EXAFS STUDIES OF FLUORITE OXIDES

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EXAFS STUDIES OF FLUORITE OXIDES

P. Battle, C. Catlow, A. Chadwick, G. Greaves, L. Moroney

To cite this version:

P. Battle, C. Catlow, A. Chadwick, G. Greaves, L. Moroney. EXAFS STUDIES OF FLUORITE OX- IDES. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-669-C8-673. �10.1051/jphyscol:19868126�.

�jpa-00226025�

JOURNAL DE PHYSIQUE

Colloque C8, supplbment au n o 12, Tome 47, dbcembre 1986

EXAFS STUDIES OF FLUORITE OXIDES

P.D. BATTLE, C.R.A. CATLOW*, A.V. CHADWICK**, G. N. GREAVES* * * and L

.

Chemistry Department, Leeds University, GB-Leeds LS2 9 J T , Great-Britain

"chemistry Department, Keele University, GB-Keele ST5 5AG, Great-Britain

* I

Chemistry Department, Kent University, GB-Canterbury CT2 7NH, Great-Britain

* ^{* X} S.E.R.C., Daresbury Laboratory, GB-Daresbury WA4 4AD,

Great-Britain

'Brookhaven National Laboratory, Upton, NY 11973, U . S . A .

Abstract - Many of the unique possibilities of EXAFS as a structural technique are illustrated in the problem of determining the local interactions between defects and anion vacancies in the fluorite oxide systems, yttria stabilised zirconia and Y2OS-doped BizOs. From the measured EXAFS on the Y, Zr and Bi absorption edges, it is possible to compare directly the host and dopant cation's different local structural environments. This shows that, in both systems, the YS+

ions adopt a more ordered and isotropic local environment than the host cation. Both Big+ and Zr4+ tend to be displaced from their centrosymmetric sites and the static disorder in the nearest neighbour oxygen shell is comparatively extensive. For yttria-stabilised zirconia, this behaviour can be attributed to ionic relaxations in response to anion vacancies in the Zr-0 shell, the anion vacancies being necessary for charge compensation of the trivalent YS+ ions. The disorder in the BizOs samples is greater than in yttria-stabilised zirconia which can be correlated with the higher anion conductivity in the former. The preference that the host cations exhibit for anisotropic co-ordination geometries can be explained by their high polarisabilities due to the high charge and small size of the Zr4+ ion and the lone electron pair on the BiS+ ion.

1. lutroduction

The fluorite oxides are an interesting class of compound, including such examples as stabilised zirconia, doped BizOs, U02, and CeOz, for which many technological uses have been demonstrated. All of the above materials show enhanced anion conductivity at elevated temperatures which is attributed to the inherent ability of the fluorite lattice to accommodate defects. The fluorite structure (Figure 1) consists of a cubic array of anions with the cations located at the centre of every alternate anion cube. Those anion cubes which do not contain cations provide space in the framework for interstitial species and the relative openness of the structure allows often extensive anion static dislacements to occur without destroying the fluorite structure. As a result of this, it is possible to substitute dopant cations for the host cation in substantial concentrations and, in the case of stabilised zirconia and Bi20s, the dopant cations are necessary to stabilise the fluorite phase, the pure ZrOz and BiaOs adopting monoclinic structures at room temperature, otherwise. Charge compensation for the aliovalent dopant ions is achieved by oxygen vacancies in the lattice.

A usual requirement of these materials is compositional homogeneity and long term stability. Un- der certain preparative conditions or with time, dopant aggregation or anion/vacancy ordering can occur resulting ultimately in the formation of microdomains of second phases within the material usually de- creasing its usefulness. An understanding of the short range structural interactions of the dopant ions and any charge compensating anion vacancies within the host is important for attempting to control these phenomena.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868126

JOURNAL DE PHYSIQUE

.oxygen vacancy

anion

0

large

interstitiai site

Fig.1 The fluorite structure showing intersti.ta1 and oxygen vacancy defect sites.

EXAFS is a particularly useful technique for such a study notably because of its unique ability to provide separate structural data for the dopant and host cations and much information can often be deduced from a purely qualitative comparison of the data a t the two absorption edges. Although, obtaining reliable quantitative values for such parameters as co-ordination number is always problematic, changes in co-ordination number in ionic compounds are often accompanied by large ion relaxations causing marked changes in radial distance which are generally easily detected with the EXAFS technqiue. Furthermore, the periodicity of the structures necessarily limits the values possible for the various structural parameters, also aiding quantitative analysis.

We describe here two series of experiments on two fluorite oxide systems: yttria stabilised zirconia and Y2OS-doped Bi2OS. Both are anion conductors, the conductivity of yttria-stabilised zirconia being about 10-sfl-l cm-' a t 500 "[I], and Y20s-doped BizOs approximately 10-2fl-1 cm-' [2] a t the same temperature. The substitution of Ys+ for Zr4+ in yttria-stabilised zirconia requires the formation of one anion vacancy for each Ys+ pair in order that electroneutrality is maintained. Thus, a t the upper dopant concentration limit for the phase where the stoichiometry is 15% of the anion sites are vacant. The anion vacancy concentration in the fluorite phase of Y20s/Bi20Sis a remarkable 25%.

Diffraction data obtained for these materials show, in both cases, large anion displacements predominantly in the (111) direction [3][4][5][6] as well as cation displacements in the (111) direction for yttria-stabilised zirconia and the (100) direction for YzOs/Bi2Os. Both materials exhibit large diffuse scattering [6] [7][8][9]

indicative of systematic short range deviations from the periodicity of the average unit cell.

Figures 2 and 3 show the ks-weighted EXAFS data obtained a t the Y and Zr K edges for 18 wt%

YZOs/Zr02 a t 80 K [la]. Direct comparison of these data sets is possible by virtue of the fact that Zr4+

and YS+, being adjacent in the periodic table, are isoelectronic. Therefore, differences in the central atom's phaseshift and backscattering functions for the two cations will be negligible. Comparing the Fourier filtered first shell data for the two cations reveals that the peak in the Zr-0 radial distribution function is a t a shorter distance than for Y-0 and fitting gives values of 2.11 A (Zr-0) and 2.28 A (Y-0) which fall within the range of cation-oxygen distances seen in the parent oxides, Y20s and ZrOz. The amplitude of the EXAFS envelope is less for Zr-0 and the results of fitting show that this amplitude difference causes the co-ordination number for Zr to be lower than Y and the disorder in the ZrO distance to be greater. It is the combination of these two facts which provide evidence that the anion vacancies are first neighbour to Zr rather than Y, for, although the accuracy of the fitted values of Debye-Waller factor and occupation number is not high owing to the large degree of correlation between the two values, there clearly is an increased degree of disorder in the Zr shell and it is reasonable to attribute this to ionic relaxations in response to anion vacancies first neighbour to Zr4+. The second shell data, too, supports

this interpretation. Figures 2b and 3b show that the peak representing the 12 second nearest neighbours of Y is twice the intensity of that for Zr. In both cases, the number of cation neighbours must be the same, as must the back scattering factor for the two cations comprising the shell. The only possible explanation for the reduction in amplitude of the Zr-cation shell is a large amount of static disorder in the Zr-cation distance. Since, of course, many of the second neighbours of Zr must also be second neighbours of Y, this static disorder cannot be attributed to general cation displacements but must be due to displacement of Zr ions only from their centrosymmetric lattice sites. Displacements of this nature are reasonable in response to nearest neighbour anion vacancies. These results indicate that it is the Zr4+ ions which undergo the (111) cation dispacements observed in the diffraction experiments.

Computer simulation methods have been used to determine the relative stability of vacancy-Y and vacancy-Zr pairs in yttria-stabilised zirconia and confirm that the latter is energetically favoured. The predicted relaxations show that Zr4+ moves ^N 0.06 A away from the vacancy in a (111) direction and the three oxygen neighbours of the vacancy are displaced

-

EXAFS data has been obtained for yttria-stabilised zirconia over the temperature range 80 K to 1000 K. Increasing temperature has a more marked effect on the Y data than the Zr data. The shorter Zr-0 bonds would be expected to be more strongly correlated than Y-0 and this is the pattern that is observed.

Radial Distance / A

Fig. 2a - k s x ( k ) data for yttria-stabilised zirconia Fig. 2b - Fourier transform of data in 2a.

at the Y edge a t 80 K.

8 - t 1 i ~ / l i 1 r l i i l l j i 8 r 1

6 - -

4 -

0 2 4 6 8

Radial Distance / A

Fig. 3a - k s x ( k ) data for yttriestabilised zirconia Fig. 3b - Fourier transform of data in 3a.

at the Zr edge a t 80 K.

C8-672 JOURNAL DE PHYSIQUE

With the measured Zr-0 distance being approximately 2.11 A and the evidence that anion vacancies are first neighbour to Zr, the model which emerges is that the local structural environment for Zr in yttria-stabilised zirconia resembles that of monoclinic ZrO, in which Zr is 7-co-ordinate. The driving force for this configuration may be the increased contact that the small Zr ion (0.97 A) 1111 achieves with its oxygen neighbours.

It is interesting to compare the forgoing data with the data for 40% YzOs in BizOs (Figures 4 and 5).

While the magnitude of the Y-0 contribution is similar to that previously observed, the Bi-0 EXAFS is much reduced and both cations display substantially reduced second shell EXAFS. The differences in the EXAFS data collected a t 80 K and room temperature are very slight indicating that the disorder is predominantly static in origin.

Comparing the two local structural environments of Ys+ and BiS+, we find that the disorder surround- ing the Bis+ ions is substantially greater than for Ys+. For example, it is possible to fit the first shell of neighbours for Y with six oxygens a t a distance of 2.27 A. The environment of Ys+ is, in fact, more similar to the parent material YzOs than it is to BizOs in spite of the two cations occupying crystallographically indistinguishable sites in the fluorite structure. An adequate fit involving six oxygen neighbours for the Bis+ ions could not be obtained owing to the very large asymmetry in the Bi-0 shell. Moreover, the fact that the 12 cation neighbours comprising the second shell of both Bis+ and Ys+ yield a much larger amplitude in the Y data compared with Bi, suggests that part of their anisotropy can be attributed to the displacement of the Bi3+ ions from their centrosymmetric sites. Thus, we can attribute the (100) cation displacements seen in the diffraction experiments to Bis+ displacements. The peak in the distribution of oxygens about Bis+ is close to 2.11 A.

We have also obtained data for the lower dopant concentrations of 27% YzOsand 34% YaOs. These showed only slight differences from the data plotted in Figures 4 and 5 which could be accounted for by a slightly increased amount of disorder in the more dilute samples. This is in accord with the diffraction data [6] which showed an increase in the long range and medium range order as the dopant concentration increases. Data for Er20s-doped and YbzOs-doped BiaOssysterns showed that, a t the local structural level, these materials are analogous to Y208/Bi20s. As with yttria-stabilised zirconia, increasing temperature had a more marked effect on the amplitude of the Y data compared with the Bi data.

4. Discussion

The similarities between the Biz08 and ZrO, systems are apparent with the Bis+ and Zr4+ adopting sites characterised by local anisotropy and the dopant ion, a more ordered site. This is in spite of the fact that Zr4+ is smaller than the dopant cation whereas Bis+ is nominally larger [ll]. In both cases the distribution of nearest oxygen neighbours is asymmetric and peaks close to the value of 2.12 A. This distance is, in fact, less than the sum of the ionic radii for 0'- and Bis+. It has been noted, however, that obtaining an accurate value for the ionic radius of Bis+ is difficult owing to its tendency to adopt a highly anisotropic co-ordination geometry undoubtedly because of its stereochemically-active lone electron pair Ill].

The structural behaviour of both Zr4+ and Bis+ in these fluorite oxides is more typically covalent than ionic. The high charge and very small ion size of Zr4+ must result in a highly polarisable interaction with its oxygen neighbours. Similarly, the lone electron pair on the Bis+ ions is very polarisable. An increase in the covalent nature of the interaction would tend to increase the relative importance of the orbital configurations thus promoting the anisotropy in the local structural environment. Of the two systems, Bi20s displays more disorder which can be correlated with its greaater ionic conductivity.

The dopant ions adopt more nearly isotropic sites and it may be that the dopant cation stabilises the fluorite structure by absorbing the anion disorder and thus, rectifying local deviations from the fluorite structure's ~eriodicity. Diffraction data for the BizOs system showed that as the dopant concentration increased there was an increase in diffuse scattering attributed to a growth of a second ordered phase and it was predicted that this phase involved a symmetric distribution of the anion vacancies along the (111) body diagonal of the anion cube about the dopant cations. The order observed for the dopant's local structural environment as determined from the EXAFS data supports this model.

Although, the very short Bi-0 and Zr-0 bonds are clearly strong and the vibrations highly correlated, the more remote oxygen neighbours within the asymmetric %st shell' may be the source of the mobile species in the conducting phase. This would accord with the observation in both cases, that increasing the dopant concentration, decreases conductivity.

' ' ' h " " ' ' ' " ' " ' 2 ' ' '

2.5 5 7.5 10 12.5 15 k /

A-'

Fig. 4a - ksx(k) data for 40% Y20s/Bi20s a t the Y edge a t 80 K.

Radial Distance /

A

Fig. 4b - Fourier transform of data in 4a.

k / A-' Radial Distance /

A

Fig. 5a - kSx(k) data for 40% Y20s/Bi20s a t the Fig. 5b - Fourier transform of data in 5a.

Bi edge a t 80 K.

Acknowledgements

We thank the S.E.R.C. and staff of Daresbury Laboratory for the provision of synchrotron radiation and associated facilities used in the course of this work.

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[ll] R.D. Shannon, Acta Cryst. AS2 (1976) 751-67.