J. Phys. Chem. B 2001, 105, 379-391
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A Multi-nuclear Multiple-Field Nuclear Magnetic Resonance Study of the Y2O3-Al2O3 Phase Diagram Pierre Florian,* Monique Gervais, Andre´ Douy, Dominique Massiot, and Jean-Pierre Coutures CNRS-CRMHT, 1D AVenue de la Recherche Scientifique, 45071 Orle´ ans Cedex 2, France ReceiVed: March 7, 2000; In Final Form: September 20, 2000
We have been investigating several compounds belonging to the Y2O3-Al2O3 phase diagram by means of aluminum-27, oxygen-17, and yttrium-89 high-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy. All aluminum, oxygen and yttrium sites of the five crystalline compounds C-Y2O3, Y4Al2O9, YAlO3, Y3Al5O12, and R-Al2O3) have been resolved and the NMR parameters deduced. AlVI sites exhibit isotropic chemical shift δiso between 0 and 10 ppm and small quadrupolar coupling constant CQ, whereas AlIV are located between 70 and 80 ppm and show large CQ. 17O resonances are characterized by small CQ and δiso ranging from 72 ppm (Al2O3) to 358 ppm (Y2O3) and showing a strong sensitivity to the nature of the second coordination sphere of oxygen as well as its coordination number. 89Y δiso are found between 184 and 314 ppm and are not strongly correlated to the coordination state of yttrium, the chemical shift anisotropy being found moderate (∼100 ppm). On the basis of the results obtained with the crystalline phases, we have characterized the oxygen environment in a glassy sample of composition Y3Al5O12 and described the variation of the local structure during the crystallization process of YAlO3 from the sol-gel raw product. In the vitreous state, oxygen’s environments can be described in terms of OYk Al4-k sites, with respective populations not distributed in a purely random fashion, and two types of 5-fold-like aluminum environments can be evidenced. A quantitative description of the heat treatments of the YAlO3 precursor is given by 17O NMR experiments in terms of Y4Al2O9, Y3Al5O12, and amorphous phases formed. New oxygen environments are also evidenced and may be attributed to the hexagonal metastable form of YAlO3.
1. Introduction Within the Y2O3-Al2O3 pseudo-binary system and in addition to the two end-members alumina and yttria, three crystallographically defined compounds are known to exist: Y3Al5O12, YAlO8, and Y4Al2O9. Several of these compounds have found practical applications:1 alumina is widely used as a refractory material, yttria and Y3Al5O12 can be components of phosphores used in cathode ray tubes, and Al2O3, YAlO3, and particularly Y3AlO12 (“YAG”) are possible host matrix for solid-state lasers. The latter application is technologically challenging because it requires the growth of single crystals which is well known to be sluggish. The crystallization behavior of Y3Al5O12 strongly depends on experimental conditions:2-4 stable or metastable crystalline Y3Al5O12 form, vitreous Y3Al5O12 or two-phase mixture YAlO3/R-Al2O3 can be obtained from the hightemperature liquid phase. Moreover, the vitreous state of this composition is expected to show some peculiar phase-separationlike behavior.5 A nuclear magnetic resonance NMR investigation could potentially lead to some insights into those problems, providing that a careful study of known crystalline phases has been performed. Aluminosilicate glasses containing rare-earth or rare-earth analogue cations have major technological importance as optical component as well as unusual properties.6,7 It has also been suggested that their structures are significantly different from more “classical” alkali and alkaline-earth aluminosilicate glasses. This suggestion has triggered high-resolution magic-angle spinning (MAS) NMR studies using 29Si and 27Al8 and more * Corresponding author (E-mail:
[email protected]; Fax: +33 (0) 238 638 103).
recently 17O9 in order to investigate, at the local atomic scale, the environments of those nuclei. Nevertheless, if alkali or alkaline-earth aluminosilicates have now been investigated by means of 27Al10 and 17O11,12 NMR, only few compounds of the Y2O3-Al2O3 system have been characterized by high-resolution NMR. It seems therefore important to perform such a detailed study, using 27Al and 17O, but also 89Y whenever possible, to lay the basis of forthcoming NMR experiments on yttrium aluminosilicates. In this paper, we explore the five crystalline compounds belonging to the Y2O3-Al2O3 phase diagram by means of 27Al, 17O, and 89Y MAS NMR. From those results, we empirically correlate some NMR parameters to the local structure around the nucleus under investigation. This result then is applied to the analysis by 17O MAS NMR of the crystallization behavior of YAlO3 as well as vitreous Y3Al5O12 investigated by means of 17O and 27Al MAS and multiple quantum magic-angle spinning (MQMAS) NMR. 2. Experimental Section 2.1. Synthesis. All experiments were run on 17O-enriched samples that were synthesized as follows. The precursor powders were prepared following the procedure described by Kim et al.13 Starting materials were yttrium isopropoxide, aluminum isopropoxide (Aldrich) and 46% 17O-enriched water (Commissariat a` l’Energie Atomique, Saclay, France). All preparations and thermal treatments were carried out under an argon atmosphere to prevent exchange of 17O with 16O. The two alkoxides were dissolved in 2-propanol (refluxed on sodium and distilled) in ratios dictated by the stoichiometry of the final compound, and
10.1021/jp0008851 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/22/2000
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the solution was heated at 60 °C in a water bath. The 17O-enriched water was then dropped into this vigorously stirred solution, the amount of added water being calculated according to:
{
Al(OC3H7)3 + 2 H2O f AlOOH + 3C3H7OH Y(OC3H7)3 + 3H2O f Y(OH)3 + 3C3H7OH
}
(1)
and an excess of 5% was used. After a precipitate formed, the mixture was refluxed for 1 h and the solvent was then distilled. The remaining powder was dried under vacuum. The initial powders obtained were colored, indicating that some organic residues remained in the sample. This coloration disappeared after annealing at 2 °C/min up 1250 °C for 2 h. The crystallization behavior, as followed by differential thermal analysis (DTA) on nonenriched samples, closely matching that described by Yamaguchi et al.14 showed that all starting powders crystallized before 1200 °C. All samples were analyzed by X-ray diffraction to check for proper compositions and crystallographic phases, and ∼500 mg of 17O-enriched samples of each compositions were made according to this procedure. The glassy Y3Al5O12 samples were made by compacting the obtained crystalline powders in an isostatic press under 5 kBar to obtain solid chunks of materials that were placed in the nozzle of an aerodynamic levitator15 using Argon as the levitation gas. One CO2 laser was then used to melt the solid piece that formed a droplet (due to its surface tension), levitating on the gas flow (i.e., without contact with the nozzle). This liquid droplet was heated at 2400 °C and the laser is shut down, allowing the sample to cool at a rate of approximately 300 °C/s and always under contactless conditions. This procedure allowed us to obtain three glassy (transparent) samples of ∼40 mg each. Because only a very small amount of 17O-enriched sample was obtained this way, no further analyses were carried out. 2.2. NMR. Most of the experiments were carried out on a Bruker MSL 300 spectrometer operating at a principal field of 7.0 T, with Larmor frequencies for 89Y, 17O, and 27Al of 14.7, 40.7, and 78.2 MHz, respectively. Additional experiments were also performed on a Bruker MSL 360 (principal field of 8.4 T), DSX 400 (principal field of 9.4 T), and MSL 500 (principal field of 11.7 T) spectrometers. Double-bearing, 4-mm, highspeed Bruker MAS probeheads were used for 17O and 27Al, whereas a 7-mm, low-frequency tunable probehead was used for 89Y. MAS experiments on 17O and 27Al were acquired with spinning speeds of 15 kHz under single-pulse excitation, with typically 0.6-1 µs (i.e., less than the liquid π/12) pulse width to ensure quantitative irradiation,16 recycle times of 1 to 10 s, and spectral width up to 2.5 MHz, with up to 16 K points in the time domain to avoid folding of the spinning sidebands back into the spectrum. Chemical shifts are reported relative to tap water for 17O, and a 1 M solution of Al(NO3)3 in nitric solution for 27Al. Static spectra were acquired under single pulse or Hahn echo under selective irradiation conditions when baseline correction could not be satisfactorily reached. MAS experiments on 89Y were acquired under single-pulse excitation, with a 16 µs (i.e. π/2) pulse width, recycle times of 440 s, spectral width of 25 kHz, and typically 1000 scans accumulated. The recycle delay was chosen considering a spinlattice relaxation time T1 of 350 s, as measured by a saturationrecovery sequence on Y3Al5O12. Chemical shifts are reported relative to a 1 M solution of YCl3. Typical spinning speeds were 1 and 4 kHz. Triple quantum 17O and 27Al MQMAS experiments were performed at 9.4 T, spinning at 15 kHz, using a shifted echo
sequence17 recording the full echo signal. Radio frequency field strength was 100 kHz, leading to excitation pulses of 4.2 µs and 5.0 µs and mixing pulses of 1.5 µs and 2.0 µs for 17O and 27Al, respectively, selective π with pulses of 15 µs used for both nuclei. Three hundred eighty four and 576 transients were acquired with 32 and 24 t1 increments of 33.3 µs and recycle delays of 20 and 1.6 s for 17O and 27Al, respectively. For the quadrupolar nuclei (both I ) 5/2 spins) 17O and 27Al, we focused on the measurement of three NMR parameters: the isotropic chemical shift δiso, the quadrupolar coupling frequency νQ (or the quadrupolar coupling constant CQ ) 2I (2I - 1)νQ/ 3) and the quadrupolar asymmetry parameter ηQ. Those parameters were extracted from a careful simulation of the spectra, using a modified version of the Bruker Winfit program to handle the finite spinning speed in MAS experiments.18 Those last two parameters can nevertheless be retrieved only when the electric field gradient at the nucleus is strong enough to produce second-order line shapes on the central transition. When this is not the case, the reduced quadrupolar coupling frequency νQ,η ) νQx1+η2Q/3 is a reliable parameter obtained by analyzing the position of the spinning sidebands arising from the various transitions19,20 and the help of a SORGE diagram.21 In the case of 89Y (I ) 1/2) and δiso, one can also attempt to evaluate the two parameters describing the anisotropy of the chemical shift tensor (CSA): the axiality ∆csa ) δiso - δzz and the asymmetry parameter ηcsa ) (δyy - δxx)/∆csa, with δxx, δyy, and δzz being the principal component of the CSA tensor ordered such as |δzz - δiso| G |δyy - δiso| g |δxx - δiso|. Those parameters can be extracted from a usual Herzfeld and Berger type analysis22 of the spinning sidebands intensities of slowspinning experiments. 2.3. Crystallography. There are five definite crystalline compounds in the pseudo-binary Y2O3-Al2O3 system: Y2O3, Y4Al2O9, YAlO3, Y3Al5O12, and Al2O3. R-Al2O3 has a wellknown trigonal structure, with one aluminum and one oxygen crystallographic site of respective environments AlO6 and OAl4. The Yttrium Aluminum Garnet (“YAG”) Y3Al5O12 has a rather complex cubic structure,24 with 160 atoms per unit cell (Z ) 8). There are two aluminum environments: Al(1)O6 and Al(2)O4, one OY2Al2 oxygen, and a unique yttrium site of the YO8 type. The Yttrium Aluminum Perovskite (“YAP”) YAlO3 is orthorhombic25 with one AlO6, one YO8-type site, and two different oxygen environments (O(1)Y2Al2 and O(2)Y3Al2). The structure of Yttrium Aluminum Monoclinic (“YAM”) Y4Al2O9 has been more recently refined26 and has been shown to contain two different AlO4 environments, four yttriums (two YO6 and two YO7), and nine distinct oxygens sites that can be described as four OY3Al, two OY2Al, two OY4 and one OY2Al2. Yttria Cs Y2O3 is also cubic,27 with a unique OY4 type site and two YO6 environments. The aluminum sites found in those crystalline compounds correspond to the usual AlIV and AlVI coordinations, whereas yttriums are 6-, 7-, or 8-fold coordinated. Oxygens, on the other hand, span a wider range of environments, mainly 4-fold coordinated in the form OYxAl4-x (x ) 0, 2, 3, or 4) and also a 3-fold coordinated OY2Al and a 5-fold-coordinated OY3Al2. Typical AlIV-O and AlVI-O distances are found in the range 1.65 to 1.90 Å and 1.85 to 1.97 Å, respectively. Yttriumoxygen distances are found between 2.17 and 2.44 Å, with a slight decrease of the mean bond length with decreasing coordination number. It has to be noted that the aforementioned coordination numbers were obtained by assuming a yttriums oxygen cutoff distance of 2.7 Å, which is in agreement with
NMR Study of Y2O3-Al2O3
J. Phys. Chem. B, Vol. 105, No. 2, 2001 381
Figure 1. 27Al MAS NMR experiments at 7.0 T. Left: experimental spectra (continuous line) along with simulations (dashed line); right: details of the simulations for (a) Y4Al2O9, (b) YAlO3, (c) Y3Al5O12, and (d) Al2O.
the crystal radii of OIV (1.24 Å) and YVI, VII, VIII (1.032, 1.155, and 1.24 Å, respectively).28 Nevertheless, for the yttrium site in YAlO3 and the Y(3,4) sites in Y4Al2O9, this cutoff distance leaves some oxygen atoms located at distances closer than the first cation (typically at 2.9 to 3.2 Å) from the central yttrium atom. Those oxygens would change the coordination number obtained for those sites if they were to be taken into account,
but we do not think that they are relevant from the NMR point of view. 3. Results and Discussion 3.1. Aluminum-27. Spectra obtained by 27Al MAS NMR at 7.0 T of all the crystalline compounds are shown in Figure 1. Spectra are well resolved for the compounds Al2O3, YAlO3,
382 J. Phys. Chem. B, Vol. 105, No. 2, 2001 TABLE 1:
27Al, 17O,
Florian et al.
and 89Y NMR Parameters for All Definite Compounds of the Y2O3-Al2O3 Pseudo-Binary System
compound
site
% crystal.
δiso (ppm) ( 1
νQ (kHz) ( 10
ηQ ( 0.05
νQ,η (kHz) ( 10
% calc. ( 1
Y4Al2O9
Al(1)O4 Al(2)O4 AlO6 Al(1)O6 Al(2)O4 AlO6
50 50 100 40 60 100
78.2 76.2 10.7 2.1 77.5 10.7
1622 ((30) 1554 ((30) 923 -
0.48 0.77 0.05 -
(1747) (1861) 241 169 (924) 237
42 58 100 42.5 57.5 100
ηQ ( 0.05
YAlO3 Y3Al5O12 R-Al2O3 compound
site
% crystal.
δiso (ppm) ( 1
νQ,η (kHz) ( 10
% calc. ( 1
C-Y2O3 Y4Al2O9
OY4 O(5)Y2Al2 O(1)Y3Al O(2)Y3Al O(4)Y3Al O(6)Y3Al O(3)Y2Al O(7)Y2Al O(8)Y4 O(9)Y4 O(2)Y3Al2 O(1)Y2Al2 OY2Al2 OAl4
100 11.1 11.1 11.1 11.1 11.1 11.1 11.1 11.1 11.1 66.7 33.3 100 100
358 126
-
-
135 110
100 10.3
175
-
-
136
25.2
180 194 198 345 372 143 165 142 72
236 247 223 319
112 124 133 101 118 (272) (252) (257) (332)
20.2 10.9 10.9 11.5 10.9 66.5 33.5 100 100
YAlO3 Y3Al5O12 R-Al2O3 compound C-Y2O3 Y4Al2O9
YAlO3 Y3Al5O12
νQ (kHz) ( 5
1.00 0.35 0.99 0.50
site
% crystal.
δiso (ppm) (1
fwhm (Hz) ( 2
∆csa (ppm) ( 10
ηcsa ( 0.2
% calc. ( 3
Y(1)O6 Y(1)O6 Y(1)O7 Y(2)O6 Y(3)O7 Y(4)O6 YO8 YO8
25 75 25 25 25 25 100 100
273 314 184 216 195 231 215 222
54 47 20 19 16 21 17 15
e50 83 e50 117 -131 -90 -108 96
0.3 0.45 0.6 0.8 0.45 0.65
23 76 25 22 29 25 100 100
and Y3Al5O12, for which a narrow line located around 0 ppm is displayed and can be very well simulated by a Gaussian line. The full set of external 〈(3/2T(3/2〉 transitions spinning sidebands are clearly visible and can serve as a means to extract δiso and νQ,η, as already described above. From the isotropic position as well as the crystallographic data, this line can be attributed to the AlVI component present in those three compositions. Their respective NMR parameters are summarized in Table 1. Values found for Al2O3 and Y3Al5O12 are in good agreement with previously published values16,19,29 as well as for YAlO3,30 and overall spectral features are consistent with previously published spectra.31 The spectra obtained for Y3Al5O12 display a second line with several discontinuities and for which NMR parameters are extracted by a simulation of this second-order broadened line shape. The results are given in Table 1 and allows assignment of this line to the AlIV environment found in this compound. On the other hand, the 27Al MAS spectrum of Y4Al2O9 is totally unresolved at 7.0 T because of the presence of strongly overlapping spinning sidebands of the central 〈1/2T-1/2〉 transition, and a detailed study is needed. Figure 2 gives the spectra of this compound obtained at 7.0, 8.4, and 11.7 T, with the sample spinning at 15 kHz. This figure clearly demonstrates that broadening of the central transition is due to second-order quadrupolar interaction because it decreases on increasing the principal field. From the simulation of the 11.7 T spectra, assuming pure second-order quadrupolar line shapes under finite spinning speed conditions, one extracts the isotropic chemical shifts and quadrupolar parameters for the two sites given in Table 1. This set of parameters also gives also excellent simulations for the 8.4 and 7.0 T spectra, as shown in Figure 2. Because of the large νQ values obtained for this compound, MQMAS was not successful at 7.0 and 8.4 T, and we did not have enough spectrometer time to perform this
experiment at high field (11.7 T) for which it could have been feasible. It is nevertheless interesting to note that those parameters do not render properly the static line shape obtained at 7.0 T. This result is undoubtedly related to the presence of chemical shift anisotropy observed under static conditions and known to affect the shape of the central transition line.32 A fit for ∆csa and ηcsa, while keeping δiso, νQ, and ηQ constant, gives an excellent simulation, shown in Figure 2a. We obtained values of ∆csa and ηcsa of 38 ( 10 ppm and 0.1 ( 0.2 for Al(1) and 33 ( 10 ppm, and 0.8 ( 0.2 for Al(2), respectively. Those values of ∆csa are significantly higher than the one found by more reliable singlecrystal studies of Al2O333 (-17.3 ( 0.6 ppm) and Y3Al5O1234 (0 ( 3 ppm for both sites), this result probably can be attributed to the higher degree of distortion of the AlIV environments found in Y4Al2O9. We did not attempt to extract the angles describing the relative orientation of the two tensors (quadrupolar/CSA) because the uncertainty related to those parameters was too high to give meaningful values. The isotropic chemical shift is found to be, as usual for aluminum, mainly sensitive to the coordination number of this cation: AlVI gives δiso ≈ 0-20 ppm and AlIV gives δiso ≈ 7080 ppm. Because the coordination number is itself related to the Al-O distances dAlO, one also find δiso to be related to those distances, or more precisely to 1/dAlO. It is extremely difficult to relate the quadrupolar parameters to relevant structural information. The distortion of the aluminum environments quantified by the empirical shear strain parameter35,36 does not, for instance, adequately describe the electric field gradient (EFG) at the aluminum sites for the compositions under study. This result is probably because the shear strain parameter takes into account only the first aluminum coordination sphere and therefore neglects the part coming from the network.37
NMR Study of Y2O3-Al2O3
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Figure 2. 27Al (a) static and (b) MAS NMR experiments of Y4Al2O9 obtained at 7.0 T, and MAS experiments at (c) 8.4 T and (d) 11.7 T. Experimental (solid line) and simulated (dashed line) spectra are given on the left, and details of simulations are on the right.
3.2. Oxygen-17. Spectra obtained by 17O MAS NMR at 7.0 T of all the crystalline compounds are shown in Figure 3. Only one line is observed for, Al2O3, CsY2O5, and Y3Al5O12, and two lines are observed for YAlO3, which is in agreement with the cyrstallographic data. The nine oxygen sites of the Y4Al2O9 structure are not fully resolved in our experiments, neither at 7.0 T nor at 8.4 T (data not shown): only eight components are clearly distinguished, among which one is a trace of C-Y2O3 (as evidenced by X-ray diffraction). Two lines in the spectra present areas that are twice the one of the other lines. Given
this result and knowing that all sites are equally populated, each of those two lines are therefore, the sum of two components that cannot be separated. The simulations were performed in a manner similar to the one described for 27Al results, and the resulting parameters are listed in Table 1. The NMR parameters for R-Al2O3 are in very good agreement with previously published values,38 as well as δiso found in C-Y2O3.39,40 The assignment of the NMR lines to the crystallographic sites is easily achieved for the YAlO3 composition based on the observed population of each site. The situation for Y4Al2O9 is
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Figure 3. 17O MAS NMR experiments at 7.0 T. Left: experimental spectra (continuous line) along with simulations (dashed line); right: details of the simulations for (a) C-Y2O3, (b) Y4Al2O9, (c) YAlO3, (d) Y3Al5O12, and (e) Al2O3.
NMR Study of Y2O3-Al2O3
J. Phys. Chem. B, Vol. 105, No. 2, 2001 385
Figure 4. 17O isotropic chemical shift plotted versus the sum of the inverse oxygen cation distances. Key (square) Al2O3; (diamond) Y3Al5O12; (circles) YAlO3; (triangles up) Y4Al2O9; (triangle down) C-Y2O3; (crosses) B-Y2O3. The lower (resp. upper) straight line is a linear fit through the points corresponding to oxygen surrounded by both yttrium and aluminum atoms (resp. yttrium atoms only). The dashed line is a power fit through the points representing four-fold coordinated oxygens.
less obvious because all sites are equally populated. We can nevertheless propose an attribution based on the following. The unique OY2Al2 site is assigned to the isolated line at 127 ppm, and the group around 350 ppm is undoubtedly assigned to the OY4 sites according to their high chemical shift values (see results on Y2O3) with the middle one being the impurity. Considering the integrated intensities of the remaining lines, the large peak around 180 ppm showing a discontinuity is attributed to the four OY3Al sites and the line around 196 ppm is assigned to the two OY2Al sites. This latter attribution also assumes that same types of oxygen environments will produce nearly identical δiso. Figure 4 shows a plot of δiso versus the sum of the inverse NAl NY (1/dOiAl) + ∑j)1 (1/dOjY), oxygen-cation distances 1/d ) ∑i)1 with the data obtained on B-Y2O3 also included.40 When oxygen is surrounded by yttrium only, δiso decreases nearly linearly as the coordination number increases. For a given value of 1/d, δiso increases with the increase of the yttrium fraction in the oxygen environment. Taking into account the well-defined 4-fold coordinated oxygens in the compounds Y2O3, Al2O3, Y3Al5O12, and Y4Al2O9, there is a decrease of δiso with the increase of the number of aluminum atoms in the environment, as shown by the dashed line in Figure 4. It is seen that the δiso values determined for OIV in Y4Al2O9 are also distributed along this line and that they decrease with the number of yttrium present in the oxygen environment. These results support the assignments proposed for the oxygen sites in Y4Al2O9. Considering only the 4-fold coordinated oxygen for which we have the larger range of environments, one can find different ranges of δiso: the OAl4 site is found at 72 ppm, OY2Al2 at 126, 142, and 165 ppm, OY3 Al at 175 and 180 ppm, and OY4
at 345, 372, and 358 ppm (there is a strong nonlinear effect in substituting one type of cation for another). One also observes a shift toward lower δiso values on increasing the coordination number of oxygen, with an overlap of the ranges due to changes in the nature of the first neighbors, the OIV environments alone covering a range of 300 ppm. For the same reasons as for aluminum, it is extremely difficult to relate the quadrupolar parameters to structural variables. The strong influence of the network contribution (as opposed to local configuration) has already been observed for 17O quadrupolar parameters of aluminum oxides and hydroxides.38 Using a point monopole model, the summation has to be taken up to a 50 Å distance from the central oxygen to properly reproduce the observed values for R-Al2O3. It is nevertheless qualitatively observed that the quadrupolar coupling constants decreases on increasing the average ionicity of the oxygenscation bond.41 3.3. Yttrium-89. Spectra obtained by 89Y MAS NMR at 7.0 T and a spinning speed of 1.5 kHz of all the studied crystalline compounds are shown in Figure 5. The isotropic chemical shift is easily retrieved from higher spinning speed experiments (i.e., 4 kHz) in which no spinning sidebands are observed. The rather low signal-to-noise ratio of the spectra as well as the low intensities of the spinning sidebands, as seen in Figure 5, do not allow a very precise determination of the CSA parameters. When no spinning sidebands are present, only an upper limit of 50 ppm for ∆csa can be given, and ηcsa cannot be obtained. The results of all simulations are given in Table 1. The assignment of the two NMR lines for the compound C-Y2O3 is straightforwardly obtained by considering the difference in population. This case is again not that of Y4Al2O9 for which all four sites are equally populated. Assuming that
386 J. Phys. Chem. B, Vol. 105, No. 2, 2001
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Figure 5. 89Y slow MAS NMR experiments at 7.0 T. Left: experimental spectra (continuous line) along with simulations (dashed line); right: details of the simulations for (a) C-Y2O3, (b) Y4Al2O9, (c) YAlO3, and (d) Y3Al5O12.
an increase in coordination number will result in a decrease of δiso, the two lines located around 220 ppm can be ascribed to the YO6 sites and the two located around 190 ppm to the remaining two YO7 sites. Further assignment is, at this stage, impossible. All values of δiso obtained in this study are in excellent agreement with previous results,42 the spectrum showing an improved line width. The recently reported values of ∆csa and
ηcsa for the Y(1) and Y(2) site of C-Y2O3 (respectively, -68 ppm, 0.0 and 72 ppm, 0.24)43 are also consistent with our results. As opposed to the previous nuclei, the 89Y isotropic chemical shift does not seem to be strongly correlated to the sum of the CN 1/dYOi (graph not inverse yttrium-oxygen distances ∑i)1 shown). There is an overall tendency to decrease δiso upon increasing the coordination number, CN, of yttrium, but there
NMR Study of Y2O3-Al2O3 is also a strong overlap between the various CN. For instance, CN ) 6 is found as low as 216 ppm in Y4Al2O9 (i.e., in the range found for CN ) 8 in YAlO3). This result is probably the consequence of the 74% ionic character of the yttrium-oxygen bond that leads to nondirectional bonding schemes and therefore a “loose” definition of CN and/or could be due to the influence of higher coordination spheres. 3.4. Crystallization of YAlO3. The formation of YAlO3 monophasic from amorphous precursors occurs after heating to ∼1200 °C through the crystallization of several intermediate phases. X-ray diffraction experiments show the existence of both the compounds Y4Al2O9 and Y3Al5O12 in powders heat treated at temperatures between 600 and 1100 °C. In addition, a hexagonal phase denoted “H” being possibly the metastable form H-YAlO3 as described by Bertaut et al.44 always appears as a minor component in the mixture of amorphous and intermediate phases, and the low intensities of the peaks prevented a complete determination of the structure. We have investigated a sample of composition YAlO3 heat treated at 670, 970, and 1000 °C. Spectra were compared with those obtained for YAlO3 crystallized from the high-temperature liquid state (2100 °C) with YAlO3 or Y3Al5O12 composition, the latter presenting a phase separation into YAlO3 and Al2O3. The very large quadrupolar broadening exhibited by the composition (vide supra) complicates greatly the 27Al MAS spectrum, and we will focus on the 17O MAS results shown in Figure 6. The stable form of YAlO3 is recovered from the hightemperature liquid state and the corresponding spectra (Figure 6d) is identical to the one obtained in the previous section (i.e., after a heat treatment at 1250 °C of the amorphous powder). The same phase is also obtained after the phase separation of a liquid with a Y3Al5O12 composition, as shown in Figure 6e. In this case, the experimental ratio YAlO3:Al2O3 is 76:24, which is in excellent agreement with the expected 3:1 ratio deduced from the composition. The simulations shown in Figures 6d and 6e use the experimental spectra of the individual phases obtained in the previous section. After a heat treatment at 670 °C (Figure 6a), the sample remains amorphous and the 17O MAS spectrum is composed of three very broad resonances covering from 50 to 400 ppm. All previously observed oxygen environments can therefore be present in this sample, with the addition of those related to the remaining hydroxyl groups still entrapped in the powder. For the compounds heat treated at 970 and 1000 °C, a large amount of amorphous phase is still present and can be simulated as the sum of, respectively, four or three large Gaussian lines representing 70% and 40% of the total intensity (see Figures 6b and 6c). Using the experimental spectra obtained previously, one can also show that Y4Al2O9 account, for 9% of the sample treated at 970 °C and that after heating at 1000 °C, Y4Al2O9 and Y3Al5O12 account, respectively, for 11% and 36% of the sample. Finally, some remaining intensity can be taken into account by three narrow Gaussian lines centered at 174, 109, and 93 ppm in a 1:1:1 ratio and representing 21% and 13% of the sample treated at 970 and 1000 °C, respectively. X-ray diffraction experiments show the existence of the aforementioned crystalline phases at each calcination step as well as the presence of the phase “H” after heating at the 970 °C and 1000 °C. According to the results of Bertaut et al.44 this compound exhibit two oxygen environments: O(1)Y2Al3 and O(2)Y3Al in a 1:2 ratio. The OY2Al3 type of sites has not been observed in any crystalline compound but according to the trends evidenced here, its isotropic chemical shift should be below the one of OY3Al2, probably around 120 ppm. The OY3Al type of environments have, on the other hand, been observed and found
J. Phys. Chem. B, Vol. 105, No. 2, 2001 387 to be around 175 ppm with a small νQ (see Table 1). The three unassigned lines evidenced in the spectra shown in Figures 6b and 6c would then correspond to this “H” phase, the peak at 173 ppm being assigned to O(2)Y3Al and the two peaks at 110 and 90 ppm being probably part of a single second-order quadrupolar broadened line shape related to O(1)Y2Al3. There is not enough resolution to perform a satisfactory simulation with such a quadrupolar line shape, but if we assume that the two peaks are the positions of the major discontinuities and that ηQ ≈ 0.2, then one can estimate δiso ≈ 119 ppm and νQ ≈ 310 kHz, which is in agreement with an environment of the type OY2Al3. The calculated O(1):O(2) ratio is, nevertheless, 2:1, and this disagreement might come from the difficulty of simulating the line shape resulting from the amorphous components. Moreover, our results are obtained on a mixture of phases containing only a small fraction of hexagonal compound. This metastable phase is probably not well crystallized and might contain some disorder leading to slightly different oxygen environments than the one expected. 3.5. Vitreous 3Y2O3‚5Al2O3. Among the pseudo-binary system only compositions very close to Y3Al5O12 can vitrify, and only if cooled from above the melting point (1950 °C) under contactless conditions.45 We have formed three glassy 17Olabeled samples of ∼40 mg each with a laser heated aerodynamic levitation system15 and performed NMR experiments at 9.4 T on the powder obtained after grinding and mixing those samples. The small quantity of sample and the broad lines associated with the glassy state prevented us from obtaining a 89Y MAS NMR spectrum even after 4 days of acquisition. For 17O and 27Al, we performed MAS one-pulse experiments and a triple quantum MAS, shifted-echo experiment, the resulting one and two-dimensional spectra are shown in Figure 7. The 27Al MAS spectrum shows the existence of overlapping line shapes with a sharp rising left edge and a slow dying right tail arising from a distribution of quadrupolar parameters.46,47 The 27Al 3Q-MAS spectrum displays more striking features: there are obviously five different lines that, according to their positions, can be assigned to one 4-fold (AlIV), two 5-fold (AlV(1), AlV(2)) and two 6-fold coordinated environments (AlVI (1), VI Al(2)). The AlVI(2) site is characterized by a very small EFG because this peak lies completely on the line of equation δ1 ) -17/ 31δ2, with identical positions in the isotropic and MAS dimension close to 0 ppm. This peak is then assigned to the AlVI site of some crystalline phase because it is known that small crystallites can be found in vitreous matrix if this compound is not quenched fast enough. The corresponding AlIV site could not be observed because of its quadrupolar broadening and the very small amount of crystalline phase (
