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Solubility of SrAl2O4 in CaAl2O4—a high resolution powder diffraction study
Materials Research Bulletin 38 (2003) 79±87
Solubility of SrAl2O4 in CaAl2O4Ða high resolution powder diffraction study A.K. Prodjosantosoa,b, B.J. Kennedya,* a
The Centre for Heavy Metals Research, School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia b Jurusan Kimia, Universitas Negeri Yogyakarta, Yogyakarta DIY 55281, Indonesia Received 17 July 2002; accepted 9 October 2002
Abstract The solid solubility of the SrAl2O4±CaAl2O4 system has been investigated using high resolution synchrotron powder diffraction methods. Analysis of the patterns shows there is a limited composition range at which single phase samples can be obtained, x < 0:25 and x > 0:75 for the series Ca1 xSrxAl2O4. At intermediate compositions up to three phases are observed, two monoclinic and one hexagonal. The temperature dependence of the structures is also described. Crown Copyright # 2002 Published by Elsevier Science Ltd. All rights reserved. Keywords: Oxides; X-ray diffraction; Phase transition; Crystal structure
1. Introduction There is considerable interest in the structure and properties in the SrAl2O4±CaAl2O4 system. In part this is related to the importance of CaAl2O4 in high-alumina cement [1]. More recently interest in this system has been fueled by the long-lasting phosphorescence properties of the Eu2 doped materials [2]. In principle, substitution of Ca by Sr would enable the energy of the electronic transitions responsible for the phosphorescence, and thus, the color of the oxides, to be altered, provided the solid solutions remain isostructural with the end-member oxides. Whilst the structures of the two oxides SrAl2O4 and CaAl2O4 contain very similar structural motifs they are not isostructural [3,4]. Ju et al. [5] has reported that a single phase of the type Ca1 xSrxAl2O4 exists for x 0±0.5. They have however not described the phase chemistry at the Sr rich end, that is for x > 0:5. In the present paper, we report the results of a high-resolution powder synchrotron X-ray diffraction study of a series *
Corresponding author. E-mail address: [email protected] (B.J. Kennedy).
0025-5408/02/$ ± see front matter. Crown Copyright # 2002 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 1 0 0 9 - 7
80
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
of oxides of the form Ca1 xSrxAl2O4. This study demonstrates a more complex phase distribution than suggested by Ju et al. [5]. 2. Experimental 2.1. Synthesis Polycrystalline samples of Ca1 xSrxAl2O4 (x 0, 0.125, 0.25, 0.5, 0.75, 0.875 and 1) were prepared by solid state reaction of an appropriate stoichiometric mixture of analytical grade CaCO3 (Univar), SrCO3 (Merck), and Al(NO3)39H2O (Aldrich). After being thoroughly mixed in an agate mortar for several minutes the mixtures were transferred to alumina crucibles and heated in air successively at 700, 800, 900 and 1000 8C, for 24 h each with regrinding after each heating step. Finally, the samples were heated in air at 1300 8C for 72 h. 2.2. X-ray powder diffraction High-resolution synchrotron powder X-ray diffraction data were collected at a wavelength of Ê using the Debye Scherrer camera at the Australian National Beamline Facility, Photon 0.9987 A Factory, Japan [6]. Samples were loaded into 0.5-mm glass capillaries that were rotated during the measurements. These measurements were performed under vacuum to minimize air scatter. Diffraction data were recorded at room temperature over the range 5 < 2y < 85 in 0.018 steps. Temperature control was achieved using a custom built furnace, and for the variable temperature runs data were collected over the range 5 < 2y < 65 in 0.018 steps 2.3. Structural re®nements The structural re®nements were performed using the Rietveld method with the PC version of the program Rietica [7]. The background was by interpolation between up to 50 points. A pseudo-Voigt function was chosen to generate the pro®les. Selected results of the Rietveld re®nements at room temperature are summarised in Table 1. 3. Results and discussion Portions of the synchrotron X-ray diffraction patterns for Ca1 xSrxAl2O4 at room temperature are shown in Fig. 1. Examination of the diffraction patterns con®rmed the successful preparation of CaAl2O4 and SrAl2O4. The samples of CaAl2O4 and Ca0.875Sr0.125Al2O4 were single-phase oxides and the structures for both were re®ned in the monoclinic space group P21/n. This phase type is referred to below as M-I. Likewise the samples SrAl2O4 and Ca0.125Sr0.875Al2O4 and Ca0.25Sr0.75Al2O4 were single phase oxides and the structures of these were re®ned in the monoclinic space group P21, referred to as M-II. The diffraction patterns of the remaining two compounds studied, Ca0.5Sr0.5Al2O4 and Ca0.75Sr0.25Al2O4 could not be reproduced with models in either of these monoclinic space groups, nor was a mixture of these two phases suitable. Examination of the
x
0.0 0.125 0.25 0.5 0.75 0.875 1.0
Phase M-I
Phase M-II
Ê) a (A
Ê) b (A
Ê) c (A
b (8)
8.4605(2) 8.4317(4) 8.3998(5)
8.8408(2) 8.8247(4) 8.8062(6)
5.1680(1) 5.1563(2) 5.1437(4)
93.417(1) 93.532(1) 93.599(4)
Ê) a (A
8.8438(8) 8.7842(5) 8.7521(4) 8.7307(4) 8.6998(2)
Phase H-I Ê) b (A
8.3558(5) 8.2225(6) 8.1582(4) 8.1273(3) 8.0946(2)
Ê) c (A
15.3155(16) 15.3193(9) 15.2613(7) 15.2394(6) 15.2075(4)
b (8)
90.196(7) 90.019(6) 90.041(3) 90.091(2) 90.156(1)
Ê) a (A
5.1092(2) 5.1003(3)
Weight percent composition Ê) c (A
M-I
8.3916(3) 8.3482(4)
100 100 21.6(8) 0
M-II
17.7(7) 44.8(1.9)
Rp (%)
RWP (%)
4.73 3.87 7.45 5.94 4.60 3.97 6.14
4.89 4.48 9.98 5.92 4.64 4.46 7.71
H-I
60.7(1.4) 55.2(1.6)
Phase M-I is monoclinicÐspace group P21/n. The structural model based on this is in [3]; phase M-II is monoclinicÐspace group P21. The structural model based on this is in [4]; phase H-I is hexagonalÐspace group P6322. The structural model based on this is in [9].
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
Table 1 Re®ned lattice parameters for the compounds Ca1 xSrxAl2O4
81
82
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
Fig. 1. Portions of Rietveld plots for ®ve samples in the series Ca1 xSrxAl2O4. In all cases the crosses are the observed data and the solid line ®ts to the model described in Table 1.
difference pro®les for Ca0.5Sr0.5Al2O4 suggested that in addition to the M-II phase a hexagonal phase was also present. SrAl2O4 has a polymorph with a hexagonal structure although this is only observed at temperatures above 700 8C [8]. This hexagonal structure is also observed in BaAl2O4 at room temperature [9], and a model assuming a mixture of M-II and a hexagonal phase in P6322 (labeled H-I) proved satisfactory, Table 1. All three phases, M-I, M-II and H-I were observed in the Ca0.75Sr0.25Al2O4 sample. Although there is no simple group±subgroup relationship between the three structures these are clearly related. They all contain a similar motif consisting of rings formed by six-corner sharing AlO4 tetrahedra, Fig. 2. The major difference between them is in the arrangement of the cations within the rings. It is this ordering of the cations which is responsible for the altered crystal symmetry. Considering the three phases then it is apparent that the following approximate relationships exist: a M-I b M-II c H-I; b M-I a M-II and c M-I c=5 M-II a H-I. Using these relationships we have illustrated in Fig. 3 the expansion of the cells as Sr is added to them. In each structure the cell size increases approximately linearly as the Sr content increases and equally signi®cantly across the three structures the same general trend is observed. However, there are clear discontinuities between the three structures. This is not surprising given their crystal symmetries. Considering ®rstly the M-II phase the systematic increase in the size of the cell as the Sr content Ê ) relative to Ca2 (1.00 A Ê ). The Sr increases is in accordance with the larger ionic radii of Sr2 (1.18 A substitution also results in a small increase in the monoclinic angle b. As illustrated in Fig. 4, the
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
83
Fig. 2. Dependence of the lattice parameters on composition for the series Ca1 xSrxAl2O4. The lattice parameters have been transformed between space groups using the following relationship a M-I b M-II c H-I; b M-I a M-II and c M-I c=5 M-II a H-I.
84
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
Fig. 3. Representation of the structure of (a) CaAl2O4, (b) SrAl2O4 and (c) BaAl2O4 highlighting the common structural motifs.
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
85
Fig. 4. Temperature dependence of the lattice parameters for the M-II phase observed in Ca1 xSrxAl2O4 samples with x 0, 0.125, 0.5 and 0.75.
temperature dependence of the lattice parameters for the M-II phase in the four samples was unremarkable and the three cell lengths all show an approximately linear increase as the temperature is increased. This thermally induced expansion results in a small decrease in the monoclinic angle. The same general trend in the cell length is observed for the Sr-rich M-I phase; that is as the Sr content increases the cell lengths increases, Fig. 5. In this phase the monoclinic angle is reduced slightly as the Sr content is increased. The b- and c-parameters for the x 0:875 and 0.75 samples are reasonably similar and at elevated temperatures these become effectively equal. The monoclinic angle is observed to decrease smoothly in all cases as the sample is heated. In the two samples where the hexagonal phase is observed both the a- and c-parameters are larger in the x 0:5 sample compared to the x 0:75 sample and these both increase smoothly as the temperature is increased. The temperature dependence of the abundance of the three phases observed in the x 0:75 sample is illustrated in Fig. 6. Here we observe the mass ratios of the three phases to be reasonably constant as the sample is heated from room temperature to near 400 8C. Above this the amount of the H-I phase present is seen to increase while the amount of the M-II phase clearly decreases. The co-existence of these phases and the gradual increase in the abundance of the H-I is consistent with a ®rst-order phase transition between these two phases. Within the precision of the present re®nements the amount of the
86
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
Fig. 5. Temperature dependence of the lattice parameters for the M-I phase observed in Ca1 xSrxAl2O4 samples with x 1, 0.75 and 0.875.
M-I phase present in the sample does not signi®cantly decrease as the temperature is increased. We are unaware of any reports of a high temperature phase transition in pure CaAl2O4, although as noted above SrAl2O4 is reported to be hexagonal above 700 8C [8]. In the case of the x 0:5 sample where both the M-I and H-I phases are present the relative masses of these two phases remain essentially constant over the entire temperature range. By extrapolation of the results for the x 0:75 sample, a ®rst-order transition to the H-I phase may be present at still higher temperatures. In conclusion, we have demonstrated the crystal chemistry in the SrAl2O4±CaAl2O4 system to be more complex than that described by Ju et al. [5]. The fact that the two end-members are not isostructural and that there is no continuous structural pathway linking these results in a miscibility gap near both the Sr-rich, M-I, and Ca-rich, M-II, ends of the phase diagram. This implies that it will not be possible to tune the energy of the phosphorescence across the entire series of oxides. An unexpected feature of this study is the formation of appreciable amounts of a hexagonal phase at intermediate compositions. In the x 0:25 sample, it appears that the stability of the H-I phase increases as the temperature is increased; there apparently is a ®rst-order M-II to H-I phase transition. Further studies at still higher temperatures are required to fully understand this transition.
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
87
Fig. 6. Temperature dependence of the mass abundance of the M-I, M-II and H-I phases in Ca0.75Sr0.25Al2O4.
Acknowledgements The synchrotron measurements at the Australian National Beamline Facility were supported by the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program. These measurements bene®ted from the technical assistance of Dr. J.R. Hester. References F. Guirado, S. GalõÂ, S. ChinchoÂn, Cem. Concr. Res. 30 (2000) 1023. H. Yamamoto, T. Matsuzawa, J. Lumin. 72/74 (1997) 287. W. Horkner, H.K. Muller-Buschbaum, J. Inorg. Nucl. Chem. 38 (1976) 983. V.A.R. Schulze, H.K. Muller-Buschbaum, Z. Anorg. Allg. Chem. 475 (1981) 205. S.H. Ju, S.G. Kim, J.C. Choi, H.L. Park, S.-I. Mho, T.W. Kim, Mater. Res. Bull. 34 (1999) 1905. T.M. Sabine, B.J. Kennedy, R.F. Garrett, G.J. Foran, D.J. Cookson, J. Appl. Crystallogr. 28 (1995) 513. B.A. Hunter, C.J. Howard, A Computer Program for the Rietveld Analysis of X-ray and Neutron Powder Diffraction Patterns, 1996. [8] S. Ito, S. Banno, K. Suzuki, M. Inagaki, Z. Physik. Chem. Neu. Folg. 105 (1977) 173. [9] A.J. Perrota, J.V. Smith, Bull. Soc. Fr. Mineral Cristallogr. 91 (1968) 85. [1] [2] [3] [4] [5] [6] [7]
Solubility of SrAl2O4 in CaAl2O4Ða high resolution powder diffraction study A.K. Prodjosantosoa,b, B.J. Kennedya,* a
The Centre for Heavy Metals Research, School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia b Jurusan Kimia, Universitas Negeri Yogyakarta, Yogyakarta DIY 55281, Indonesia Received 17 July 2002; accepted 9 October 2002
Abstract The solid solubility of the SrAl2O4±CaAl2O4 system has been investigated using high resolution synchrotron powder diffraction methods. Analysis of the patterns shows there is a limited composition range at which single phase samples can be obtained, x < 0:25 and x > 0:75 for the series Ca1 xSrxAl2O4. At intermediate compositions up to three phases are observed, two monoclinic and one hexagonal. The temperature dependence of the structures is also described. Crown Copyright # 2002 Published by Elsevier Science Ltd. All rights reserved. Keywords: Oxides; X-ray diffraction; Phase transition; Crystal structure
1. Introduction There is considerable interest in the structure and properties in the SrAl2O4±CaAl2O4 system. In part this is related to the importance of CaAl2O4 in high-alumina cement [1]. More recently interest in this system has been fueled by the long-lasting phosphorescence properties of the Eu2 doped materials [2]. In principle, substitution of Ca by Sr would enable the energy of the electronic transitions responsible for the phosphorescence, and thus, the color of the oxides, to be altered, provided the solid solutions remain isostructural with the end-member oxides. Whilst the structures of the two oxides SrAl2O4 and CaAl2O4 contain very similar structural motifs they are not isostructural [3,4]. Ju et al. [5] has reported that a single phase of the type Ca1 xSrxAl2O4 exists for x 0±0.5. They have however not described the phase chemistry at the Sr rich end, that is for x > 0:5. In the present paper, we report the results of a high-resolution powder synchrotron X-ray diffraction study of a series *
Corresponding author. E-mail address: [email protected] (B.J. Kennedy).
0025-5408/02/$ ± see front matter. Crown Copyright # 2002 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 1 0 0 9 - 7
80
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
of oxides of the form Ca1 xSrxAl2O4. This study demonstrates a more complex phase distribution than suggested by Ju et al. [5]. 2. Experimental 2.1. Synthesis Polycrystalline samples of Ca1 xSrxAl2O4 (x 0, 0.125, 0.25, 0.5, 0.75, 0.875 and 1) were prepared by solid state reaction of an appropriate stoichiometric mixture of analytical grade CaCO3 (Univar), SrCO3 (Merck), and Al(NO3)39H2O (Aldrich). After being thoroughly mixed in an agate mortar for several minutes the mixtures were transferred to alumina crucibles and heated in air successively at 700, 800, 900 and 1000 8C, for 24 h each with regrinding after each heating step. Finally, the samples were heated in air at 1300 8C for 72 h. 2.2. X-ray powder diffraction High-resolution synchrotron powder X-ray diffraction data were collected at a wavelength of Ê using the Debye Scherrer camera at the Australian National Beamline Facility, Photon 0.9987 A Factory, Japan [6]. Samples were loaded into 0.5-mm glass capillaries that were rotated during the measurements. These measurements were performed under vacuum to minimize air scatter. Diffraction data were recorded at room temperature over the range 5 < 2y < 85 in 0.018 steps. Temperature control was achieved using a custom built furnace, and for the variable temperature runs data were collected over the range 5 < 2y < 65 in 0.018 steps 2.3. Structural re®nements The structural re®nements were performed using the Rietveld method with the PC version of the program Rietica [7]. The background was by interpolation between up to 50 points. A pseudo-Voigt function was chosen to generate the pro®les. Selected results of the Rietveld re®nements at room temperature are summarised in Table 1. 3. Results and discussion Portions of the synchrotron X-ray diffraction patterns for Ca1 xSrxAl2O4 at room temperature are shown in Fig. 1. Examination of the diffraction patterns con®rmed the successful preparation of CaAl2O4 and SrAl2O4. The samples of CaAl2O4 and Ca0.875Sr0.125Al2O4 were single-phase oxides and the structures for both were re®ned in the monoclinic space group P21/n. This phase type is referred to below as M-I. Likewise the samples SrAl2O4 and Ca0.125Sr0.875Al2O4 and Ca0.25Sr0.75Al2O4 were single phase oxides and the structures of these were re®ned in the monoclinic space group P21, referred to as M-II. The diffraction patterns of the remaining two compounds studied, Ca0.5Sr0.5Al2O4 and Ca0.75Sr0.25Al2O4 could not be reproduced with models in either of these monoclinic space groups, nor was a mixture of these two phases suitable. Examination of the
x
0.0 0.125 0.25 0.5 0.75 0.875 1.0
Phase M-I
Phase M-II
Ê) a (A
Ê) b (A
Ê) c (A
b (8)
8.4605(2) 8.4317(4) 8.3998(5)
8.8408(2) 8.8247(4) 8.8062(6)
5.1680(1) 5.1563(2) 5.1437(4)
93.417(1) 93.532(1) 93.599(4)
Ê) a (A
8.8438(8) 8.7842(5) 8.7521(4) 8.7307(4) 8.6998(2)
Phase H-I Ê) b (A
8.3558(5) 8.2225(6) 8.1582(4) 8.1273(3) 8.0946(2)
Ê) c (A
15.3155(16) 15.3193(9) 15.2613(7) 15.2394(6) 15.2075(4)
b (8)
90.196(7) 90.019(6) 90.041(3) 90.091(2) 90.156(1)
Ê) a (A
5.1092(2) 5.1003(3)
Weight percent composition Ê) c (A
M-I
8.3916(3) 8.3482(4)
100 100 21.6(8) 0
M-II
17.7(7) 44.8(1.9)
Rp (%)
RWP (%)
4.73 3.87 7.45 5.94 4.60 3.97 6.14
4.89 4.48 9.98 5.92 4.64 4.46 7.71
H-I
60.7(1.4) 55.2(1.6)
Phase M-I is monoclinicÐspace group P21/n. The structural model based on this is in [3]; phase M-II is monoclinicÐspace group P21. The structural model based on this is in [4]; phase H-I is hexagonalÐspace group P6322. The structural model based on this is in [9].
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
Table 1 Re®ned lattice parameters for the compounds Ca1 xSrxAl2O4
81
82
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
Fig. 1. Portions of Rietveld plots for ®ve samples in the series Ca1 xSrxAl2O4. In all cases the crosses are the observed data and the solid line ®ts to the model described in Table 1.
difference pro®les for Ca0.5Sr0.5Al2O4 suggested that in addition to the M-II phase a hexagonal phase was also present. SrAl2O4 has a polymorph with a hexagonal structure although this is only observed at temperatures above 700 8C [8]. This hexagonal structure is also observed in BaAl2O4 at room temperature [9], and a model assuming a mixture of M-II and a hexagonal phase in P6322 (labeled H-I) proved satisfactory, Table 1. All three phases, M-I, M-II and H-I were observed in the Ca0.75Sr0.25Al2O4 sample. Although there is no simple group±subgroup relationship between the three structures these are clearly related. They all contain a similar motif consisting of rings formed by six-corner sharing AlO4 tetrahedra, Fig. 2. The major difference between them is in the arrangement of the cations within the rings. It is this ordering of the cations which is responsible for the altered crystal symmetry. Considering the three phases then it is apparent that the following approximate relationships exist: a M-I b M-II c H-I; b M-I a M-II and c M-I c=5 M-II a H-I. Using these relationships we have illustrated in Fig. 3 the expansion of the cells as Sr is added to them. In each structure the cell size increases approximately linearly as the Sr content increases and equally signi®cantly across the three structures the same general trend is observed. However, there are clear discontinuities between the three structures. This is not surprising given their crystal symmetries. Considering ®rstly the M-II phase the systematic increase in the size of the cell as the Sr content Ê ) relative to Ca2 (1.00 A Ê ). The Sr increases is in accordance with the larger ionic radii of Sr2 (1.18 A substitution also results in a small increase in the monoclinic angle b. As illustrated in Fig. 4, the
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
83
Fig. 2. Dependence of the lattice parameters on composition for the series Ca1 xSrxAl2O4. The lattice parameters have been transformed between space groups using the following relationship a M-I b M-II c H-I; b M-I a M-II and c M-I c=5 M-II a H-I.
84
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
Fig. 3. Representation of the structure of (a) CaAl2O4, (b) SrAl2O4 and (c) BaAl2O4 highlighting the common structural motifs.
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
85
Fig. 4. Temperature dependence of the lattice parameters for the M-II phase observed in Ca1 xSrxAl2O4 samples with x 0, 0.125, 0.5 and 0.75.
temperature dependence of the lattice parameters for the M-II phase in the four samples was unremarkable and the three cell lengths all show an approximately linear increase as the temperature is increased. This thermally induced expansion results in a small decrease in the monoclinic angle. The same general trend in the cell length is observed for the Sr-rich M-I phase; that is as the Sr content increases the cell lengths increases, Fig. 5. In this phase the monoclinic angle is reduced slightly as the Sr content is increased. The b- and c-parameters for the x 0:875 and 0.75 samples are reasonably similar and at elevated temperatures these become effectively equal. The monoclinic angle is observed to decrease smoothly in all cases as the sample is heated. In the two samples where the hexagonal phase is observed both the a- and c-parameters are larger in the x 0:5 sample compared to the x 0:75 sample and these both increase smoothly as the temperature is increased. The temperature dependence of the abundance of the three phases observed in the x 0:75 sample is illustrated in Fig. 6. Here we observe the mass ratios of the three phases to be reasonably constant as the sample is heated from room temperature to near 400 8C. Above this the amount of the H-I phase present is seen to increase while the amount of the M-II phase clearly decreases. The co-existence of these phases and the gradual increase in the abundance of the H-I is consistent with a ®rst-order phase transition between these two phases. Within the precision of the present re®nements the amount of the
86
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
Fig. 5. Temperature dependence of the lattice parameters for the M-I phase observed in Ca1 xSrxAl2O4 samples with x 1, 0.75 and 0.875.
M-I phase present in the sample does not signi®cantly decrease as the temperature is increased. We are unaware of any reports of a high temperature phase transition in pure CaAl2O4, although as noted above SrAl2O4 is reported to be hexagonal above 700 8C [8]. In the case of the x 0:5 sample where both the M-I and H-I phases are present the relative masses of these two phases remain essentially constant over the entire temperature range. By extrapolation of the results for the x 0:75 sample, a ®rst-order transition to the H-I phase may be present at still higher temperatures. In conclusion, we have demonstrated the crystal chemistry in the SrAl2O4±CaAl2O4 system to be more complex than that described by Ju et al. [5]. The fact that the two end-members are not isostructural and that there is no continuous structural pathway linking these results in a miscibility gap near both the Sr-rich, M-I, and Ca-rich, M-II, ends of the phase diagram. This implies that it will not be possible to tune the energy of the phosphorescence across the entire series of oxides. An unexpected feature of this study is the formation of appreciable amounts of a hexagonal phase at intermediate compositions. In the x 0:25 sample, it appears that the stability of the H-I phase increases as the temperature is increased; there apparently is a ®rst-order M-II to H-I phase transition. Further studies at still higher temperatures are required to fully understand this transition.
A.K. Prodjosantoso, B.J. Kennedy / Materials Research Bulletin 38 (2003) 79±87
87
Fig. 6. Temperature dependence of the mass abundance of the M-I, M-II and H-I phases in Ca0.75Sr0.25Al2O4.
Acknowledgements The synchrotron measurements at the Australian National Beamline Facility were supported by the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program. These measurements bene®ted from the technical assistance of Dr. J.R. Hester. References F. Guirado, S. GalõÂ, S. ChinchoÂn, Cem. Concr. Res. 30 (2000) 1023. H. Yamamoto, T. Matsuzawa, J. Lumin. 72/74 (1997) 287. W. Horkner, H.K. Muller-Buschbaum, J. Inorg. Nucl. Chem. 38 (1976) 983. V.A.R. Schulze, H.K. Muller-Buschbaum, Z. Anorg. Allg. Chem. 475 (1981) 205. S.H. Ju, S.G. Kim, J.C. Choi, H.L. Park, S.-I. Mho, T.W. Kim, Mater. Res. Bull. 34 (1999) 1905. T.M. Sabine, B.J. Kennedy, R.F. Garrett, G.J. Foran, D.J. Cookson, J. Appl. Crystallogr. 28 (1995) 513. B.A. Hunter, C.J. Howard, A Computer Program for the Rietveld Analysis of X-ray and Neutron Powder Diffraction Patterns, 1996. [8] S. Ito, S. Banno, K. Suzuki, M. Inagaki, Z. Physik. Chem. Neu. Folg. 105 (1977) 173. [9] A.J. Perrota, J.V. Smith, Bull. Soc. Fr. Mineral Cristallogr. 91 (1968) 85. [1] [2] [3] [4] [5] [6] [7]