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Preparation by pyrolysis of aerosols and structural characterization of Fe-doped mullite powders
Pergamon
Materials Research Bulletin 35 (2000) 775–788
Preparation by pyrolysis of aerosols and structural characterization of Fe-doped mullite powders M. Ocan˜aa,*, A. Caballeroa, T. Gonza´lez-Carren˜ob, C.J. Sernab a
Instituto de Ciencia de Materiales de Sevilla, C.S.I.C.-Universidad de Sevilla, Americo Vespucio s/n, Isla de La Cartuja, 41092 Sevilla, Spain b Instituto de Ciencia de Materiales de Madrid, C.S.I.C., Campus Universitario de Cantoblanco, 28049 Madrid, Spain (Refereed) Received 11 May 1999; accepted 21 June 1999
Abstract Amorphous Fe-doped 2SiO2 䡠3Al2O3 powders were prepared by pyrolysis of aerosols generated from solutions of TEOS, Al(III) nitrate, and Fe(III) nitrate in methanol. The study of the thermal evolution of these powders revealed that the presence of Fe(III) promotes mullite formation and that these cations enter into the mullite lattice at the first stages of crystallization at ⬃900°C. It was also found that the amount of Fe(III) in solid solution with mullite increased as increasing temperature from 900 to 1400°C. The limit of Fe(III) incorporation into mullite in the as-prepared powders was in the range previously reported for samples prepared by the ceramic procedure (⬃11 Fe2O3 wt%). The additional iron phase appearing in samples with an iron content above this limit consisted of an iron oxide spinel, which probably contained some Al(III) cations in solid solution. Finally, the analysis of the EXAFS spectra of the heated samples showed that the iron cations incorporated into the mullite lattice are mainly located at octahedral positions. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramics; A. Oxides; A. Structural materials; B. Chemical synthesis; C. XAFS (EXAFS and XANES)
* Corresponding author. E-mail address: [email protected] (M. Ocan˜a). 0025-5408/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 0 ) 0 0 2 5 6 - 7
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1. Introduction The incorporation of impurities in the mullite (2SiO2 䡠3Al2O3) structure has been the subject of much study [1,2] due to the important consequences that it may have on the physical properties of the final products. In particular, it has been shown [3] that the formation of solid solutions between mullite and transition metal cations affects the thermal expansion of mullite through distortion of the Al–O octahedra. Several authors have amply reported that mullite can incorporate different amounts of transition metal cations such as Ti3⫹, Ti4⫹, V3⫹, V4⫹, V5⫹, Cr3⫹, Mn2⫹, Co2⫹, Fe2⫹, and Fe3⫹ [1,2]. In the case of Fe-doped mullite, most of the studies have been conducted on samples prepared by the traditional ceramic method, which requires high temperatures (ⱖ1300°C) for mullite formation [1,4,5]. Under these conditions, it was found that the limit of Fe3⫹ incorporation was about 11 wt% expressed as Fe2O3 [5]. From the electron spin resonance (ESR) spectra of the samples, it was suggested that the Fe3⫹ cations are located in different positions of the mullite lattice: substituting for Al3⫹ in isolated octahedral and tetrahedral sites and in clusters [5]. It is very well known that mullite may crystallize at temperatures as low as ⬃900°C by annealing amorphous precursors having a high chemical homogeneity (Si–Al mixing), which can be synthesized by sol-gel or spray pyrolysis procedures [6,7]. The initially crystallized mullite phase is characterized by a low SiO2 content, accompanied by a decrease in the b/a unit-cell parameters ratio to ⬃1, for which it is called pseudo-tetragonal mullite [7]. This phase can be transformed to the stoichiometric (3:2) orthorhombic mullite by further heating at higher temperatures (ⱖ1200°C). The aim of this work was to study the pathway of mullite formation from Fe-doped 2SiO2 䡠3Al2O3 precursors prepared by pyrolysis of aerosols and to analyze the effects of temperature and raw iron content on the incorporation of Fe3⫹ into the mullite structure. In addition, the localization of Fe3⫹ in the mullite framework was investigated using EXAFS spectroscopy, which has been shown to be a suitable tool to obtain structural information on solid solutions between other transition metal cations and ceramic silicates [8,9], including mullite [10].
2. Experimental 2.1. Powders preparation Fe-doped 2SiO2 䡠 3Al2O3 powders were prepared by pyrolysis of aerosols in an apparatus described elsewhere [11]. The aerosols were generated using a nozzle atomizer and air at constant pressure (1.6 kg cm⫺2) as a carrier gas, from solutions in methanol of tetraethyl orthosilicate (TEOS, Fluka, 98%), Al(NO3)3 䡠9H2O (Aldrich, 98%), and Fe(NO3)3 䡠9H2O (Fluka, 99%). The total concentration of M3⫹ salts (M ⫽ Al ⫹ Fe) and TEOS were, in all experiments, 0.075 and 0.025 mol dm⫺3, respectively, thus keeping constant the stoichiometry of mullite (M/Si molar ratio ⫽ 3) for all samples. The Fe3⫹/Al3⫹ ratio was varied around the limit of Fe incorporation previously reported [5] for samples prepared by the conven-
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tional ceramic procedure (⬃11 wt%, expressed as Fe2O3). The as-generated aerosols were first introduced into a furnace heated at 250°C, to evaporate the solvent from the droplets and afterwards into a second furnace at 600°C, to decompose the metal precursors. The resulting solid particles of the corresponding metal oxides were collected with an electrostatic precipitator. 2.2. Analyses The particle size and shape of the powders were examined by transmission electron microscopy (TEM; Philips 200 CM). The composition of the solids (Al/Si ratio and Fe content) was determined by plasma emission (ICP, Perkin-Elmer 5500). For this purpose, 100 mg of sample were first fused at 1100°C with a NaKCO3:Na2B4O7 mixture and then extracted with a HCl solution. Differential thermal (DTA) and thermogravimetric (TGA) analyses (Setaram 92-16.18) were carried out in air at a heating rate of 10°C min⫺1. The infrared spectra of the samples diluted in KBr were recorded in a Nicolet 510 FT-IR spectrometer. The crystalline phases present in the solids were identified by X-ray diffraction (XRD) (Siemens D501). The unit-cell parameters of the powders were measured by Rietveld analysis of the X-ray diffraction data following a previously described procedure [12]. In all experiments, a silicon standard (20% by weight) was mechanically mixed with the sample. The crystallographic data for mullite and silicon were taken from refs. 13 and 14, respectively. Extended X-ray absorption fine structure (EXAFS) measurements at the Fe K-edge were collected at Station 7.1 at the Synchrotron Radiation Source, Daresbury Laboratory (UK), with an average current of 250 mA at 2 GeV. Energy selection was accomplished by a double-crystal Si(111) monochromator. The XAS spectra were collected in the transmission mode using two gas ionization chambers as detectors. The experimental data were analyzed with the software developed by Bonin et al. [15], using the theoretical amplitude and phases shift functions proposed by Rehr et al. (version 6.01) [16]. Hematite (␣-Fe2O3), maghemite (␥-Fe2O3) and iron(III) acetylacetonate (Fe-acac) were used as reference compounds.
3. Results and discussion 3.1. Preparation and characterization of the starting powders Three Fe-doped silica–alumina samples were prepared with an iron content close to the limit of solid solubility (⬃11 wt%, expressed as Fe2O3) previously reported [5] for Fe-doped mullites prepared by the traditional ceramic procedure. The compositions obtained from the ICP analyses for these samples are shown in Table 1. As illustrated for sample 8.5Fe (Fig. 1A), these powders consisted of spherical particles of a broad size distribution (⬃0.05– 0.7 m), which were amorphous to X-ray diffraction. The IR spectra of the as-prepared samples were similar to that shown in Fig. 2 corresponding to sample 8.5Fe, which showed two broad
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Table 1 Composition of the as-prepared Fe-doped silica–alumina samples Sample
Nominal composition
Measured composition
Iron content (wt% Fe2O3)
8.5Fe 11.7Fe 14.9Fe
Fe0.5Al5.5Si2O13 Fe0.7Al5.3Si2O13 Fe0.9Al5.1Si2O13
Fe0.47Al5.6Si1.98O13 Fe0.65Al5.42Si1.96O13 Fe0.84Al5.2Si1.98O13
8.5 11.7 14.9
Fig. 1. Transmission electron micrographs of sample 8.5Fe as prepared (A) and heated at 900 (B), 1200 (C), and 1400°C (D).
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Fig. 2. Infrared spectra of sample 8.5Fe as prepared and after heating at different temperatures.
bands at 1055 and 615 cm⫺1, attributed to Te–O (Te ⫽ Si or Al in tetrahedral coordination) and Oc–O (Oc ⫽ Al or Fe in octahedral coordination) vibrations, respectively [17]. An intense absorption at 1385 cm⫺1 was also observed (Fig. 2) due to nitrate anions [18], indicating that they were not fully decomposed in the pyrolysis furnace. 3.2. Thermal analyses Fig. 3 shows the DTA curves obtained for the Fe-doped silica–alumina samples along with that corresponding to an undoped sample with mullite composition (blank) prepared using a similar procedure, which is included for the sake of comparison. As observed, all curves display an exothermic effect between 200 and 250°C due to the decomposition of the nitrate anions remaining in the samples, which is substantiated by the disappearance of the band at 1385 cm⫺1 in the IR spectrum of the samples heated at 400°C (Fig. 2). At higher temperatures, only an exothermic peak at about 900°C was detected, which, as is later confirmed, is associated with the crystallization of pseudo-tetragonal mullite. It is interesting to note that this peak was progressively shifted to lower temperature with increasing Fe content in the doped samples, indicating a catalytic effect of Fe in mullite formation. This finding is contrary to previously reported observations [19] carried out on samples prepared by the
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Fig. 3. Differential thermal analyses obtained for the Fe-doped mullite samples and for an undoped mullite powder (blank).
sol-gel process, according to which the presence of iron retarded the formation of mullite up to ⬃1200°C. This behavior was explained [19] by the presence of heterogeneities in the silica–alumina matrix promoted by the iron(III) cations, which induced the previous crystallization (⬃900°C) of an Fe/Al spinel. It should be noted that the particles still retained the spherical shape after heating at 900°C, whereas the beginning of sintering was observed at ⱖ1200°C (Fig. 1). 3.3. Phases evolution with temperature X-ray diffraction (Fig. 4) showed that sample 8.5Fe remained amorphous after heating up to 800°C. The crystallization of mullite was observed at 900°C. As expected, the pattern obtained at this temperature did not show the splitting of the peak at 2 ⬃26°, corresponding to the 120 and 210 reflections [13]. This suggests the pseudo-tetragonal character of this phase. Such a splitting was detected after calcination at higher temperature (ⱖ1200°C), indicating the development of orthorhombic mullite. It should be noted that for this sample, mullite was the only crystalline phase detected by X-ray diffraction during the calcination process from 900 to 1400°C. The crystallization of pseudo-tetragonal mullite and its evolution to the orthorhombic phase was also studied by infrared spectroscopy (Fig. 2). The spectrum obtained for the sample heated at 900°C was similar to that previously reported for tetragonal mullite [20].
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Fig. 4. X-ray diffraction patterns of sample 8.5Fe heated at different temperatures.
The only appreciable difference between the latter and that of the orthorhombic mullite developed at 1400°C was the relative intensity of the doublet at 1165–1130 cm⫺1 [20]. The crystallization behavior of sample 11.7Fe (Fig. 5) was similar to that of sample 8.5Fe. The main difference between the X-ray diffraction patterns of both samples was the appearance at 1100°C of two reflections at d ⫽ 2.94 and 2.50 Å, in addition to those of mullite. The position of these peaks was slightly shifted to lower d-spacing values with respect to the most intense reflections of ␥-Fe2O3 [21] or Fe3O4 [22]. Therefore, they are attributed to an iron oxide spinel, probably containing some Al3⫹ cations in solid solution. The magnetic character observed for the sample is in agreement with such an assignment. It should be noted that the phase diagram reported for the SiO2–Al2O3–Fe2O3 system [23] predicts a similar spinel phase to that suggested here, although at higher temperatures (ⱖ1380°C). The relative intensity of the spinel peaks decreased with increasing temperature, finally disappearing after heating at 1400°C (Fig. 5). The increase of the iron content in Fe-doped mullite up to 14.9 wt% (as Fe2O3) (sample 14.9Fe) gave rise to an increase of the relative intensities of the peaks corresponding to the spinel phase (Fig. 6), with respect to those of sample 11.7Fe (Fig. 5).
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Fig. 5. X-ray diffraction patterns of sample 11.7Fe heated at different temperatures. The peaks corresponding to the spinel phase are labeled with an asterisk.
These peaks were still observed for sample 14.9Fe after heating at 1400°C, although with lower relative intensity than at 1200°C. The IR spectra of samples 11.7Fe and 14.9Fe heated at different temperatures did not show noticeable differences with respect to those of sample 8.5Fe (Fig. 2). 3.4. Unit-cell parameters The unit-cell parameters measured for the Fe-doped mullite samples heated at different temperatures are shown in Table 2, in which those corresponding to the blank and the ASTM file [13] have also been included. As expected, the a value obtained for undoped mullite heated at 900°C was much higher (7.615 Å) than that reported in the ASTM file for orthorhombic mullite (7.5456 Å), being closer to the value of b (7.680 Å), which shows the pseudo-tetragonal character of this phase [7]. This behavior has been reported [7] to be due to a deficiency in the silica content of the mullite phase initially crystallized from amorphous precursors having a high chemical homogeneity. When the calcination temperature is further
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Fig. 6. X-ray diffraction patterns of sample 14.8Fe heated at different temperatures.
Table 2 Unit-cell parameters of the Fe-doped mullite samples heated at different temperatures and their relative increment with respect to the blank heated at the same temperature Sample ASTM Blank 8.5Fe “ “ 11.7Fe “ 14.9Fe
Temperature (°C) 900 1200 1400 900 1200 1400 1200 1400 1400
a (Å)
b (Å)
c (Å)
7.5456 7.609(2) 7.553(2) 7.545(1) 7.615(2) 7.576(1) 7.578(1) 7.580(2) 7.581(1) 7.581(1)
7.6898 7.680(2) 7.686(2) 7.697(2) 7.720(2) 7.722(2) 7.723(1) 7.724(2) 7.727(1) 7.732(1)
2.8842 2.8847(9) 2.8850(8) 2.8851(5) 2.8957(7) 2.8984(5) 2.8993(3) 2.8989(6) 2.9012(3) 2.9030(4)
⌬a/a (%)
⌬b/b (%)
⌬c/c (%)
0.437
0.338
0.492
0.477 0.477
0.390 0.455
0.554 0.620
The cell parameters given in the ASTM file of mullite are also included.
b/a 1.0191 1.0093 1.0176 1.0201 1.0137 1.0192 1.0192 1.0190 1.0190 1.0199
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increased, the incorporation of silica to the mullite lattice progresses, finally yielding the stoichiometry of mullite. This process is accompanied by a decrease of a to the value that progressively approaches that corresponding to orthorhombic mullite, which took place for our blank after heating at 1400°C (Table 2). As observed in Table 2, the values of a, b, and c obtained after heating sample 8.5Fe at different temperatures are higher than those of the blank heated at the same temperatures. This finding reveals that the incorporation of iron cations into the mullite structure is parallel to the formation of the mullite framework at 900°C, and the existence of a solid solution during the whole studied temperature range. It was also noted that, as in the blank, the ratio b/a for this sample heated at 900°C was lower (1.0137) than that corresponding to orthorhombic mullite (1.0201) and that the a value decreased from 900 to 1200°C, which as stated above can be attributed to the incorporation of silica into the mullite lattice. Although the b parameter did not significantly change during this treatment, the slight increase observed for the c parameter seems to indicate a progress in the incorporation of iron into mullite. Therefore, the amount of iron in solid solution in the sample heated at 900°C must be lower than 8.5 wt% (as Fe2O3). It should be noted that the unit-cell parameters did not show important variations when sample 8.5Fe was heated at higher temperature (1400°C). This finding, along with the absence of crystalline phases other than mullite (Fig. 4), suggests that the whole iron content of this sample was incorporated into the mullite lattice at 1200°C. As shown in Table 2, the differences between the cell parameters obtained for sample 11.7Fe (11.7 wt% Fe2O3) heated at 1200°C and those of sample 8.5Fe (8.5 wt% Fe2O3) heated at the same temperature are within experimental errors. Therefore, both samples contained a similar amount of iron in solid solution (8.5 wt% Fe2O3). In sample 11.7Fe, this involves the presence of an additional Fe phase, consisting of a Fe–Al spinel, as revealed by X-ray diffraction (Fig. 5). This result suggests that the limit of iron incorporation into mullite at 1200°C is ⬃8.5 wt% Fe2O3. The heating of the sample at 1400°C gave rise to an increase of the c parameter indicating a progress in the incorporation of iron into the Fe-mullite solid solution. This would be in agreement with the disappearance of the X-ray diffraction peaks corresponding to the spinel phase (Fig. 5). The latter behavior suggests that most of the iron cations of the sample were incorporated into the mullite lattice at this temperature. It should be noted that the iron content of this sample (11.7 wt% Fe2O3) is similar to the maximum amount of iron incorporated to mullites prepared by the conventional ceramic procedure (⬃11 wt% Fe2O3) [5]. The increase of the raw iron content in the doped samples from 11.7 (sample 11.7Fe) to 14.9 wt% Fe2O3 (sample 14.9Fe) also gave rise to an increase of the b and c parameters of mullite after calcination at 1400°C (Table 2). This suggests an increase of the iron amount in the solid solution. Such an increase must be very small, since the spinel phase was still observed for sample 14.9Fe heated at this temperature, in addition to mullite (Fig. 6). Finally, it should be noted that for all studied compositions, the incorporation of iron into mullite gave rise to a larger relative expansion for c than for a or b (Table 2), as it has been previously reported for samples prepared by the ceramic procedure [2].
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Fig. 7. F.T. functions obtained from the Fe K-edge oscillations for sample 8.5Fe heated at different temperatures.
3.5. Localization of Fe3⫹ into the mullite lattice To elucidate the position that Fe3⫹ cations occupy in the mullite lattice, we selected sample 8.5Fe, which according to X-ray diffraction (Fig. 4), did not show additional iron oxide phases. Fig. 7 displays the pseudo-radial distribution functions (F.T. curves) obtained for this sample heated at different temperatures. The functions corresponding to the iron compounds used as references (␣-Fe2O3, ␥-Fe2O3, and Fe-acac) are presented in Fig. 8. As observed, after heating at 800°C, the curve of the Fe-doped sample showed an intense peak at ⬃1.53 Å (uncorrected for the phase shift) corresponding to the first coordination shell around the Fe3⫹ cations. The peak associated with the second shell (2– 4 Å region) however, was very weak, which is characteristic of molecular (as is the case of Fe-acac) or amorphous compounds. This is in agreement with the X-ray diffraction observations (Fig. 4). The presence of a more intense peak at 2.88 Å in the F.T. curve of the sample heated at 900°C indicates the crystallization of an iron compound, which, according to X-ray diffraction (Fig. 4) and the unit-cell parameters of the sample (Table 2), can be ascribed to the Fe-mullite solid solution. The slight increase in intensity of this peak with heat treatment at 1200°C (Fig. 7) could be explained by the further incorporation of Fe into mullite, as revealed by the increase of the unit-cell parameters detected after this treatment (Table 2).
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Fig. 8. F.T. functions obtained from the Fe K-edge oscillations for the reference iron compounds.
A quantitative analysis of the EXAFS data was conducted on sample 8.5Fe heated at 1200°C, since at this temperature the Fe-mullite solid solution was fully developed (Table 2). Because of the rather complex atomic environments around the cationic sites in the mullite structure, especially for the 2– 4 Å region of the F.T. curve in which contributions from several kinds of O, Al, and Si atoms are included [24], the fitting procedure of the EXAFS data was only accomplished for the first coordination shell (0.82–2.15 Å). The best fitting attained (Fig. 9) resulted in a Fe–O distance of 1.95 Å, which is slightly higher than that for the octahedral Al3⫹ cations in mullite (1.89 –1.94 Å) [24]. This finding, along with the coordination number obtained from the fitting (5.6), clearly indicates that the Fe3⫹ ions are located at the octahedral positions of Al in the mullite lattice, which induces a cell expansion. This is in agreement with the measured unit-cell parameters (Table 2). Finally, it should be noted that the pseudo-radial distribution functions of the 11.7Fe and 14.9Fe samples after heating at 1400°C were similar to those of sample 8.5Fe heated at 1200°C, showing small differences in the intensity of the peak corresponding to the second shell (2– 4 Å region). This behavior can be attributed to the different Fe amounts in solid solution with mullite, as revealed by the unit-cell parameters shown in Table 2, as well as the presence of the additional spinel phase (Fig. 6).
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Fig. 9. Inverse Fourier transform (dotted line) and curve obtained by fitting analysis (solid line) of the Fe K-edge EXAFS spectrum registered for sample 8.5Fe heated at 1200°C.
Acknowledgments This work was financially supported by the Spanish DGCYT under Project PB95-0225.
References [1] M. Krishna Murthy, F.A. Hummel, J Am Ceram Soc 43 (1960) 267. [2] H. Schneider, in: S. Somiya, R.F. Davic, J.A. Pask (Eds.), Ceramic Transactions Vol. 6, American Ceramic Society, Westerville, OH, 1990, p. 135. [3] H. Schneider, E. Eberhard, J Am Ceram Soc 73 (1990) 2073. [4] W.E. Brownell, J Am Ceram Soc 41 (1958) 226. [5] H. Scheneider, H. Rager, Ceram Int 12 (1986) 117. [6] I.A. Aksay, D.M. Dabbs, M. Sarikaya, J Am Ceram Soc 74 (1991) 2343. [7] K. Okada, N. Otsuka, S. Somiya, Ceram Bull 70 (1991) 1633. [8] M. Ocan˜a, A.R. Gonza´lez-Elipe, V.M. Orera, P. Tartaj, C.J. Serna, J Am Ceram Soc 81 (1998) 395. [9] M. Ocan˜a, A. Caballero, A.R. Gonza´lez-Elipe, P. Tartaj, C.J. Serna, J Solid State Chem 139 (1998) 412. [10] K.R. Bauchspieß, H. Schneider, A. Kulikov, J Eur Ceram Soc 16 (1996) 203.
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M. Ocan˜a et al. / Materials Research Bulletin 35 (2000) 775–788 T. Gonza´lez-Carren˜o, A. Mifsud, J.M. Palacios, C.J. Serna, Mater Chem Phys 27 (1991) 287. P. Tartaj, T. Gonza´lez-Carren˜o, C.J. Serna, M. Ocan˜a, J Solid State Chem 128 (1997) 102. ASTM file no. 15-776. Nat. Bur. Stand. Monogr. (U.S.) 25 (1976) 13. D. Bonin, P. Kaiser, C. Fretigny, J. Desbarres, in: H. Dexpert, A. Michalowicz, M. Verdaguer (Eds.), Structures Fines d’Absorption des Rayons X en Chimie, Vol. 3, Orsay, France, 1989. J.J. Rehr, S.I. Zabinsky, R.C. Albers, Phys Rev Lett 69 (1992) 3397. M. Ocan˜a, J. Sanz, T. Gonzalez-Carren˜o, C.J. Serna, J Am Ceram Soc 76 (1993) 2081. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1986, p. 476. J. Parmentier, S. Vilminot, Eur J Solid State Inorg Chem, 34 (1997) 197. K. Okada, Y Hoshi, N. Otsuka, J Mater Sci Lett 5 (1986) 1315. ASTM file no. 24-81. ASTM file no. 19-629. A. Muan, J Am Ceram Soc 40 (1957) 121. D. Balzar, H. Ledbetter, Am Mineral 78 (1993) 1192.
Materials Research Bulletin 35 (2000) 775–788
Preparation by pyrolysis of aerosols and structural characterization of Fe-doped mullite powders M. Ocan˜aa,*, A. Caballeroa, T. Gonza´lez-Carren˜ob, C.J. Sernab a
Instituto de Ciencia de Materiales de Sevilla, C.S.I.C.-Universidad de Sevilla, Americo Vespucio s/n, Isla de La Cartuja, 41092 Sevilla, Spain b Instituto de Ciencia de Materiales de Madrid, C.S.I.C., Campus Universitario de Cantoblanco, 28049 Madrid, Spain (Refereed) Received 11 May 1999; accepted 21 June 1999
Abstract Amorphous Fe-doped 2SiO2 䡠3Al2O3 powders were prepared by pyrolysis of aerosols generated from solutions of TEOS, Al(III) nitrate, and Fe(III) nitrate in methanol. The study of the thermal evolution of these powders revealed that the presence of Fe(III) promotes mullite formation and that these cations enter into the mullite lattice at the first stages of crystallization at ⬃900°C. It was also found that the amount of Fe(III) in solid solution with mullite increased as increasing temperature from 900 to 1400°C. The limit of Fe(III) incorporation into mullite in the as-prepared powders was in the range previously reported for samples prepared by the ceramic procedure (⬃11 Fe2O3 wt%). The additional iron phase appearing in samples with an iron content above this limit consisted of an iron oxide spinel, which probably contained some Al(III) cations in solid solution. Finally, the analysis of the EXAFS spectra of the heated samples showed that the iron cations incorporated into the mullite lattice are mainly located at octahedral positions. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramics; A. Oxides; A. Structural materials; B. Chemical synthesis; C. XAFS (EXAFS and XANES)
* Corresponding author. E-mail address: [email protected] (M. Ocan˜a). 0025-5408/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 0 ) 0 0 2 5 6 - 7
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1. Introduction The incorporation of impurities in the mullite (2SiO2 䡠3Al2O3) structure has been the subject of much study [1,2] due to the important consequences that it may have on the physical properties of the final products. In particular, it has been shown [3] that the formation of solid solutions between mullite and transition metal cations affects the thermal expansion of mullite through distortion of the Al–O octahedra. Several authors have amply reported that mullite can incorporate different amounts of transition metal cations such as Ti3⫹, Ti4⫹, V3⫹, V4⫹, V5⫹, Cr3⫹, Mn2⫹, Co2⫹, Fe2⫹, and Fe3⫹ [1,2]. In the case of Fe-doped mullite, most of the studies have been conducted on samples prepared by the traditional ceramic method, which requires high temperatures (ⱖ1300°C) for mullite formation [1,4,5]. Under these conditions, it was found that the limit of Fe3⫹ incorporation was about 11 wt% expressed as Fe2O3 [5]. From the electron spin resonance (ESR) spectra of the samples, it was suggested that the Fe3⫹ cations are located in different positions of the mullite lattice: substituting for Al3⫹ in isolated octahedral and tetrahedral sites and in clusters [5]. It is very well known that mullite may crystallize at temperatures as low as ⬃900°C by annealing amorphous precursors having a high chemical homogeneity (Si–Al mixing), which can be synthesized by sol-gel or spray pyrolysis procedures [6,7]. The initially crystallized mullite phase is characterized by a low SiO2 content, accompanied by a decrease in the b/a unit-cell parameters ratio to ⬃1, for which it is called pseudo-tetragonal mullite [7]. This phase can be transformed to the stoichiometric (3:2) orthorhombic mullite by further heating at higher temperatures (ⱖ1200°C). The aim of this work was to study the pathway of mullite formation from Fe-doped 2SiO2 䡠3Al2O3 precursors prepared by pyrolysis of aerosols and to analyze the effects of temperature and raw iron content on the incorporation of Fe3⫹ into the mullite structure. In addition, the localization of Fe3⫹ in the mullite framework was investigated using EXAFS spectroscopy, which has been shown to be a suitable tool to obtain structural information on solid solutions between other transition metal cations and ceramic silicates [8,9], including mullite [10].
2. Experimental 2.1. Powders preparation Fe-doped 2SiO2 䡠 3Al2O3 powders were prepared by pyrolysis of aerosols in an apparatus described elsewhere [11]. The aerosols were generated using a nozzle atomizer and air at constant pressure (1.6 kg cm⫺2) as a carrier gas, from solutions in methanol of tetraethyl orthosilicate (TEOS, Fluka, 98%), Al(NO3)3 䡠9H2O (Aldrich, 98%), and Fe(NO3)3 䡠9H2O (Fluka, 99%). The total concentration of M3⫹ salts (M ⫽ Al ⫹ Fe) and TEOS were, in all experiments, 0.075 and 0.025 mol dm⫺3, respectively, thus keeping constant the stoichiometry of mullite (M/Si molar ratio ⫽ 3) for all samples. The Fe3⫹/Al3⫹ ratio was varied around the limit of Fe incorporation previously reported [5] for samples prepared by the conven-
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tional ceramic procedure (⬃11 wt%, expressed as Fe2O3). The as-generated aerosols were first introduced into a furnace heated at 250°C, to evaporate the solvent from the droplets and afterwards into a second furnace at 600°C, to decompose the metal precursors. The resulting solid particles of the corresponding metal oxides were collected with an electrostatic precipitator. 2.2. Analyses The particle size and shape of the powders were examined by transmission electron microscopy (TEM; Philips 200 CM). The composition of the solids (Al/Si ratio and Fe content) was determined by plasma emission (ICP, Perkin-Elmer 5500). For this purpose, 100 mg of sample were first fused at 1100°C with a NaKCO3:Na2B4O7 mixture and then extracted with a HCl solution. Differential thermal (DTA) and thermogravimetric (TGA) analyses (Setaram 92-16.18) were carried out in air at a heating rate of 10°C min⫺1. The infrared spectra of the samples diluted in KBr were recorded in a Nicolet 510 FT-IR spectrometer. The crystalline phases present in the solids were identified by X-ray diffraction (XRD) (Siemens D501). The unit-cell parameters of the powders were measured by Rietveld analysis of the X-ray diffraction data following a previously described procedure [12]. In all experiments, a silicon standard (20% by weight) was mechanically mixed with the sample. The crystallographic data for mullite and silicon were taken from refs. 13 and 14, respectively. Extended X-ray absorption fine structure (EXAFS) measurements at the Fe K-edge were collected at Station 7.1 at the Synchrotron Radiation Source, Daresbury Laboratory (UK), with an average current of 250 mA at 2 GeV. Energy selection was accomplished by a double-crystal Si(111) monochromator. The XAS spectra were collected in the transmission mode using two gas ionization chambers as detectors. The experimental data were analyzed with the software developed by Bonin et al. [15], using the theoretical amplitude and phases shift functions proposed by Rehr et al. (version 6.01) [16]. Hematite (␣-Fe2O3), maghemite (␥-Fe2O3) and iron(III) acetylacetonate (Fe-acac) were used as reference compounds.
3. Results and discussion 3.1. Preparation and characterization of the starting powders Three Fe-doped silica–alumina samples were prepared with an iron content close to the limit of solid solubility (⬃11 wt%, expressed as Fe2O3) previously reported [5] for Fe-doped mullites prepared by the traditional ceramic procedure. The compositions obtained from the ICP analyses for these samples are shown in Table 1. As illustrated for sample 8.5Fe (Fig. 1A), these powders consisted of spherical particles of a broad size distribution (⬃0.05– 0.7 m), which were amorphous to X-ray diffraction. The IR spectra of the as-prepared samples were similar to that shown in Fig. 2 corresponding to sample 8.5Fe, which showed two broad
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Table 1 Composition of the as-prepared Fe-doped silica–alumina samples Sample
Nominal composition
Measured composition
Iron content (wt% Fe2O3)
8.5Fe 11.7Fe 14.9Fe
Fe0.5Al5.5Si2O13 Fe0.7Al5.3Si2O13 Fe0.9Al5.1Si2O13
Fe0.47Al5.6Si1.98O13 Fe0.65Al5.42Si1.96O13 Fe0.84Al5.2Si1.98O13
8.5 11.7 14.9
Fig. 1. Transmission electron micrographs of sample 8.5Fe as prepared (A) and heated at 900 (B), 1200 (C), and 1400°C (D).
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Fig. 2. Infrared spectra of sample 8.5Fe as prepared and after heating at different temperatures.
bands at 1055 and 615 cm⫺1, attributed to Te–O (Te ⫽ Si or Al in tetrahedral coordination) and Oc–O (Oc ⫽ Al or Fe in octahedral coordination) vibrations, respectively [17]. An intense absorption at 1385 cm⫺1 was also observed (Fig. 2) due to nitrate anions [18], indicating that they were not fully decomposed in the pyrolysis furnace. 3.2. Thermal analyses Fig. 3 shows the DTA curves obtained for the Fe-doped silica–alumina samples along with that corresponding to an undoped sample with mullite composition (blank) prepared using a similar procedure, which is included for the sake of comparison. As observed, all curves display an exothermic effect between 200 and 250°C due to the decomposition of the nitrate anions remaining in the samples, which is substantiated by the disappearance of the band at 1385 cm⫺1 in the IR spectrum of the samples heated at 400°C (Fig. 2). At higher temperatures, only an exothermic peak at about 900°C was detected, which, as is later confirmed, is associated with the crystallization of pseudo-tetragonal mullite. It is interesting to note that this peak was progressively shifted to lower temperature with increasing Fe content in the doped samples, indicating a catalytic effect of Fe in mullite formation. This finding is contrary to previously reported observations [19] carried out on samples prepared by the
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Fig. 3. Differential thermal analyses obtained for the Fe-doped mullite samples and for an undoped mullite powder (blank).
sol-gel process, according to which the presence of iron retarded the formation of mullite up to ⬃1200°C. This behavior was explained [19] by the presence of heterogeneities in the silica–alumina matrix promoted by the iron(III) cations, which induced the previous crystallization (⬃900°C) of an Fe/Al spinel. It should be noted that the particles still retained the spherical shape after heating at 900°C, whereas the beginning of sintering was observed at ⱖ1200°C (Fig. 1). 3.3. Phases evolution with temperature X-ray diffraction (Fig. 4) showed that sample 8.5Fe remained amorphous after heating up to 800°C. The crystallization of mullite was observed at 900°C. As expected, the pattern obtained at this temperature did not show the splitting of the peak at 2 ⬃26°, corresponding to the 120 and 210 reflections [13]. This suggests the pseudo-tetragonal character of this phase. Such a splitting was detected after calcination at higher temperature (ⱖ1200°C), indicating the development of orthorhombic mullite. It should be noted that for this sample, mullite was the only crystalline phase detected by X-ray diffraction during the calcination process from 900 to 1400°C. The crystallization of pseudo-tetragonal mullite and its evolution to the orthorhombic phase was also studied by infrared spectroscopy (Fig. 2). The spectrum obtained for the sample heated at 900°C was similar to that previously reported for tetragonal mullite [20].
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Fig. 4. X-ray diffraction patterns of sample 8.5Fe heated at different temperatures.
The only appreciable difference between the latter and that of the orthorhombic mullite developed at 1400°C was the relative intensity of the doublet at 1165–1130 cm⫺1 [20]. The crystallization behavior of sample 11.7Fe (Fig. 5) was similar to that of sample 8.5Fe. The main difference between the X-ray diffraction patterns of both samples was the appearance at 1100°C of two reflections at d ⫽ 2.94 and 2.50 Å, in addition to those of mullite. The position of these peaks was slightly shifted to lower d-spacing values with respect to the most intense reflections of ␥-Fe2O3 [21] or Fe3O4 [22]. Therefore, they are attributed to an iron oxide spinel, probably containing some Al3⫹ cations in solid solution. The magnetic character observed for the sample is in agreement with such an assignment. It should be noted that the phase diagram reported for the SiO2–Al2O3–Fe2O3 system [23] predicts a similar spinel phase to that suggested here, although at higher temperatures (ⱖ1380°C). The relative intensity of the spinel peaks decreased with increasing temperature, finally disappearing after heating at 1400°C (Fig. 5). The increase of the iron content in Fe-doped mullite up to 14.9 wt% (as Fe2O3) (sample 14.9Fe) gave rise to an increase of the relative intensities of the peaks corresponding to the spinel phase (Fig. 6), with respect to those of sample 11.7Fe (Fig. 5).
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Fig. 5. X-ray diffraction patterns of sample 11.7Fe heated at different temperatures. The peaks corresponding to the spinel phase are labeled with an asterisk.
These peaks were still observed for sample 14.9Fe after heating at 1400°C, although with lower relative intensity than at 1200°C. The IR spectra of samples 11.7Fe and 14.9Fe heated at different temperatures did not show noticeable differences with respect to those of sample 8.5Fe (Fig. 2). 3.4. Unit-cell parameters The unit-cell parameters measured for the Fe-doped mullite samples heated at different temperatures are shown in Table 2, in which those corresponding to the blank and the ASTM file [13] have also been included. As expected, the a value obtained for undoped mullite heated at 900°C was much higher (7.615 Å) than that reported in the ASTM file for orthorhombic mullite (7.5456 Å), being closer to the value of b (7.680 Å), which shows the pseudo-tetragonal character of this phase [7]. This behavior has been reported [7] to be due to a deficiency in the silica content of the mullite phase initially crystallized from amorphous precursors having a high chemical homogeneity. When the calcination temperature is further
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Fig. 6. X-ray diffraction patterns of sample 14.8Fe heated at different temperatures.
Table 2 Unit-cell parameters of the Fe-doped mullite samples heated at different temperatures and their relative increment with respect to the blank heated at the same temperature Sample ASTM Blank 8.5Fe “ “ 11.7Fe “ 14.9Fe
Temperature (°C) 900 1200 1400 900 1200 1400 1200 1400 1400
a (Å)
b (Å)
c (Å)
7.5456 7.609(2) 7.553(2) 7.545(1) 7.615(2) 7.576(1) 7.578(1) 7.580(2) 7.581(1) 7.581(1)
7.6898 7.680(2) 7.686(2) 7.697(2) 7.720(2) 7.722(2) 7.723(1) 7.724(2) 7.727(1) 7.732(1)
2.8842 2.8847(9) 2.8850(8) 2.8851(5) 2.8957(7) 2.8984(5) 2.8993(3) 2.8989(6) 2.9012(3) 2.9030(4)
⌬a/a (%)
⌬b/b (%)
⌬c/c (%)
0.437
0.338
0.492
0.477 0.477
0.390 0.455
0.554 0.620
The cell parameters given in the ASTM file of mullite are also included.
b/a 1.0191 1.0093 1.0176 1.0201 1.0137 1.0192 1.0192 1.0190 1.0190 1.0199
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increased, the incorporation of silica to the mullite lattice progresses, finally yielding the stoichiometry of mullite. This process is accompanied by a decrease of a to the value that progressively approaches that corresponding to orthorhombic mullite, which took place for our blank after heating at 1400°C (Table 2). As observed in Table 2, the values of a, b, and c obtained after heating sample 8.5Fe at different temperatures are higher than those of the blank heated at the same temperatures. This finding reveals that the incorporation of iron cations into the mullite structure is parallel to the formation of the mullite framework at 900°C, and the existence of a solid solution during the whole studied temperature range. It was also noted that, as in the blank, the ratio b/a for this sample heated at 900°C was lower (1.0137) than that corresponding to orthorhombic mullite (1.0201) and that the a value decreased from 900 to 1200°C, which as stated above can be attributed to the incorporation of silica into the mullite lattice. Although the b parameter did not significantly change during this treatment, the slight increase observed for the c parameter seems to indicate a progress in the incorporation of iron into mullite. Therefore, the amount of iron in solid solution in the sample heated at 900°C must be lower than 8.5 wt% (as Fe2O3). It should be noted that the unit-cell parameters did not show important variations when sample 8.5Fe was heated at higher temperature (1400°C). This finding, along with the absence of crystalline phases other than mullite (Fig. 4), suggests that the whole iron content of this sample was incorporated into the mullite lattice at 1200°C. As shown in Table 2, the differences between the cell parameters obtained for sample 11.7Fe (11.7 wt% Fe2O3) heated at 1200°C and those of sample 8.5Fe (8.5 wt% Fe2O3) heated at the same temperature are within experimental errors. Therefore, both samples contained a similar amount of iron in solid solution (8.5 wt% Fe2O3). In sample 11.7Fe, this involves the presence of an additional Fe phase, consisting of a Fe–Al spinel, as revealed by X-ray diffraction (Fig. 5). This result suggests that the limit of iron incorporation into mullite at 1200°C is ⬃8.5 wt% Fe2O3. The heating of the sample at 1400°C gave rise to an increase of the c parameter indicating a progress in the incorporation of iron into the Fe-mullite solid solution. This would be in agreement with the disappearance of the X-ray diffraction peaks corresponding to the spinel phase (Fig. 5). The latter behavior suggests that most of the iron cations of the sample were incorporated into the mullite lattice at this temperature. It should be noted that the iron content of this sample (11.7 wt% Fe2O3) is similar to the maximum amount of iron incorporated to mullites prepared by the conventional ceramic procedure (⬃11 wt% Fe2O3) [5]. The increase of the raw iron content in the doped samples from 11.7 (sample 11.7Fe) to 14.9 wt% Fe2O3 (sample 14.9Fe) also gave rise to an increase of the b and c parameters of mullite after calcination at 1400°C (Table 2). This suggests an increase of the iron amount in the solid solution. Such an increase must be very small, since the spinel phase was still observed for sample 14.9Fe heated at this temperature, in addition to mullite (Fig. 6). Finally, it should be noted that for all studied compositions, the incorporation of iron into mullite gave rise to a larger relative expansion for c than for a or b (Table 2), as it has been previously reported for samples prepared by the ceramic procedure [2].
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Fig. 7. F.T. functions obtained from the Fe K-edge oscillations for sample 8.5Fe heated at different temperatures.
3.5. Localization of Fe3⫹ into the mullite lattice To elucidate the position that Fe3⫹ cations occupy in the mullite lattice, we selected sample 8.5Fe, which according to X-ray diffraction (Fig. 4), did not show additional iron oxide phases. Fig. 7 displays the pseudo-radial distribution functions (F.T. curves) obtained for this sample heated at different temperatures. The functions corresponding to the iron compounds used as references (␣-Fe2O3, ␥-Fe2O3, and Fe-acac) are presented in Fig. 8. As observed, after heating at 800°C, the curve of the Fe-doped sample showed an intense peak at ⬃1.53 Å (uncorrected for the phase shift) corresponding to the first coordination shell around the Fe3⫹ cations. The peak associated with the second shell (2– 4 Å region) however, was very weak, which is characteristic of molecular (as is the case of Fe-acac) or amorphous compounds. This is in agreement with the X-ray diffraction observations (Fig. 4). The presence of a more intense peak at 2.88 Å in the F.T. curve of the sample heated at 900°C indicates the crystallization of an iron compound, which, according to X-ray diffraction (Fig. 4) and the unit-cell parameters of the sample (Table 2), can be ascribed to the Fe-mullite solid solution. The slight increase in intensity of this peak with heat treatment at 1200°C (Fig. 7) could be explained by the further incorporation of Fe into mullite, as revealed by the increase of the unit-cell parameters detected after this treatment (Table 2).
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Fig. 8. F.T. functions obtained from the Fe K-edge oscillations for the reference iron compounds.
A quantitative analysis of the EXAFS data was conducted on sample 8.5Fe heated at 1200°C, since at this temperature the Fe-mullite solid solution was fully developed (Table 2). Because of the rather complex atomic environments around the cationic sites in the mullite structure, especially for the 2– 4 Å region of the F.T. curve in which contributions from several kinds of O, Al, and Si atoms are included [24], the fitting procedure of the EXAFS data was only accomplished for the first coordination shell (0.82–2.15 Å). The best fitting attained (Fig. 9) resulted in a Fe–O distance of 1.95 Å, which is slightly higher than that for the octahedral Al3⫹ cations in mullite (1.89 –1.94 Å) [24]. This finding, along with the coordination number obtained from the fitting (5.6), clearly indicates that the Fe3⫹ ions are located at the octahedral positions of Al in the mullite lattice, which induces a cell expansion. This is in agreement with the measured unit-cell parameters (Table 2). Finally, it should be noted that the pseudo-radial distribution functions of the 11.7Fe and 14.9Fe samples after heating at 1400°C were similar to those of sample 8.5Fe heated at 1200°C, showing small differences in the intensity of the peak corresponding to the second shell (2– 4 Å region). This behavior can be attributed to the different Fe amounts in solid solution with mullite, as revealed by the unit-cell parameters shown in Table 2, as well as the presence of the additional spinel phase (Fig. 6).
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Fig. 9. Inverse Fourier transform (dotted line) and curve obtained by fitting analysis (solid line) of the Fe K-edge EXAFS spectrum registered for sample 8.5Fe heated at 1200°C.
Acknowledgments This work was financially supported by the Spanish DGCYT under Project PB95-0225.
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