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The defect structure of yttria-stabilized zirconia, studied by quasielastic diffuse neutron scattering
Physica 136B (1986) 315-317 North-Holland, Amsterdam
THE DEFECT STRUCTURE OF YTTRIA-STABILIZED ZIRCONIA, STUDIED BY QUASIELASTIC DIFFUSE NEUTRON SCATTERING N.H. A N D E R S E N b, K. C L A U S E N b, M.A. H A C K E T T a, W. H A Y E S a, M.T. H U T C H I N G S c, J.E. M A C D O N A L D a* and R. O S B O R N a aClarendon Laboratory, Parks Road, Oxford, 0)(1 3PU, UK bDepartment of Physics, Riso National Laboratory, DK-4000 Roskilde, Denmark CMaterials Physics & Metallurgy Division, AERE Harwell, Didcot, OXll ORA, UK
The static defect structure of the oxygen ion conductor YzO3 stabilized zirconia has been studied at room temperature by coherent diffuse neutron scattering from single crystal samples containing nominally9.4, 12, 15 and 18 mol% YzO3. There are two principal contributions to the observed diffuseintensity. The first arises from tetrahedral distortions in small vacancy free regions of the crystal which decrease in volume as the dopant level increases. The second arises from correlated vacancies and their associated relaxed ions in the remainder of the crystal. The 9.4 mol% sample has been studied at elevated temperatures. The scattering becomes partly quasielastic, but the correlations persist to the highest temperatures studied (1900°C). The temperature and Q-dependence of the energy width has been studied at selected positions in reciprocal space.
1. Introduction
2. Experiment
Zirconia possesses the cubic fluorite structure, Fm3m, just below its melting temperature T m = 2988 K. On cooling it transforms initially to a tetragonal structure at 2643 K and finally below - 1 2 7 3 K it is monoclinic. However, by alloying with a wide range of concentrations of divalent (CaO, MgO) or trivalent (Y203, rare earth sesquioxides) oxides, the cubic structure can be stabilized at ambient temperature. The high oxygen ion conductivity of Y203 stabilized Z r O 2 at elevated temperatures arises from its defect fluorite structure. The structure of stabilized zirconia has been extensively studied at 293 K by X-ray, electron and neutron diffraction and diffuse scattering (see Osborn et al. [2] and references therein). In this paper we will briefly describe the static defect structure as determined by elastic diffuse neutron scattering and then discuss the high temperature diffuse scattering which becomes partly quasielastic, giving information on the dynamics of the disorder. For further details see Osborn et al. [2] and Andersen et al. [3].
The samples of nominal composition ZrO 2 ( y mol% Y203) with y = 9.4, 12, 15 and 18 were supplied by J.F. Wenckus of the Ceres Corporation and consisted of large transparent single crystals grown by the skull melting technique. For the high temperature measurements, crystals with y = 9.4 were utrasonically cut into cylinders of 1 5 m m diameter before being encapsulated in molybdenum tubes. A contour map of the diffuse intensity in the (110) plane was measured at 293 K using the triple axis spectrometer (TAS) in D I D O at Harwell. High temperature surveys of this scattering were made using the TAS at Harwell, Ris0 and ILL, Grenoble. At the latter two reactors, cold moderated neutron beams enabled the quasielastic nature of some of the scattering to be studied with energy resolution of 0.13 meV.
3. Elastic diffuse scattering The distribution of diffuse scattering intensity I ( Q ) in the (110) plane at room temperature for
* Present address: School of Physics, University of Bath, Bath, BA2 7AY, UK.
y = 9.4 is shown in fig. 1. Crystals with y = 9.4 and 12 were found to give rise to broadened peaks
0378-4363/86/$03.50 © Elsevier Science Publishers BN. (North-Holland Physics Publishing Division)
316
N . H . Andersen et al. / Defect structure o f stabilized zirconia
centred at the 'forbidden' positions (112) and (114), but those with y = 15 and 18 have negligible peak intensity in these positions. On the other hand, the peaks occurring at superlattice positions (~- fluorite -+(0.4, 0.4, -+0.8)) such as (0.6, 0.6, 3.8) and (1.4, 1.4, 3.8) become stronger and sharper with increasing y. Analysis of these results in terms of possible models of the defect structure suggests that the scattering arises from relatively vacancy free tetrahedrally distorted regions and regions with aggregates of clusters [2, 3]. The basic building block is a single vacancy with relaxation of the nearest neighbours. However, there is a strong tendency for the vacancies to cluster in pairs as shown in fig. 2, and a weaker tendency for these clusters to aggregate with separations given by the primitive lattice vectors of the Zr3Y4012 structure (ref. 3 and references therein). The cluster-cluster correlation length is of the order of 5-10 ,~ [2], and the defect structure can be visualized as an assembly of tetrahedrally distorted regions of varying size and regions containing aggregates of defects ranging from single vacancies to vacancy aggregates of spatial extent up to a few times the correlation length. These regions can be considered as randomly filling the crystal, and hence
0 Cation O Relaxed cation (111) [ ] Oxygen vacancy
Oxygen on regular site • Relaxed oxygen (100) •
j~
J/~
Fig. 2. Defect cluster consisting of a vacancy pair plus relaxation of nearest-neighbour oxygens towards the vacancy along (100) directions and relaxation of the cations away from the vacancy along (111) directions.
the intensity of the diffuse scattering can be calculated as the incoherent summation of the intensities from the different regions.
4. Quasielastic diffuse scattering z~.O:
3.0
©
0 o
2.0 ~
1.0
0.0
1.0
2.0
3.0
~
4.0
Fig. 1. Contour diagram of the coherent diffuse neutron scattering in the (110) plane of ZrO 2 doped with 9.4 mol% Y203 as observed at room temperature.
The part of the contour diagram shown in fig. 1 extending from Q = 0 to Q ~ 4 / ~ k -1 has been studied at 1145°C. Apart from a Debye-Waller factor the intensity integrated over energy transfer retained the same overall distribution as at room temperature, but the scattering becomes partly quasielastic. Energy scans at a selection of Q points and temperatures have been performed and the analysis is still in progress. The preliminary results suggest that the scattering can be fitted to a sum of three components a) a resolution limited component, b) a Lorentzian shaped energybroadened quasielastic component convolved with the instrumental resolution function, and c) a weak incoherent resolution limited contribution from the sample container. A thermal history dependent component- resolution limited in both Q and
N.H. Andersen et al. / Defect structure o f stabilized zirconia i
0./.,
I
..{/
(1.8,1.8,0.8)
/
~-
"I-
0.2
Q/
0
- - ~ 1 ~ 1
0
1
_
o/
I
500 1000 TENPERATUR£ (°C 1
--__
1500
Fig. 3, Temperature dependence of the linewidth (FWHM) of the coherent quasielastic diffuse scattering observed at the (1.8, 1.8, 0.8) reciprocal lattice point. The broken line is a guide to the eye.
E - evolves at the forbidden reflections (112) and (114) if the sample is left at intermediate temperatures for longer times. This scattering disappears again once the sample has been heated to high temperatures for a few minutes. No thermal history dependence has so far been observed elsewhere. Three different regions of scattering in the (110) plane could be identified: 1) The scattering around the (112) 'forbidden' reflection shows marked energy-broadening at lower temperatures than elsewhere. 2) Close to the superlattice points ~"- (0.4, 0.4, ---0.8) the scattering consists of an energy broadened Lorentzian component whose intensity and linewidth only change marginally with Q, and a resolution limited Gaussian component which increases in intensity as the superlattice point is approached. 3) Outside the areas 1) and 2) the scattering can be fitted to an energy broadened Lorentzian plus the incoherent Gaussian contribution. The temperature dependence of the linewidth observed at the Q = (1.8, 1.8, 0.8) point is shown in fig. 3. The general trend is for the linewidth of the Lorentzian component to increase slowly with Q.
5. Discussion The strong energy-broadening of the scattering observed around the (112) point, where the vacancy-free tetrahedrally distorted regions dominate the scattering, indicates that these parts of the sample have the shortest lifetime. The origin of this broadening may be due either to the destruction of the tetrahedral symmetry in a region because a single vacancy is diffusing
317
through it, or to fluctuations between cubic and tetrahedral symmetry. Close to the superlattice positions the scattering is dominated by the largest aggregates of dusters. The appearance of a large resolution-limited Gaussian component in these regions suggests that these aggregates, on the time scales that can be determined by neutron scattering, are static up to the highest temperatures investigated. Outside the two regions mentioned above, the scattering arises from small defect clusters or single vacancies, and is quasielastic, i.e. diffusion, or at least reorientation of these small clusters can be inferred. In conclusion we can say that vacancies seem to be trapped in the large aggregates. The small clusters are dynamic, and the highest mobility is probably through the tetrahedrally distorted regions. This explains why the ionic conductivity starts to decrease on doping in excess of 8 mol% Y203 (see S~rensen et al. [4] and references therein). With increasing y the lattice becomes gradually filled with 'static' defects.
Acknowledgements We have benefitted from several very helpful discussions with Dr. J.K. Kjems. This work is supported by the UK. SERC, Harwell EMR Contracts, a Twinning Grant from the EEC STI032-J-C, and the Danish Ministry of Energy.
References [1] E.C. Subbarao, in: Science and Technology of Zirconia, A.H. Heura, ed. (America Ceramic Society, Columbus, Ohio, 1981). [2] R. Osborn, N.H. Andersen, K. Clausen, M.A. Hackett, W. Hayes, M.T. Hutchings and J.E. Macdonald, to be published in Mat. Science Forum 5 (1985). [3] N.H. Andersen, K. Clausen, M.A. Hackett, W. Hayes, M.T. Hutchings, J.E. Macdonald and R. Osborn, in: Transport-Structure Relations in Fast Ion and Mixed Conductors, F.W. Poulsen, N.H. Andersen, K. Clausen, S. Skaarup and O.T. S0rensen, eds. (Ris0, 1985) p. 279, [4] O.T. SOrensen, ~. Johannesen and K. Clausen, in: Transport-Structure Relations in Fast Ion and Mixed Conductors, F.W. Poulsen, N.H. Andersen, K. Clausen, S. Skaarup and O.T. Sorensen, eds. (Ris0, 1985) p. 93.
THE DEFECT STRUCTURE OF YTTRIA-STABILIZED ZIRCONIA, STUDIED BY QUASIELASTIC DIFFUSE NEUTRON SCATTERING N.H. A N D E R S E N b, K. C L A U S E N b, M.A. H A C K E T T a, W. H A Y E S a, M.T. H U T C H I N G S c, J.E. M A C D O N A L D a* and R. O S B O R N a aClarendon Laboratory, Parks Road, Oxford, 0)(1 3PU, UK bDepartment of Physics, Riso National Laboratory, DK-4000 Roskilde, Denmark CMaterials Physics & Metallurgy Division, AERE Harwell, Didcot, OXll ORA, UK
The static defect structure of the oxygen ion conductor YzO3 stabilized zirconia has been studied at room temperature by coherent diffuse neutron scattering from single crystal samples containing nominally9.4, 12, 15 and 18 mol% YzO3. There are two principal contributions to the observed diffuseintensity. The first arises from tetrahedral distortions in small vacancy free regions of the crystal which decrease in volume as the dopant level increases. The second arises from correlated vacancies and their associated relaxed ions in the remainder of the crystal. The 9.4 mol% sample has been studied at elevated temperatures. The scattering becomes partly quasielastic, but the correlations persist to the highest temperatures studied (1900°C). The temperature and Q-dependence of the energy width has been studied at selected positions in reciprocal space.
1. Introduction
2. Experiment
Zirconia possesses the cubic fluorite structure, Fm3m, just below its melting temperature T m = 2988 K. On cooling it transforms initially to a tetragonal structure at 2643 K and finally below - 1 2 7 3 K it is monoclinic. However, by alloying with a wide range of concentrations of divalent (CaO, MgO) or trivalent (Y203, rare earth sesquioxides) oxides, the cubic structure can be stabilized at ambient temperature. The high oxygen ion conductivity of Y203 stabilized Z r O 2 at elevated temperatures arises from its defect fluorite structure. The structure of stabilized zirconia has been extensively studied at 293 K by X-ray, electron and neutron diffraction and diffuse scattering (see Osborn et al. [2] and references therein). In this paper we will briefly describe the static defect structure as determined by elastic diffuse neutron scattering and then discuss the high temperature diffuse scattering which becomes partly quasielastic, giving information on the dynamics of the disorder. For further details see Osborn et al. [2] and Andersen et al. [3].
The samples of nominal composition ZrO 2 ( y mol% Y203) with y = 9.4, 12, 15 and 18 were supplied by J.F. Wenckus of the Ceres Corporation and consisted of large transparent single crystals grown by the skull melting technique. For the high temperature measurements, crystals with y = 9.4 were utrasonically cut into cylinders of 1 5 m m diameter before being encapsulated in molybdenum tubes. A contour map of the diffuse intensity in the (110) plane was measured at 293 K using the triple axis spectrometer (TAS) in D I D O at Harwell. High temperature surveys of this scattering were made using the TAS at Harwell, Ris0 and ILL, Grenoble. At the latter two reactors, cold moderated neutron beams enabled the quasielastic nature of some of the scattering to be studied with energy resolution of 0.13 meV.
3. Elastic diffuse scattering The distribution of diffuse scattering intensity I ( Q ) in the (110) plane at room temperature for
* Present address: School of Physics, University of Bath, Bath, BA2 7AY, UK.
y = 9.4 is shown in fig. 1. Crystals with y = 9.4 and 12 were found to give rise to broadened peaks
0378-4363/86/$03.50 © Elsevier Science Publishers BN. (North-Holland Physics Publishing Division)
316
N . H . Andersen et al. / Defect structure o f stabilized zirconia
centred at the 'forbidden' positions (112) and (114), but those with y = 15 and 18 have negligible peak intensity in these positions. On the other hand, the peaks occurring at superlattice positions (~- fluorite -+(0.4, 0.4, -+0.8)) such as (0.6, 0.6, 3.8) and (1.4, 1.4, 3.8) become stronger and sharper with increasing y. Analysis of these results in terms of possible models of the defect structure suggests that the scattering arises from relatively vacancy free tetrahedrally distorted regions and regions with aggregates of clusters [2, 3]. The basic building block is a single vacancy with relaxation of the nearest neighbours. However, there is a strong tendency for the vacancies to cluster in pairs as shown in fig. 2, and a weaker tendency for these clusters to aggregate with separations given by the primitive lattice vectors of the Zr3Y4012 structure (ref. 3 and references therein). The cluster-cluster correlation length is of the order of 5-10 ,~ [2], and the defect structure can be visualized as an assembly of tetrahedrally distorted regions of varying size and regions containing aggregates of defects ranging from single vacancies to vacancy aggregates of spatial extent up to a few times the correlation length. These regions can be considered as randomly filling the crystal, and hence
0 Cation O Relaxed cation (111) [ ] Oxygen vacancy
Oxygen on regular site • Relaxed oxygen (100) •
j~
J/~
Fig. 2. Defect cluster consisting of a vacancy pair plus relaxation of nearest-neighbour oxygens towards the vacancy along (100) directions and relaxation of the cations away from the vacancy along (111) directions.
the intensity of the diffuse scattering can be calculated as the incoherent summation of the intensities from the different regions.
4. Quasielastic diffuse scattering z~.O:
3.0
©
0 o
2.0 ~
1.0
0.0
1.0
2.0
3.0
~
4.0
Fig. 1. Contour diagram of the coherent diffuse neutron scattering in the (110) plane of ZrO 2 doped with 9.4 mol% Y203 as observed at room temperature.
The part of the contour diagram shown in fig. 1 extending from Q = 0 to Q ~ 4 / ~ k -1 has been studied at 1145°C. Apart from a Debye-Waller factor the intensity integrated over energy transfer retained the same overall distribution as at room temperature, but the scattering becomes partly quasielastic. Energy scans at a selection of Q points and temperatures have been performed and the analysis is still in progress. The preliminary results suggest that the scattering can be fitted to a sum of three components a) a resolution limited component, b) a Lorentzian shaped energybroadened quasielastic component convolved with the instrumental resolution function, and c) a weak incoherent resolution limited contribution from the sample container. A thermal history dependent component- resolution limited in both Q and
N.H. Andersen et al. / Defect structure o f stabilized zirconia i
0./.,
I
..{/
(1.8,1.8,0.8)
/
~-
"I-
0.2
Q/
0
- - ~ 1 ~ 1
0
1
_
o/
I
500 1000 TENPERATUR£ (°C 1
--__
1500
Fig. 3, Temperature dependence of the linewidth (FWHM) of the coherent quasielastic diffuse scattering observed at the (1.8, 1.8, 0.8) reciprocal lattice point. The broken line is a guide to the eye.
E - evolves at the forbidden reflections (112) and (114) if the sample is left at intermediate temperatures for longer times. This scattering disappears again once the sample has been heated to high temperatures for a few minutes. No thermal history dependence has so far been observed elsewhere. Three different regions of scattering in the (110) plane could be identified: 1) The scattering around the (112) 'forbidden' reflection shows marked energy-broadening at lower temperatures than elsewhere. 2) Close to the superlattice points ~"- (0.4, 0.4, ---0.8) the scattering consists of an energy broadened Lorentzian component whose intensity and linewidth only change marginally with Q, and a resolution limited Gaussian component which increases in intensity as the superlattice point is approached. 3) Outside the areas 1) and 2) the scattering can be fitted to an energy broadened Lorentzian plus the incoherent Gaussian contribution. The temperature dependence of the linewidth observed at the Q = (1.8, 1.8, 0.8) point is shown in fig. 3. The general trend is for the linewidth of the Lorentzian component to increase slowly with Q.
5. Discussion The strong energy-broadening of the scattering observed around the (112) point, where the vacancy-free tetrahedrally distorted regions dominate the scattering, indicates that these parts of the sample have the shortest lifetime. The origin of this broadening may be due either to the destruction of the tetrahedral symmetry in a region because a single vacancy is diffusing
317
through it, or to fluctuations between cubic and tetrahedral symmetry. Close to the superlattice positions the scattering is dominated by the largest aggregates of dusters. The appearance of a large resolution-limited Gaussian component in these regions suggests that these aggregates, on the time scales that can be determined by neutron scattering, are static up to the highest temperatures investigated. Outside the two regions mentioned above, the scattering arises from small defect clusters or single vacancies, and is quasielastic, i.e. diffusion, or at least reorientation of these small clusters can be inferred. In conclusion we can say that vacancies seem to be trapped in the large aggregates. The small clusters are dynamic, and the highest mobility is probably through the tetrahedrally distorted regions. This explains why the ionic conductivity starts to decrease on doping in excess of 8 mol% Y203 (see S~rensen et al. [4] and references therein). With increasing y the lattice becomes gradually filled with 'static' defects.
Acknowledgements We have benefitted from several very helpful discussions with Dr. J.K. Kjems. This work is supported by the UK. SERC, Harwell EMR Contracts, a Twinning Grant from the EEC STI032-J-C, and the Danish Ministry of Energy.
References [1] E.C. Subbarao, in: Science and Technology of Zirconia, A.H. Heura, ed. (America Ceramic Society, Columbus, Ohio, 1981). [2] R. Osborn, N.H. Andersen, K. Clausen, M.A. Hackett, W. Hayes, M.T. Hutchings and J.E. Macdonald, to be published in Mat. Science Forum 5 (1985). [3] N.H. Andersen, K. Clausen, M.A. Hackett, W. Hayes, M.T. Hutchings, J.E. Macdonald and R. Osborn, in: Transport-Structure Relations in Fast Ion and Mixed Conductors, F.W. Poulsen, N.H. Andersen, K. Clausen, S. Skaarup and O.T. S0rensen, eds. (Ris0, 1985) p. 279, [4] O.T. SOrensen, ~. Johannesen and K. Clausen, in: Transport-Structure Relations in Fast Ion and Mixed Conductors, F.W. Poulsen, N.H. Andersen, K. Clausen, S. Skaarup and O.T. Sorensen, eds. (Ris0, 1985) p. 93.