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Al2O3 solubility in the fast ion conductor 0.88ZrO2–(0.12−x)Sc2O3–xAl2O3 determined by 27Al NMR
Solid State Ionics 175 (2004) 415 – 417 www.elsevier.com/locate/ssi
Al2O3 solubility in the fast ion conductor 0.88ZrO2–(0.12 x)Sc2O3–xAl2O3 determined by 27Al NMR T.J. Bastowa,*, Tom Mathewsb, J.R. Sellarb a
CSIRO Manufacturing and Infrastructure Technology, Private Bag 33, South Clayton MDC, Clayton 3169, Australia b School of Physics and Materials Engineering, Monash University, Clayton, Victoria 3168, Australia Received 10 July 2003; received in revised form 6 November 2003; accepted 19 December 2003
Abstract 27
Al NMR has been used to investigate the solubility limit of alumina (a-Al2O3) in the fast ion conductor 0.88ZrO2–0.12Sc2O3. It is found that approximately 0.6 mol% alumina is retained in the ZrO2/Sc2O3 host lattice and that at higher doping levels the alumina exists in the specimen as a separate a-Al2O3 phase. Further 27Al NMR evidence is presented to support a view that the incorporated Al replaces Sc in the cubic zirconia-scandia structure. D 2004 Elsevier B.V. All rights reserved. Keywords: Nuclear magnetic resonance; Al solubility; Doped zirconia-scandia
1. Introduction The ZrO2–Sc2O3 solid solution is reported to have the highest ionic conductivity of the zirconia solid solutions [1,2]. As a function of temperature, the ionic conductivity of the ZrO2–Sc2O3 system exhibits a discontinuous change at around 650 8C [3], which accompanies the reversible transition from rhombohedral (low temperature) to cubic (high temperature). It has been shown that the addition of a second dopant, a-Al2O3, to 0.88ZrO2–0.12Sc2O3 stabilises the high conductivity cubic phase at room temperature and enables its use as an efficient oxygen conductor at temperatures below 650 8C [4]. Nuclear magnetic resonance provides a technique that enables the atomic environment to be characterised by means of line shift relative to a reference frequency or, for nuclei with spin IN1/2, line structure provided by the nuclear quadrupole interaction [5]. In this work, 27Al NMR has been used to investigate a series of compounds of the form 0.88ZrO2–(0.12 x)Sc2O3–xAl2O3 (x=0.006–0.04) at * Corresponding author. Tel.: +61 3 9544 2680; fax: +61 3 9544 1128. E-mail address: [email protected] (T.J. Bastow). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.12.043
room temperature in order to determine the role of Al, and the extent of its incorporation, in the zirconia-scandia structure. An additional preparation of 0.995ZrO 2 – 0.005Al2O3 was investigated to observe to what extent, at this dilution, Al will substitute in the (monoclinic) zirconia structure. The random substitution of Sc in the metal lattice of the cubic 0.88ZrO2–(0.12)Sc2O3 structure was verified by observing the 45Sc NMR spectrum and comparing with that for pure Sc2O3.
2. Experimental The ceramic specimens were prepared by standard powder ceramic techniques. The final heat treatment for all powder specimens was an annealing at 1600 8C for 10 h. The 27Al and 45Sc NMR spectra, both static and magic angle spinning (MAS), were collected on a Bruker Avance 400 spectrometer using an MAS probes with 4 mm PSZ rotors. The MAS frequency was 12 kHz. The NMR zero reference frequency for 27Al and 45Sc were derived from Al(NO3)3 (aq) and ScCl3 (aq).
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3. Results and discussion Al2O3 is soluble in ZrO2 and ZrO2–Sc2O3 only to a very limited extent. It is shown below that even down to x=0.01 an appreciable fraction of the dopant alumina exists in the 0.88ZrO2–0.12Sc2O3 lattice as a separate undissolved alumina phase. The specimens were first examined by magic angle spinning (MAS) 27Al NMR spectroscopy which indicated (Fig. 1) that at the highest concentrations, x=0.04 (not shown) and x=0.02 the spectrum consisted of one narrow dominant line at a shift of 11.7 ppm, consistent with the presence of undissolved Al2O3 [5]. Note that the 27Al MAS spectra in Fig. 1 exhibit only the partially narrowed central (1/2, 1/2) transition [5]. Since many alumina-based oxide materials in which the Al is octahedrally coordinated by oxygen have a very similar 27Al isotropic shift for the centra (1/2, 1/2) transition, the chemical identity with a-Al2O3 of the phase giving rise to the major 27Al signal at x=0.04 is provided by matching the nuclear quadrupole perturbed lineshapes in static spectra for the two samples. The nuclear quadrupole interaction is sensitive to the immediate atomic environment and provides a clear identification of the crystalline phase present. Since 27Al has nuclear spin I=5/2 the static 27Al spectrum for Al2O3 (Fig. 2, upper trace) consists of a central line flanked by two pairs of satellites corresponding to the F(1/2,3/2) and F(3/2,5/2) transitions associated with a first order nuclear quadrupole interaction at the Al site [5]. The
Fig. 1. 27Al MAS spectrum for 0.88ZrO2–(0.12 x)Sc2O3–xAl2O3; (a) x=0.02, (b) x=0.015, (c) x=0.01 and (d) x=0.006.
Fig. 2. 27Al static spectrum for (a) 0.88ZrO2–0.08Sc2O3–0.04Al2O3 and (b) pure a-Al2O3. Note that the peak of the central transition has been truncated to clearly display the satellites.
satellite spacing is characteristic of a-Al2O3. The same satellite structure can be identified in the 27Al spectrum for 0.88ZrO2–0.08Sc2O3–0.04Al2O3 (Fig. 2, lower trace) confirming the identity of the impurity phase. However, at concentrations below about x=0.02, an additional 27Al signal can be detected in the MAS spectrum which is different to that for a-Al2O3, and when the concentration has been reduced to x=0.006 only this new signal is present, and the 27Al signal for a-Al2O3 cannot be resolved. It is inferred that x=0.006 is a lower limit for the solubility of alumina in this host material, and that at this concentration the Al has been fully incorporated into the host lattice. It is interesting to note that for temperatures below 500 8C the previously reported [6] ionic conductivity of the title material, in series comprising x=0.003, 0.006, 0.01, 0.02, 0.05, is a maximum for x=0.006. For x=0.003, the material is
Fig. 3.
27
Al MAS spectrum for 0.995ZrO2–0.005Al2O3.
T.J. Bastow et al. / Solid State Ionics 175 (2004) 415–417
still in the rhombohedral phase at this temperature, while for the three highest concentrations the regions of undissolved a-Al2O3 presumably only add another source of oxygen (O2 )scattering, thus decreasing the ionic conductivity. An interpretation of the structure that develops around the base of the 27Al MAS line at low alumina doping levels (Fig. 2) appears most likely in terms of Al sites in which Al is substituting for either Sc or Zr in a few well defined sites. Similar valency would favour Sc, while similar ionic size would favour Zr; the ionic radii are as follows Al3+ 0.51 2, Sc3+ 0.73 2, Al3+ 0.79 2. However, the 27Al spectrum of a preparation 0.995ZrO2–0.005Al2O3 (Fig. 4) shows only a line centred at the a-Al2O3 frequency, which strongly suggests that the Al is substituting for Sc in a number of distinct sites. An alternative (probably less likely) interpretation is that the line structure is due to a second order quadrupolar lineshape. A simulation (not shown) of the 27Al MAS lineshape of 0.88ZrO2–0.114Sc2O3–0.006Al2O3 (Fig. 3) yielded the approximate value for the 27Al nuclear quadrupole coupling constant of 9.4 MHz. However in view of the atomically disordered nature of the metal sublattice of the zirconia-scandia the observation of a well-defined nuclear quadrupolar structure in the 27Al lineshape seems implausible. The extent to which Sc (the major dopant) has become part of the ceramic balloyQ can be illustrated by comparing the 45 Sc NMR spectrum of 0.88ZrO 2 –0.117Sc 2 O 3 – 0.003Al2O3 (Fig. 4a) with that from pure (99%) Sc2O3 (Fig. 4b). The complex 45Sc spectrum for Sc2O3 can be decomposed into two separate second order quadrupole perturbed lineshapes for the two metal sites in the cubic bixbyite structure: Sc(1) in 8b and Sc(2) in 24d [7]. The structure marked 1 in Fig. 4b belongs to the lineshape for Sc(1) and that marked 2 belongs to the lineshape for Sc(2). The 45Sc NMR spectrum of 0.88ZrO2–0.117Sc2O 3–0.003Al 2O3 shows none of this fine structure, but instead shows a peaked asymmetric lineshape, characteristic of an atomically disordered structure, consistent with a random substitution of Sc for Zr in the cubic zirconia structure. A similar lineshape has been observed for 91Zr in the cubic zirconia 0.87ZrO2–0.13MgO [8]. A well-defined second order quadrupolar lineshape, such as is observed in pure
417
Fig. 4. 45Sc static spectrum for (a) 0.88ZrO2–0.117Sc2O3–0.003Al2O3 and (b) pure Sc2O3.
Sc2O3 requires a sharply peaked distribution of electric field gradients at the atomic site in the crystalline unit cell. If two (or more) atomic species are randomly distributed over this site a broad distribution of electric field gradients is generated and a lineshape such as observed in Fig. 4a results. The line broadening is thus essentially due to chemical shift dispersion and will not narrow under magic angle spinning to yield higher spectroscopic detail.
References [1] D.W. Strickler, W.G. Carlson, J. Am. Ceram. Soc. 48 (1965) 286. [2] J.M. Dixon, L.D. Lagrange, D.L. Merten, C.F. Miller, J.T. Porter, J. Electrochem. Soc. 11 (1963) 276. [3] T. Ishii, T. Iwata, Y. Tajima, A. Yamaji, Solid State Ionics 57 (1993) 153. [4] T. Ishii, Solid State Ionics 78 (1995) 333. [5] K.J.D Mackenzie, M.E. Smith, Multinuclear Solid State NMR of Inorganic Materials, Pergamon, 2002. [6] R. Chiba, F. Yoshimura, J. Yamaki, T. Ishii, T. Yonezawa, K. Endou, Solid State Ionics 104 (1997) 259. [7] D. Riou, F. Fayon, D. Massiot, Chem. Mater. 14 (2002) 2416. [8] T.J. Bastow, M.E. Smith, Solid State Nucl. Magn. Reson. 1 (1992) 165.
Al2O3 solubility in the fast ion conductor 0.88ZrO2–(0.12 x)Sc2O3–xAl2O3 determined by 27Al NMR T.J. Bastowa,*, Tom Mathewsb, J.R. Sellarb a
CSIRO Manufacturing and Infrastructure Technology, Private Bag 33, South Clayton MDC, Clayton 3169, Australia b School of Physics and Materials Engineering, Monash University, Clayton, Victoria 3168, Australia Received 10 July 2003; received in revised form 6 November 2003; accepted 19 December 2003
Abstract 27
Al NMR has been used to investigate the solubility limit of alumina (a-Al2O3) in the fast ion conductor 0.88ZrO2–0.12Sc2O3. It is found that approximately 0.6 mol% alumina is retained in the ZrO2/Sc2O3 host lattice and that at higher doping levels the alumina exists in the specimen as a separate a-Al2O3 phase. Further 27Al NMR evidence is presented to support a view that the incorporated Al replaces Sc in the cubic zirconia-scandia structure. D 2004 Elsevier B.V. All rights reserved. Keywords: Nuclear magnetic resonance; Al solubility; Doped zirconia-scandia
1. Introduction The ZrO2–Sc2O3 solid solution is reported to have the highest ionic conductivity of the zirconia solid solutions [1,2]. As a function of temperature, the ionic conductivity of the ZrO2–Sc2O3 system exhibits a discontinuous change at around 650 8C [3], which accompanies the reversible transition from rhombohedral (low temperature) to cubic (high temperature). It has been shown that the addition of a second dopant, a-Al2O3, to 0.88ZrO2–0.12Sc2O3 stabilises the high conductivity cubic phase at room temperature and enables its use as an efficient oxygen conductor at temperatures below 650 8C [4]. Nuclear magnetic resonance provides a technique that enables the atomic environment to be characterised by means of line shift relative to a reference frequency or, for nuclei with spin IN1/2, line structure provided by the nuclear quadrupole interaction [5]. In this work, 27Al NMR has been used to investigate a series of compounds of the form 0.88ZrO2–(0.12 x)Sc2O3–xAl2O3 (x=0.006–0.04) at * Corresponding author. Tel.: +61 3 9544 2680; fax: +61 3 9544 1128. E-mail address: [email protected] (T.J. Bastow). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.12.043
room temperature in order to determine the role of Al, and the extent of its incorporation, in the zirconia-scandia structure. An additional preparation of 0.995ZrO 2 – 0.005Al2O3 was investigated to observe to what extent, at this dilution, Al will substitute in the (monoclinic) zirconia structure. The random substitution of Sc in the metal lattice of the cubic 0.88ZrO2–(0.12)Sc2O3 structure was verified by observing the 45Sc NMR spectrum and comparing with that for pure Sc2O3.
2. Experimental The ceramic specimens were prepared by standard powder ceramic techniques. The final heat treatment for all powder specimens was an annealing at 1600 8C for 10 h. The 27Al and 45Sc NMR spectra, both static and magic angle spinning (MAS), were collected on a Bruker Avance 400 spectrometer using an MAS probes with 4 mm PSZ rotors. The MAS frequency was 12 kHz. The NMR zero reference frequency for 27Al and 45Sc were derived from Al(NO3)3 (aq) and ScCl3 (aq).
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T.J. Bastow et al. / Solid State Ionics 175 (2004) 415–417
3. Results and discussion Al2O3 is soluble in ZrO2 and ZrO2–Sc2O3 only to a very limited extent. It is shown below that even down to x=0.01 an appreciable fraction of the dopant alumina exists in the 0.88ZrO2–0.12Sc2O3 lattice as a separate undissolved alumina phase. The specimens were first examined by magic angle spinning (MAS) 27Al NMR spectroscopy which indicated (Fig. 1) that at the highest concentrations, x=0.04 (not shown) and x=0.02 the spectrum consisted of one narrow dominant line at a shift of 11.7 ppm, consistent with the presence of undissolved Al2O3 [5]. Note that the 27Al MAS spectra in Fig. 1 exhibit only the partially narrowed central (1/2, 1/2) transition [5]. Since many alumina-based oxide materials in which the Al is octahedrally coordinated by oxygen have a very similar 27Al isotropic shift for the centra (1/2, 1/2) transition, the chemical identity with a-Al2O3 of the phase giving rise to the major 27Al signal at x=0.04 is provided by matching the nuclear quadrupole perturbed lineshapes in static spectra for the two samples. The nuclear quadrupole interaction is sensitive to the immediate atomic environment and provides a clear identification of the crystalline phase present. Since 27Al has nuclear spin I=5/2 the static 27Al spectrum for Al2O3 (Fig. 2, upper trace) consists of a central line flanked by two pairs of satellites corresponding to the F(1/2,3/2) and F(3/2,5/2) transitions associated with a first order nuclear quadrupole interaction at the Al site [5]. The
Fig. 1. 27Al MAS spectrum for 0.88ZrO2–(0.12 x)Sc2O3–xAl2O3; (a) x=0.02, (b) x=0.015, (c) x=0.01 and (d) x=0.006.
Fig. 2. 27Al static spectrum for (a) 0.88ZrO2–0.08Sc2O3–0.04Al2O3 and (b) pure a-Al2O3. Note that the peak of the central transition has been truncated to clearly display the satellites.
satellite spacing is characteristic of a-Al2O3. The same satellite structure can be identified in the 27Al spectrum for 0.88ZrO2–0.08Sc2O3–0.04Al2O3 (Fig. 2, lower trace) confirming the identity of the impurity phase. However, at concentrations below about x=0.02, an additional 27Al signal can be detected in the MAS spectrum which is different to that for a-Al2O3, and when the concentration has been reduced to x=0.006 only this new signal is present, and the 27Al signal for a-Al2O3 cannot be resolved. It is inferred that x=0.006 is a lower limit for the solubility of alumina in this host material, and that at this concentration the Al has been fully incorporated into the host lattice. It is interesting to note that for temperatures below 500 8C the previously reported [6] ionic conductivity of the title material, in series comprising x=0.003, 0.006, 0.01, 0.02, 0.05, is a maximum for x=0.006. For x=0.003, the material is
Fig. 3.
27
Al MAS spectrum for 0.995ZrO2–0.005Al2O3.
T.J. Bastow et al. / Solid State Ionics 175 (2004) 415–417
still in the rhombohedral phase at this temperature, while for the three highest concentrations the regions of undissolved a-Al2O3 presumably only add another source of oxygen (O2 )scattering, thus decreasing the ionic conductivity. An interpretation of the structure that develops around the base of the 27Al MAS line at low alumina doping levels (Fig. 2) appears most likely in terms of Al sites in which Al is substituting for either Sc or Zr in a few well defined sites. Similar valency would favour Sc, while similar ionic size would favour Zr; the ionic radii are as follows Al3+ 0.51 2, Sc3+ 0.73 2, Al3+ 0.79 2. However, the 27Al spectrum of a preparation 0.995ZrO2–0.005Al2O3 (Fig. 4) shows only a line centred at the a-Al2O3 frequency, which strongly suggests that the Al is substituting for Sc in a number of distinct sites. An alternative (probably less likely) interpretation is that the line structure is due to a second order quadrupolar lineshape. A simulation (not shown) of the 27Al MAS lineshape of 0.88ZrO2–0.114Sc2O3–0.006Al2O3 (Fig. 3) yielded the approximate value for the 27Al nuclear quadrupole coupling constant of 9.4 MHz. However in view of the atomically disordered nature of the metal sublattice of the zirconia-scandia the observation of a well-defined nuclear quadrupolar structure in the 27Al lineshape seems implausible. The extent to which Sc (the major dopant) has become part of the ceramic balloyQ can be illustrated by comparing the 45 Sc NMR spectrum of 0.88ZrO 2 –0.117Sc 2 O 3 – 0.003Al2O3 (Fig. 4a) with that from pure (99%) Sc2O3 (Fig. 4b). The complex 45Sc spectrum for Sc2O3 can be decomposed into two separate second order quadrupole perturbed lineshapes for the two metal sites in the cubic bixbyite structure: Sc(1) in 8b and Sc(2) in 24d [7]. The structure marked 1 in Fig. 4b belongs to the lineshape for Sc(1) and that marked 2 belongs to the lineshape for Sc(2). The 45Sc NMR spectrum of 0.88ZrO2–0.117Sc2O 3–0.003Al 2O3 shows none of this fine structure, but instead shows a peaked asymmetric lineshape, characteristic of an atomically disordered structure, consistent with a random substitution of Sc for Zr in the cubic zirconia structure. A similar lineshape has been observed for 91Zr in the cubic zirconia 0.87ZrO2–0.13MgO [8]. A well-defined second order quadrupolar lineshape, such as is observed in pure
417
Fig. 4. 45Sc static spectrum for (a) 0.88ZrO2–0.117Sc2O3–0.003Al2O3 and (b) pure Sc2O3.
Sc2O3 requires a sharply peaked distribution of electric field gradients at the atomic site in the crystalline unit cell. If two (or more) atomic species are randomly distributed over this site a broad distribution of electric field gradients is generated and a lineshape such as observed in Fig. 4a results. The line broadening is thus essentially due to chemical shift dispersion and will not narrow under magic angle spinning to yield higher spectroscopic detail.
References [1] D.W. Strickler, W.G. Carlson, J. Am. Ceram. Soc. 48 (1965) 286. [2] J.M. Dixon, L.D. Lagrange, D.L. Merten, C.F. Miller, J.T. Porter, J. Electrochem. Soc. 11 (1963) 276. [3] T. Ishii, T. Iwata, Y. Tajima, A. Yamaji, Solid State Ionics 57 (1993) 153. [4] T. Ishii, Solid State Ionics 78 (1995) 333. [5] K.J.D Mackenzie, M.E. Smith, Multinuclear Solid State NMR of Inorganic Materials, Pergamon, 2002. [6] R. Chiba, F. Yoshimura, J. Yamaki, T. Ishii, T. Yonezawa, K. Endou, Solid State Ionics 104 (1997) 259. [7] D. Riou, F. Fayon, D. Massiot, Chem. Mater. 14 (2002) 2416. [8] T.J. Bastow, M.E. Smith, Solid State Nucl. Magn. Reson. 1 (1992) 165.