to an increase in the amount of Eu3+ present. Depending on the amount of Eu present the oxidation product had up to six Eu sites with an additional Eu3+ site. On oxidation of the BAM sample containing 20% Eu total, the Eu3+ concentration increased from 3% to 68% (after 12 hours at high temp.) and the mixed valence site fell from 17% to less than 1%. In the oxidation of BaAl2O4:Eu2+ in the electron beam of the TEM, reported in this work, the Eu2+ cations that were oxidised to Eu3+ were likely to have moved off the special Ba2+ positions towards the oxygen atoms of the AlO4 tetrahedra either before or as a consequence of the oxidation. In the case of the oxidised BAM lattice the Mössbauer parameters of the larger amounts of Eu3+ are not the same as that of the small amount of Eu3+ present before oxidation.
5. Summary
The crystal structure and the PL of BCA doped with Eu2+ have been described herein. The most interesting and surprising result is the odd behaviour of the cell parameters of BCA as a function of the molar fraction of Ca. For the hexagonal phase this behaviour is explained in terms of ferroelectricity, in which the spontaneous polarization is the driving force to inflate the lattice. We have planned follow up research, not only to investigate the implications of this hypothesis on (anti-)ferroelectricity, but also to clarify the differences found in the literature about the radiance of the 430 nm band in BaAl2O4:Eu2+. From the analyses of the PL spectra it has been concluded herein that the hexagonal phase of BaAl2O4:Eu2+ does not change its structure upon adding Ca, contrasting to the adding of Sr, which causes a change to the paraelectric phase [2].
The difference in oxidation behaviour of BaAl2O4:Eu2+ on the one hand and CaAl2O4:Eu2+ and SrAl2O4:Eu2+ on the other hand after prolonged shelf life (in air at room temperature) has been explained in terms of the size of the AlO4 channels, being more roomy in BaAl2O4 than those in SrAl2O4 and CaAl2O4 and hence facilitating fast(er) diffusion of O2 and H2O into the crystals.
Appendix
Table 3. Hexagonal BA in Ba0.97-xCaxEu0.03Al2O4
Table 4. Monoclinic CA in Ba0.97-xCaxEu0.03Al2O4
Table 5. Cubic C3A (tri-calcium aluminate) in Ba0.97-xCaxEu0.03Al2O4
Table 6. Cubic C12A7 (mayenite) in Ba0.97-xCaxEu0.03Al2O4
Funding
Engineering and Physical Sciences Research Council (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 (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.
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