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The application of secondary ion mass spectrometry (SIMS) to the study of high temperature proton conductors (HTPC)
Solid State Ionics 97 (1997) 409–419
The application of secondary ion mass spectrometry (SIMS) to the study of high temperature proton conductors (HTPC) a, a b R.A. De Souza *, J.A. Kilner , C. Jeynes a
Department of Materials, Prince Consort Road, Imperial College of Science, Technology and Medicine, 2 W7 2 BP London UK b EPSRC Central Ion Beam Facility, Department of Electrical and Electronic Engineering, University of Surrey, Surrey, UK
Abstract The oxygen tracer diffusivity of CaZr 0.9 In 0.1 O 2.95 , SrCe 0.95 Yb 0.05 O 2.975 , and BaCe 0.9 La 0.1 O 2.95 at a nominal oxygen partial pressure of 1 atm (PH 2 O |10 26 atm) has been measured in the temperature range 500–10008C by means of 18 O / 16 O Isotope Exchange Depth Profiling (IEDP) with SIMS. The results are interpreted in terms of the ordering of oxygen vacancies in the orthorhombic structured perovskites. Negative secondary ion mass spectra (m /z516 to 19) obtained from SrCe 0.95 Yb 0.05 O 2.975 samples are also discussed, with particular reference to the presence of fluorine. Confirmation of fluorine contamination was provided by 19 F Proton Induced Gamma-ray Emission (PIGE) analysis. Keywords: Perovskite oxides; SIMS; IEDP; Oxygen diffusion; Fluorine Materials: CaZr 0.9 In 0.1 O 2.95 ; SrCe 0.95 Yb 0.05 O 2.975 ; BaCe 0.9 La 0.1 O 2.95
1. Introduction The analysis of hydrogen in solids in notoriously difficult by traditional analytical methods: there are few techniques that can detect hydrogen let alone provide quantitative analysis. Since the primary electrical conductivity measurements on the acceptor-doped alkaline earth cerate and zirconate perovskites, a number of different techniques have been employed in the hope of characterising the transport properties and elucidating the defect chemistry of these compounds. The use of ion beam techniques, in
particular Secondary Ion Mass Spectrometry (SIMS), has been limited. SIMS is a powerful technique for providing elemental analysis of the surface and near surface region of any vacuum compatible solid, and possesses the ability to detect all elements, differentiate between isotopes, provide spatial information, and determine concentrations over a wide range, routinely down to the ppm level. These strengths have not been exploited to their full potential in the study of High Temperature Proton Conductors (HTPC).
1.1. Secondary ion mass spectrometry ( SIMS) *Corresponding author. Present Address: Institut fur ¨ Werkstoffe ¨ Karlsruhe, Hertzstr. 16, 76187 der Electrotechnik, Universitat Karlsruhe, Germany. 0167-2738 / 97 / $17.00 1997 Elsevier Science B.V. All rights reserved PII S0167-2738( 97 )00038-6
The SIMS technique consists essentially of bombarding the sample of interest with a primary, low energy ion beam, usually of energy between 3 and
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15 keV. Interaction of the primary ions with the target causes material to be ejected from the sample surface. This process is called sputtering and produces a number of atomic and molecular species, most of which are neutral. A small proportion, however, are singly or even multiply charged and it is possible to collect some of the charged species (the secondary ions) through the application of a suitable electrical potential. These are energy and mass filtered before being counted. Mass separation may be accomplished with an electric quadrupole or magnetic sector mass spectrometer. A depth profile is obtained by continuously recording the intensities of the relevant secondary ions as the primary beam erodes into the sample. In general the major problems in SIMS analysis are: quantification, the need for a standard to convert the secondary ion intensity into concentration; mass interference, the occurrence of more than one secondary ion at the same mass to charge ratio; the residual gas effect, and the adsorption of gas in the main chamber (mostly H 2 , H 2 O and hydrocarbons) onto the sample surface during analysis.
1.2. Literature review There are two means of studying hydrogen in oxides by means of SIMS: atomic ion analysis (e.g. H 2), and molecular ion analysis (e.g. OH 2). Ishigaki et al. employed the first method in their SIMS study of deuterium in SrCe 0.95 Yb 0.05 O 2.975 as a function of temperature [1]. D 2 was used instead of H 2 in order to minimise the effect of residual gas, and the measured secondary ion intensities were quantified with a Sr(OD) 2 standard. Although it is clear that the overall trend was correctly identified (the deuterium concentration fell as the anneal temperature was raised), it is possible that the deuterium concentrations obtained deviate appreciably from the true concentrations, because secondary ion intensities are known to vary with the chemical composition of the host matrix. Sr(OD) 2 was employed as the standard rather than a SrCe 0.95 Yb 0.05 O 2.975 specimen containing a known concentration of deuterium. A preferable, and indeed, more common method of quantification is to ion implant the element (isotope) of interest into the matrix at a suitable fluence and
energy. This provides a standard against which samples of unknown concentration may be calibrated with an accuracy better than 5%, provided the fluence is known accurately. Ohgi et al. [2] have concentrated on the molecular ion analysis, but have not comprehensively addressed the problems associated with measuring hydrogen in oxides with SIMS. Comparing negative secondary ion mass spectra from wet and dry annealed samples of SrCe 0.95 Yb 0.05 O 2.975 , they found that the signal at m /z517 (which they assigned to 16 O 1 H 2) was higher for the sample annealed in dry conditions. In contrast the signals at m /z518 and 19 were higher for the sample annealed in wet conditions. This led them to believe that these signals are more representative of proton concentration, and thus they assigned m /z518 to 16 O 1 H 2 2 and m /z519 to 16 O 1 H 2 3 . Besides the doubtful stability of such ions, the question of mass interference was not given adequate attention. In this case mass interferences arise from the existence of three naturally occurring oxygen isotopes ( 16 O 2 , 17 O 2 and 18 2 O ) and their respective OH 2 n species (Table 1). The possibility of fluorine contamination must also be considered (signal at m /z519 is more usually ascribed to 19 F in negative secondary ion spectra). In a previous publication [3], we reported the analysis of negative secondary ion mass spectra obtained from as received samples of SrCe 0.95 Yb 0.05 O 2.975 . It was concluded that the signals at m /z516 and 18 were due to 16 O 2 and 18 O 2 respectively. The signal at m /z517 was attributed to 16 1 2 O H ions, which originated from the sample bulk. Unequivocal assignment of m /z519 was not possible, although it was concluded that 19 F was the most probable source, as we found no evidence for
Table 1 Possible mass interferences for negative secondary ions in the m /z range 16 to 19 m /z
Ions
16 17 18 19
16
O O 18 O 19 F 17
16
O 1H O 1H 18 1 O H 17
16 17
O 1H 2 O 1H 2
16
O 1H 3
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the existence of the exotic OH 2 3 ion proposed by Ohgi et al. [2]. The electrical conductivity of the alkaline earth cerates and zirconates contains contributions from oxygen vacancies, protons, electrons and electron holes, and consequently exhibits a complex dependence upon temperature and oxygen and water partial pressures. There have been attempts to extract the various partial conductivities by fitting the experimental data to simple defect models [4–6]. In two of these studies [4,5], measurements were only made as a function of PO 2 at several temperatures, and the total conductivity partitioned into three /4 /4 components: n-type (P 21 ), p-type (P 11 ), and O2 O2 ionic (PO 2 independent). In both cases the ionic contribution was ascribed to oxygen ions, although this method cannot differentiate between oxygen ion and proton transport as neither displays a dependence upon PO 2 . In contrast Kurita et al. measured the conductivity as a function of PO 2 , PH 2 O , and PD 2 O , and thus were able to distinguish between the two ionic transport processes [6]. The activation energies for oxygen ion transport quoted in the literature show variation both within one system, e.g. 0.5 to 0.8 eV for BaCeO 3 [4,7] and for the alkaline earth cerate and zirconate perovskite family, e.g. from 0.5 / 0.8 eV for BaCeO 3 to 2.5 eV for CaZrO 3 [6,8]. In contrast the activation energies for proton transport only vary from |0.5 eV for BaCeO 3 to |0.75 eV for CaZrO 3 . Moreover, 2.5 eV appears to be far too high to solely represent oxygen vacancy migration, yet no questions have been raised regarding this in the literature.
1.3. This work In their investigations Ohgi et al. [2] carried out anneals on samples of SrCe 0.95 Yb 0.05 O 2.975 either in nominally dry, oxidizing conditions (air, undefined PH 2 O ), or in wet, reducing conditions (‘humidified’ H 2 ). Consequently the reported variations in the secondary ion intensities may have resulted from the change in the water activity, or the change in the oxygen activity, or possibly some combination thereof. It was therefore decided to investigate the effects of varying PH 2 O and PO 2 separately. The behaviour
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as a function of PH 2 O at constant (high) PO 2 is detailed here, and follows on from our previous publication on negative molecular ion analysis [3]. In the present work oxygen tracer diffusion data for CaZr 0.9 In 0.1 O 2.95 , SrCe 0.95 Yb 0.05 O 2.975 , and BaCe 0.9 La 0.1 O 2.95 are also presented, some of which were reported in a preliminary study [9]. The 18 O / 16 O IEDP SIMS technique was employed to investigate oxygen transport. This method has been successful in the unambiguous determination of oxygen tracer diffusion coefficients in mixed conducting oxides, in which sel 4 sion [10]. Here, it is applied to a different sort of mixed conductors: mixed ionic conductors. It is important to determine the oxygen transport behaviour of these compounds more thoroughly, not only in view of the differences in the reported activation energies, but also with the aim of establishing the ionic domains and of elucidating the relationship between the two ionic conduction processes.
2. Experimental Ceramic samples of CaZr 0.9 In 0.1 O 2.95 and SrCe 0.95 Yb 0.05 O 2.975 were gratefully received from Dr. T. Yajima (TYK Corp., Japan); samples of BaCe 0.9 La 0.1 O 2.95 , from Dr. K.-D. Kreuer (MPI, Stuttgart, Germany). Preparation for IEDP experiments consisted of polishing with successive grades of diamond polish (final polish ]14 mm), prior to an equilibration anneal in 16 O 2 . The isotope exchange was carried out in an atmosphere of 93% enriched 18 O 2 gas. The water content of the annealing gases was measured in situ using a moisture meter (Shaw Ltd.) and both were found to be less than 2 ppm. The diffusion profiles were determined on an Atomika 6500 SIMS instrument with a 10 keV Xe 1 or Cs 1 primary beam. A 2 keV electron beam was used to provide charge compensation. For profiles less than 10 mm the machine was operated in depth profile mode. The crater depth was measured post analysis using a surface profilometer. For penetration profiles greater than 100 mm the linescan technique was employed. Details of both are given elsewhere [10].
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The oxygen self diffusion and surface exchange coefficients are obtained by fitting C9(x), the experimental data corrected for the natural isotope concentration of 18 O (50.2%) and the concentration of the gas (593%), to
F
x C9(x) 5 erfc ]] ] Π2 D*t
G
D S F S œ DG
kx k 2t x 2 exp ] 1 ] 3 erfc ]] ] ΠD* D* 2 D*t ] t 1k ] D*
targets can be treated as the linear superposition of elemental targets.
3. Results and discussion
3.1. Wet and dry anneals
(1)
where D* is the tracer diffusion coefficient, k is the surface exchange coefficient and t is the time of the 18 O isotope anneal. Polished specimens of SrCe 0.95 Yb 0.05 O 2.975 were annealed at 6008C for 10 h, one in dry oxygen (PH 2 O ,2310 26 atm), another in wet oxygen (PH 2 O | 7310 22 atm). The water contents for humidified conditions were assumed to be determined by the water vapour pressure at the temperature of the water bath. To ensure saturation of the carrier gas, a dual Drecksel bottle arrangement was used, and the rig was heated to above the temperature of the water bath to prevent condensation. Acquisition of negative secondary ion mass spectra was accomplished with a 10 keV Cs 1 beam with a 2 keV electron beam for charge compensation. High primary ion current density conditions were used to minimise residual gas absorption [3]. The possibility of 19 F incorporation was investigated by means of a resonance method of prompt radiation analysis: Proton Induced Gamma-ray Emission (PIGE). The 872 keV resonance of the 19 F( 1 H,ga )16 O reaction of width 4.5 keV was used in analysis. The g ray yield was measured as a function of beam energy. The absolute concentration of fluorine was obtained by calibration against an AlF 3 thin film standard, which was fabricated by holding a piece of Al foil over HF for 2.5 min, and is considered to be a reasonably well defined fluoride. To establish the relationship between the beam energy and the depth of emission, it is necessary to know the stopping power of SrCe 0.95 Yb 0.05 O 2.975 for protons. This was calculated from the elemental stopping powers [11] assuming that compound
Negative secondary ion mass spectra were obtained from three specimens of SrCe 0.95 Yb 0.05 O 2.975 : received, dry annealed and wet annealed. Selected secondary ion ratios, calculated from the spectra, are given in Table 2. Unfortunately quantitative comparisons with the values determined by Ohgi et al. [2] are not possible, as signals were normalised, in their case, against the m /z532 ( 16 O 2 ) signal rather than the m /z516 ( 16 O 2) signal. First, the results for the dry annealed sample are compared to those for the as-received sample. It is seen that the S 17 / S 16 ratio fell almost to the natural isotopic level ( 17 O / 16 O50.04%), but that no change was detected for the other ratios. The decrease in S 17 / S 16 implies that the m /z517 signal is representative of bulk hydrogen (though not necessarily quantitative) and that annealing in dry conditions removes protons from the sample. Consequently the fact that no change was observed for either of the other ratios is consistent with neither the m /z518 signal nor the m /z519 signal being due to hydrogen containing species. Secondly, comparing the ratios for the wet annealed sample to those for the as received sample, it is found that S 17 / S 16 was lower for the wet annealed sample than for the as received sample, while S 19 / S 16 was higher for the wet annealed sample. Once again there was no change detected in S 18 / S 16 . Based on the conclusions reached in our previous work, the increase in the S 19 / S 16 ratio indicates that further fluorine was introduced during the wet oxyTable 2 Selected secondary ion ratios obtained from samples of SrCe 0.95 Yb 0.05 O 2.975 annealed in dry O 2 or wet O 2 at 6008C, or as-received
S 17 / S 16 S 18 / S 16 S 19 / S 16
As-received
Dry O 2 anneal
Wet O 2 anneal
0.20% 0.20% 0.04%
0.05% 0.20% 0.04%
0.14% 0.20% 0.12%
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gen anneal, while the decrease in the S 17 / S 16 ratio indicates that protons were removed from the sample during the wet anneal. These changes are identical to those seen by Ohgi et al. [2] and are discussed further in the following section. On the other hand, here the S 18 / S 16 ratio was found to be independent of the water partial pressure of the annealing gas, whereas they observed an increase in the m /z518 signal. If this is a real effect, it must be related to the use of H 2 as the annealing gas. However, the reason for this increase is not clear, given that m /z518 is due to 18 O 2 and that no sensible mass interferences can be found.
9 ] 5 2[V O?? ] 1 [OH O? ] 1 [F O? ] [Yb Ce
3.2. Fluorine contamination
while the dry annealed sample may be described by
The wet oxygen annealed sample of SrCe 0.95 Yb 0.05 O 2.975 sample was examined with PIGE. Analysis explicitly confirmed the presence of 19 F within this sample; the concentration of fluorine was calculated to maintain a constant level of 0.0860.01 at.% down to 0.7 mm. An approximate comparison with the SIMS data can be made, which yields a concentration of 0.07 at.%. The agreement between PIGE and SIMS analysis implies that the S 19 / S 16 ratio is a fair indication of the fluorine concentration. Thus, fluorine is present in the asreceived sample; it may have been introduced either during fabrication or during preparation for analysis (cutting and polishing). Although the amount of fluorine determined by PIGE analysis does not seem to be significant, the maximum vacancy concentration (dopant cations compensated entirely by anion vacancies) is only 0.5 at.%. Therefore 0.08 at.% F may have a significant effect on the defect chemistry of these perovskite oxides. For instance, fluorine contamination may explain some of the discrepancies concerning the concentration of protons within these materials. O 22 and F ions possess very similar ionic radii [12], and it is therefore considered that fluorine ¨ substitutes for oxygen, denoted F ?O in Kroger–Vink notation. Hence, the electroneutrality condition, commonly used for these materials [Yb 9Ce ] 5 2[V ??O ] 1 [OH O? ] now becomes,
(2)
(3)
where dopant cations are compensated by oxygen vacancies, hydroxide ions, and fluorine ions, and the concentration of electronic defects is assumed to be small, as before. The trends observed for the secondary ion ratios upon annealing may now be explained by making some assumptions concerning the dominant defects for each anneal state. In other words it is assumed that the as received and wet annealed samples may be described by the simplified expression [Yb 9Ce ] 5 [OH O ? ] 1 [F O ? ]
[Yb 9Ce ] 5 2[VO ? ] 1 [F O ? ].
(4)
(5)
Therefore upon annealing the as received sample in wet conditions, which increased the concentration of fluorine (S 19 / S 16 ), the concentration of protons (S 17 / S 16 ) fell as a consequence (see Eq. (4)). Annealing the as-received sample in dry conditions did not alter the fluorine concentration, but simply reduced concentration of protons, whilst increasing the concentration of oxygen vacancies (i.e. transition from Eq. (4) to Eq. (5)). Although the presence of fluorine may be regarded with some suspicion, many fluorides are prepared by the reaction of an oxide with HF, F 2 or NH 4 F [13]; moreover, the partial substitution of fluorine for oxygen in high T c superconducting oxides (perovskite related structures) has been demonstrated [14]. Clearly within perovskite structures, substitutions based on ionic radii criteria are possible on the anion sublattice (e.g. OH or F for O) as well as the cation sublattices (e.g. Yb for Ce).
3.3. Oxygen transport In view of the conclusions reached in the previous section, the fluorine concentration in each material is estimated from the S 19 / S 16 ratios obtained at background 18 O concentration (i.e. 0.2%), to avoid interference from 18 O 1 H 2 . Values were, as expected, not affected by the annealing temperature and are listed in Table 3. The materials clearly fall into two groups: BaCe 0.9 La 0.1 O 2.95 which displays high 19 F
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Table 3 Estimated fluorine concentrations compared to the acceptor dopant level for the materials of interest 19
BaCe 0.9 La 0.1 O 2.95 SrCe 0.95 Yb 0.05 O 2.975 CaZr 0.9 In 0.1 O 2.95
F (at.%)
0.38 0.02 0.01
Dopant (at.%) 2 1 2
concentrations; and SrCe 0.95 Yb 0.05 O 2.975 and CaZr 0.9 In 0.1 O 2.95 , which display low 19 F concentrations. The former was supplied by Dr. K.-D. Kreuer; the latter by Dr. T. Yajima. Therefore, the difference in fluorine concentration between the two groups may be ascribed to the different fabrication routes. In the case of SrCe 0.95 Yb 0.05 O 2.975 and CaZr 0.9 In 0.1 O 2.95 , the fluorine concentrations are negligible in comparison with the acceptor dopant concentration. In contrast BaCe 0.9 La 0.1 O 2.95 contain significant amounts of fluorine. As a consequence the concentration of oxygen vacancies is no longer defined solely by the acceptor dopant level, but is reduced by the donor doping effect of the fluorine (Eq. (5)). A typical 18 O penetration plot is given in Fig. 1. It is seen from the good agreement between the experimental data and the fitted curve that high quality diffusion data can be obtained by this method. The oxygen tracer diffusion and surface exchange coefficients for each of the three compositions are
Fig. 1. 18 O penetration profile for SrCe 0.95 Yb 0.05 O 2.975 together with fitted curve. 18 O anneal for 500 s at 6078C.
now presented in turn. The activation energies stated on the graphs were determined by regression analysis; the errors given are the standard uncertainties associated with fitting the values of D* and k to Arrhenius type behaviour. Values of D* and k for CaZr 0.9 In 0.1 O 2.95 are shown in Fig. 2 as a function of inverse temperature. Two depth profiles were carried out on the same sample for each temperature investigated. While the high temperature values for D* and k are of extremely high precision, the low temperature data shows some scatter. This is attributed to the shallow 18 O penetration depths in these samples. The Arrhenius plot for the oxygen tracer diffusion and surface exchange coefficients obtained for SrCe 0.95 Yb 0.05 O 2.975 is given in Fig. 3. As for CaZr 0.9 In 0.1 O 2.95 , two measurements were made on each sample: depth profiling was employed for the two low temperature profiles and linescanning for the high temperature ones. Besides containing high concentrations of fluorine, the samples of BaCe 0.9 La 0.1 O 2.95 suffered from poor mechanical integrity, which meant that sample preparation was fraught with problems; many of the samples also had to be analysed by linescanning (on account of the high diffusivities), which thus entailed further cutting and polishing. All this is inevitably reflected in the quality of the data, which are shown in Fig. 4. It is seen that, in the case of
Fig. 2. Oxygen tracer diffusion and surface exchange coefficients for CaZr 0.9 In 0.1 O 2.95 as a function of inverse temperature at a nominal oxygen partial pressure of 1 atm.
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Table 4 Activation energies obtained for oxygen tracer diffusion and surface exchange
BaCe 0.9 La 0.1 O 2.95 SrCe 0.95 Yb 0.05 O 2.975 CaZr 0.9 In 0.1 O 2.95
DHD * (eV)
DHk (eV)
0.960.1 1.9160.07 2.3060.05
0.660.4 1.460.1 1.960.2
Errors are standard errors.
Fig. 3. Oxygen tracer diffusion and surface exchange coefficients as a function of inverse temperature for SrCe 0.95 Yb 0.05 O 2.975 at a nominal oxygen pressure of 1 atm.
Fig. 4. Oxygen tracer diffusion and surface exchange coefficients as a function of inverse temperature for BaCe 0.9 La 0.1 O 2.95 at a nominal oxygen pressure of 1 atm.
BaCe 0.9 La 0.1 O 2.95 , a dramatic increase in D* of around five orders of magnitude (and in k of two orders of magnitude) was found between 500 and 6008C. It should be noted though that there is a large error (factor of 5, at least) associated with the values of D* and k at 5008C, due to the extremely small 18 O profile length; for the anneal time used, a far higher penetration depth was clearly expected. Nevertheless, there is a considerable change in D* and k at low temperatures.
3.3.1. Oxygen tracer diffusion The activation energies for oxygen tracer diffusion are summarised in Table 4; they are, with the exception of BaCe 0.9 La 0.1 O 2.95 , really too high to represent migration enthalpies alone, cf. [15,16]. Assuming that the oxygen vacancy concentration is constant – this point will be returned to later – it is thought that the additional contribution arises from the trapping of oxygen vacancies, and this may be either by association between dopant cations and oxygen vacancies, or by superlattice ordering. Evidence of vacancy ordering in the alkaline earth cerate perovskites was first suggested by the work of Knight and Bonanos [17]. Neutron powder diffraction indicated that at room temperature the vacancies in Y-doped BaCeO 3 were confined exclusively to one of the two oxygen positions in the orthorhombic structure. Ranlov et al., in their neutron diffraction study of SrCe 0.85 Y 0.15 O 2.925 [18], similarly found that the oxygen vacancies were situated at the apical positions of the CeO 6 octahedra. They proposed that this is due to the longer Ce–O bond length (and therefore weaker electrostatic potential) associated with the apical position. For CaZr 0.9 In 0.1 O 2.95 and SrCe 0.95 Yb 0.05 O 2.975 it is therefore believed that the measured activation energy, DHD * , is the sum of DHM , the oxygen vacancy migration enthalpy, and DH O , a term which reflects the fact that the oxygen vacancies are ordered. In defect chemistry, such systems may be regarded either as oxygen deficient perovskites with defect clustering, or as vacancy ordered structures with pseudo oxygen Frenkel disorder. Both frameworks, though, lead to the conclusion that trapping reduces the number of vacancies that contribute to diffusion, that is there is a dynamic equilibrium between the free vacancies and those that are trapped by ordering. Superlattice ordering not only accounts for the high activation energies determined for
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CaZr 0.9 In 0.1 O 2.95 (2.30 eV) and SrCe 0.95 Yb 0.05 O 2.975 (1.91 eV) but also provides an explanation for the sudden increase in D* observed for BaCe 0.9 La 0.1 O 2.95 . The room temperature crystal structure of barium cerate is known to be orthorhombic, and hence contains two distinct oxygen sites. At higher temperatures, a transformation occurs to a rhombohedral structure, for which there is but a single oxygen site. Thus, the oxygen sublattice would be expected to disorder at the transition temperature, resulting in an increase in the number of free oxygen vacancies and a decrease in the activation energy. Knight [19] reported rhombohedral symmetry above 4008C for undoped barium cerate, but around 1008C higher for BaCe 0.9 Y 0.1 O 2.95 – the temperature at which the sharp increase in D* was observed for BaCe 0.9 La 0.1 O 2.95 . It is emphasised that possible contributions from defect associates have not been ruled out: it is merely proposed that ordering provides the dominant term. Smyth [20] states that the high polarizability of perovskite oxides favours the stabilisation of ordered structures and reduces the tendency for defect association. However, atomistic computer simulations of cubic BaCeO 3 by Hill and Cormack [21] have indicated favourable binding energies (0.3 to 0.6 eV) 9 –V O?? )? associates. for (M Ce The trapping of oxygen vacancies has important implications for ionic transport in these materials, specifically the question of oxygen ion versus proton conduction. The behaviour of BaCe 0.9 La 0.1 O 2.95 indicates that the disordering of the oxygen sublattice promotes oxygen ion conduction at the expense of proton conduction (the transition from proton to oxygen ion dominated transport starts at |6008C [22]). The converse is equally significant, if not more so: vacancy ordering ‘freezes’ the oxygen sublattice, thereby allowing proton conduction to dominate. This model would explain why compounds such as CeZr 0.9 In 0.1 O 2.95 and SrCe 0.95 Yb 0.05 O 2.975 , in comparison to BaCeO 3 materials, are predominantly proton conductors even at reasonably high temperatures (|10008C), as vacancy trapping is still effective. Indeed the study by Yajima et al. [23] on Ba 12x Ca x Ce 0.9 Nd 0.1 O 2.95 demonstrates that by increasing the degree of orthorhombic distortion (by raising the level of Ca substitution, x), oxygen vacancies are hindered from migrating, without greatly affecting proton conduction.
Central to the arguments detailed above concerning the high activation energies is the assumption of a constant total vacancy concentration. In the case of these AII B IV O 3 perovskites, it is expected that the total vacancy concentration is defined by the dopant concentration. Lattice oxidation, that is the reaction of oxygen vacancies with oxygen to give electron holes, does not result in large changes in oxygen vacancy concentration for these materials. The major concern, though, is the annihilation of vacancies by hydroxide ions (Eq. (2)). Based on numerical modelling of the defect structure of SrCe 0.95 Yb 0.05 O 2.975 [24], it is believed that, in the temperature range of interest, and due to the low water partial pressure in which the isotope anneals were carried out, protons were minority species and thus the oxygen vacancy concentration was, to a first approximation, unchanged (Fig. 5). Furthermore, the S 17 / S 16 ratios from the SIMS analyses (i.e. an indication of the OH 2 concentration) were all near the natural isotopic level of 17 O / 16 O of 0.04%. The oxygen diffusivities of the three compositions are compared in Fig. 6. Curiously the two cerate compositions exhibit very similar diffusivities compared to CaZr 0.9 In 0.1 O 2.95 . This is surprising given that BaCe 0.9 La 0.1 O 2.95 is characterised by a disordered oxygen sublattice, whereas in SrCe 0.95 Yb 0.05 O 2.975 , and CaZr 0.9 In 0.1 O 2.95 , the vacancies are thought to be trapped. It may have been expected from the trend seen for the
Fig. 5. Calculated concentrations of oxygen vacancies and protons in SrCe 0.95 Yb 0.05 O 2.975 as a function of inverse temperature at PH 2 O 510 24.5 . Data taken from Larring [24].
R. A. De Souza et al. / Solid State Ionics 97 (1997) 409 – 419
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studied in several AII B IV O 3 perovskites as a function of dopant cation and concentration.
3.4. Comparison with literature data The oxygen diffusion coefficients obtained in this work are compared with reported transport data by means of the Nernst–Einstein equation Nq 2 D* s 5 ]] kT
Fig. 6. Comparison of oxygen tracer diffusivities for the perovskite compositions investigated.
activation energies that BaCe 0.9 La 0.1 O 2.95 and CaZr 0.9 In 0.1 O 2.95 would represent the extremes, with SrCe 0.95 Yb 0.05 O 2.975 exhibiting intermediate behaviour. There are, however, several factors to be taken into account, from which it may be supposed that the diffusion coefficients obtained for BaCe 0.9 La 0.1 O 2.95 are lower than expected. Firstly, there is the widely differing fluorine concentrations between compositions; as already noted, this causes a reduction in the concentration of oxygen vacancies by |25%. Secondly, it is well known that electrical conductivity of the alkaline earth cerates and zirconates is dependent upon the size of the dopant cation relative to that of the host cation, e.g. see Refs. [25,26]. Yb 2 O 3 , and Y 2 O 3 to a lesser extent, are generally regarded as optimum dopants for SrCeO 3 . The same is not true of La 2 O 3 doped BaCeO 3 : Gd 2 O 3 , Nd 2 O 3 and Sm 2 O 3 are considered to be better. Thirdly, the ionic radius of ˚ in twelve-fold co-ordination (A site) La 31 is 1.36 A ˚ and 1.03 A in six-fold coordination (B site), in˚ and Ce 41 (0.87 A) ˚ between that of Ba 21 (1.61 A) [12]; therefore, the lanthanum ions may be distributed over the two sites. Such a possibility has been put forward by Kreuer et al. [7] for 10% Gd:BaCeO 3 , which they believed corresponded to a bulk composition of (Ba 0.965 Gd 0.035 )(Ce 0.935 Gd 0.065 )O 2.985 . The net acceptor concentration is thus far lower than the nominal one and the oxygen vacancy concentration is hence greatly diminished. To solve this problem satisfactorily, oxygen diffusion must be
(6)
where q, k and T have their usual meanings and correlation factors are ignored; N is the number of anion sites per unit volume. It is stressed that this equation exclusively defines the relationship between s and D* and does not require the concentration of vacancies to be included as this is contained within the transport parameters themselves. This allows the direct comparison of activation energies obtained from IEDP experiments with those from electrical measurements.
3.4.1. CaZr0.9 In0.1 O2.95 The oxygen ion conductivity, calculated from the pre-exponential term and activation energy derived, is plotted in Fig. 7 together with the data of Kurita et al. for CaZr 0.9 In 0.1 O 2.9 [6] and Wang et al. for various Y 2 O 3 -doped CaZrO 3 compositions [8]. Excellent agreement is found between the three sets of
Fig. 7. Comparison of oxygen ion conductivities for various acceptor-doped calcium zirconate compositions: (A) CaZr 0.9 In 0.1 O 2.95 , this work; (B) CaZr 0.9 In 0.1 O 2.95 , Kurita et al. [6]; (C) Y 2 O 3 -doped CaZrO 3 , Wang et al. [8].
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data, each determined by a different method, both in terms of the magnitude of the conductivity and the activation energy. The value of DHD * 52.3 eV determined here is close to Ea 52.560.5 eV calculated by Kurita et al., and well within the 2.0 to 2.7 eV range found by Wang et al.
3.4.2. SrCe0.95 Yb0.05 O2.975 In addition to the calculated oxygen ion conductivity, the results from two other groups are also shown in Fig. 8: the partial ionic conductivity calculated by Kosacki and Tuller [5], which they ascribed to oxygen ions; and the extrapolated (proton) conductivity data of Bonanos et al. [27] from measurements in an N 2 :5%H 2 atmosphere. It is evident from Fig. 8 that the data of Kosacki and Tuller cannot be attributed to oxygen ion conduction. It is proposed that the activation energy of 0.77 eV they determined refers to mixed oxygen ion and proton transport, being only slightly higher than the activation energy of 0.59 eV reported by Bonanos et al., and considerably less than DDH * 5 1.91 eV obtained here. Still, it is not clear why the mixed conductivity is lower than the calculated oxygen ion conductivity. Perhaps, secondary effects, e.g. microstructural, play a part. Comparing the
theoretical oxygen ion conductivity with the data of Bonanos et al., it appears that in an N 2 :5%H 2 atmosphere proton transport would dominate the conductivity at least up to temperatures of 10008C.
Fig. 8. Comparison of calculated oxygen ion conductivity A of SrCe 0.95 Yb 0.05 O 2.975 with the oxygen ion conductivity B determined by Kosacki and Tuller [5], and the extrapolated proton conductivity C from Bonanos et al. [27].
3.4.3. BaCe0.9 La0.2 O2.95 The theoretical oxygen ion conductivities are compared in Fig. 9 to the converted diffusivities obtained by Kreuer et al. [7] for BaCe 0.9 Gd 0.1 O 2.95 by means of 18 O / 16 O TGA, and the extracted ionic conductivity data of Bonanos for BaCe 0.9 Gd 0.1 O 2.95 [4], and of Paria and Maiti for BaCe 0.9 La 0.1 O 2.95 [28]. It is apparent that the values measured by Bonanos are significantly higher than the other data sets, which suggests that the PO 2 independent conductivity extracted by Bonanos, though free from electronic contributions, may nevertheless contain a proton component. This is supported by examination of the activation energies: 0.52 eV determined by Bonanos is significantly lower than 0.9 eV (this work), 0.8 eV (Kreuer et al.) and 0.75 eV (Paria and Maiti). It is also found that the Gd 2 O 3 -doped sample of Kreuer et al. exhibits higher conductivities than either of the La-doped samples: this is consistent with Gd 2 O 3 being the best dopant for BaCeO 3 .
Fig. 9. Comparison of oxygen ion conductivities for BaCe 0.9 M 0.1 O 2.95 : (A) BaCe 0.9 La 0.1 O 2.95 , this work; (B) BaCe 0.9 La 0.1 O 2.95 , Paria and Maiti [28]; (C) BaCe 0.9 Gd 0.1 O 2.95 , Kreuer et al. [7]; (D) BaCe 0.9 Gd 0.1 O 2.95 , Bonanos [4].
R. A. De Souza et al. / Solid State Ionics 97 (1997) 409 – 419
4. Conclusions Analysis of negative secondary ion mass spectra (m /z516 to 19) of SrCe 0.95 Yb 0.05 O 2.975 ceramics, annealed in various conditions, provided a qualitative indication of the amount of hydrogen and fluorine within the bulk of the samples as a function of annealing atmosphere. A nuclear reaction technique (PIGE) was used to confirm the presence of fluorine; good agreement was seen between the 19 F concentrations obtained from SIMS and PIGE analyses. The oxygen diffusion data obtained for CaZr 0.9 In 0.1 O 2.95 , SrCe 0.95 Yb 0.05 O 2.975 , and BaCe 0.9 La 0.1 O 2.95 indicated that oxygen vacancy ordering plays a decisive role in the transport characteristics of these perovskite materials.
Acknowledgments Financial assistance for RADS was obtained from EPSRC and Siemens AG.
References [1] T. Ishigaki, S. Yamauchi, K. Kishio, K. Fueki, H. Iwahara, Solid State Ionics 21 (1986) 239. [2] T. Ohgi, T. Namikawa, Y. Yamazaki, in: F.W. Poulsen, J.J. ˚ Bentzen, T. Jacobsen, E. Skou, M.J.L.Q. Østergard, Eds., Proc. 14th Risø Int. Symp. on Materials Science, Risø National Laboratory, Roskilde, Denmark, 1993, p. 357. [3] R.A. De Souza, J.A. Kilner, B.C.H. Steele, Solid State Ionics 77 (1995) 180. [4] N. Bonanos, J. Phys. Chem. Solids 54 (1993) 867. [5] I. Kosacki, H.L. Tuller, Solid State Ionics 80 (1995) 223. [6] N. Kurita, N. Fukatsu, K. Ito, T. Ohashi, J. Electrochem. Soc. 142 (1995) 1552.
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¨ [7] K.D. Kreuer, E. Schonherr, J. Maier, Solid State Ionics 70 / 71 (1994) 278. [8] C. Wang, X. Xu, H. Yu, Y. Wen, K. Zeao, Solid State Ionics 28–30 (1988) 542. [9] R.A. De Souza, G. Gibson, J.A. Kilner, in: B.C.H. Steele Ed., Ceramic Oxygen Ion Conductors and their Technological Application, Br. Ceram. Proc. No. 56, Inst. Materials, 1996, p. 95. [10] S. Carter, A. Selcuk, R.J. Chater, J. Kajda, J.A. Kilner, B.C.H. Steele, Solid State Ionics 53–56 (1992) 597. [11] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Ranges of Ions in Solids, Pergamon, New York, 1985. [12] R.D. Shannon, Acta Cryst. A32 (1976) 751. [13] R. Bougon, J. Ehretsmann, J. Portier, A. Tressaud, in: P. Hagenmuller, Ed., Preparative Methods in Solid State Chemistry, Academic Press, London, 1972, p. 401. [14] C. Greaves, M. Almamouri, P.R. Slater, P.P. Edwards, Physica C 235 (1994) 158. [15] J.A. Kilner, R.J. Brook, Solid State Ionics 6 (1982) 237. [16] T. Ishihara, H. Matsuda, Y. Takita, Solid State Ionics 79 (1995) 147. [17] K.S. Knight, M. Soar, N. Bonanos, J. Mater. Chem. 2 (1992) 709. [18] J. Ranløv, B. Lebech, K. Nielsen, J. Mater. Chem. 5 (1995) 743. [19] K.S. Knight, Solid State Ionics 74 (1994) 109. [20] D.M. Smyth, Ann. Rev. Mater. Sci. 15 (1985) 329. [21] S.E. Hill, A.N. Cormack, in: T. Ramanarayanan, W.L. Worrell and H.L. Tuller, Eds., Proc. 2nd Int. Symp. on Ionic and Mixed Conducting Ceramics, Electrochemical Soc., Pennington, NJ, 1994, p. 394. [22] N. Bonanos, Solid State Ionics 53–56 (1992) 967. [23] T. Yajima, H. Iwahara, H. Uchida, Solid State Ionics 47 (1991) 117. [24] Y. Larring, University of Oslo, unpublished results. [25] T. Yajima, H. Kazeoka, T. Togo, H. Iwahara, Solid State Ionics 47 (1991) 271. [26] D.A. Stevenson, N. Jiang, R.M. Buchanan, F.E.G. Henn, Solid State Ionics 62 (1993) 279. [27] N. Bonanos, B. Ellis, M.N. Mahmood, Solid State Ionics 28–30 (1988) 579. [28] M.K. Paria, H.S. Maiti, Solid State Ionics 13 (1984) 285.
The application of secondary ion mass spectrometry (SIMS) to the study of high temperature proton conductors (HTPC) a, a b R.A. De Souza *, J.A. Kilner , C. Jeynes a
Department of Materials, Prince Consort Road, Imperial College of Science, Technology and Medicine, 2 W7 2 BP London UK b EPSRC Central Ion Beam Facility, Department of Electrical and Electronic Engineering, University of Surrey, Surrey, UK
Abstract The oxygen tracer diffusivity of CaZr 0.9 In 0.1 O 2.95 , SrCe 0.95 Yb 0.05 O 2.975 , and BaCe 0.9 La 0.1 O 2.95 at a nominal oxygen partial pressure of 1 atm (PH 2 O |10 26 atm) has been measured in the temperature range 500–10008C by means of 18 O / 16 O Isotope Exchange Depth Profiling (IEDP) with SIMS. The results are interpreted in terms of the ordering of oxygen vacancies in the orthorhombic structured perovskites. Negative secondary ion mass spectra (m /z516 to 19) obtained from SrCe 0.95 Yb 0.05 O 2.975 samples are also discussed, with particular reference to the presence of fluorine. Confirmation of fluorine contamination was provided by 19 F Proton Induced Gamma-ray Emission (PIGE) analysis. Keywords: Perovskite oxides; SIMS; IEDP; Oxygen diffusion; Fluorine Materials: CaZr 0.9 In 0.1 O 2.95 ; SrCe 0.95 Yb 0.05 O 2.975 ; BaCe 0.9 La 0.1 O 2.95
1. Introduction The analysis of hydrogen in solids in notoriously difficult by traditional analytical methods: there are few techniques that can detect hydrogen let alone provide quantitative analysis. Since the primary electrical conductivity measurements on the acceptor-doped alkaline earth cerate and zirconate perovskites, a number of different techniques have been employed in the hope of characterising the transport properties and elucidating the defect chemistry of these compounds. The use of ion beam techniques, in
particular Secondary Ion Mass Spectrometry (SIMS), has been limited. SIMS is a powerful technique for providing elemental analysis of the surface and near surface region of any vacuum compatible solid, and possesses the ability to detect all elements, differentiate between isotopes, provide spatial information, and determine concentrations over a wide range, routinely down to the ppm level. These strengths have not been exploited to their full potential in the study of High Temperature Proton Conductors (HTPC).
1.1. Secondary ion mass spectrometry ( SIMS) *Corresponding author. Present Address: Institut fur ¨ Werkstoffe ¨ Karlsruhe, Hertzstr. 16, 76187 der Electrotechnik, Universitat Karlsruhe, Germany. 0167-2738 / 97 / $17.00 1997 Elsevier Science B.V. All rights reserved PII S0167-2738( 97 )00038-6
The SIMS technique consists essentially of bombarding the sample of interest with a primary, low energy ion beam, usually of energy between 3 and
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15 keV. Interaction of the primary ions with the target causes material to be ejected from the sample surface. This process is called sputtering and produces a number of atomic and molecular species, most of which are neutral. A small proportion, however, are singly or even multiply charged and it is possible to collect some of the charged species (the secondary ions) through the application of a suitable electrical potential. These are energy and mass filtered before being counted. Mass separation may be accomplished with an electric quadrupole or magnetic sector mass spectrometer. A depth profile is obtained by continuously recording the intensities of the relevant secondary ions as the primary beam erodes into the sample. In general the major problems in SIMS analysis are: quantification, the need for a standard to convert the secondary ion intensity into concentration; mass interference, the occurrence of more than one secondary ion at the same mass to charge ratio; the residual gas effect, and the adsorption of gas in the main chamber (mostly H 2 , H 2 O and hydrocarbons) onto the sample surface during analysis.
1.2. Literature review There are two means of studying hydrogen in oxides by means of SIMS: atomic ion analysis (e.g. H 2), and molecular ion analysis (e.g. OH 2). Ishigaki et al. employed the first method in their SIMS study of deuterium in SrCe 0.95 Yb 0.05 O 2.975 as a function of temperature [1]. D 2 was used instead of H 2 in order to minimise the effect of residual gas, and the measured secondary ion intensities were quantified with a Sr(OD) 2 standard. Although it is clear that the overall trend was correctly identified (the deuterium concentration fell as the anneal temperature was raised), it is possible that the deuterium concentrations obtained deviate appreciably from the true concentrations, because secondary ion intensities are known to vary with the chemical composition of the host matrix. Sr(OD) 2 was employed as the standard rather than a SrCe 0.95 Yb 0.05 O 2.975 specimen containing a known concentration of deuterium. A preferable, and indeed, more common method of quantification is to ion implant the element (isotope) of interest into the matrix at a suitable fluence and
energy. This provides a standard against which samples of unknown concentration may be calibrated with an accuracy better than 5%, provided the fluence is known accurately. Ohgi et al. [2] have concentrated on the molecular ion analysis, but have not comprehensively addressed the problems associated with measuring hydrogen in oxides with SIMS. Comparing negative secondary ion mass spectra from wet and dry annealed samples of SrCe 0.95 Yb 0.05 O 2.975 , they found that the signal at m /z517 (which they assigned to 16 O 1 H 2) was higher for the sample annealed in dry conditions. In contrast the signals at m /z518 and 19 were higher for the sample annealed in wet conditions. This led them to believe that these signals are more representative of proton concentration, and thus they assigned m /z518 to 16 O 1 H 2 2 and m /z519 to 16 O 1 H 2 3 . Besides the doubtful stability of such ions, the question of mass interference was not given adequate attention. In this case mass interferences arise from the existence of three naturally occurring oxygen isotopes ( 16 O 2 , 17 O 2 and 18 2 O ) and their respective OH 2 n species (Table 1). The possibility of fluorine contamination must also be considered (signal at m /z519 is more usually ascribed to 19 F in negative secondary ion spectra). In a previous publication [3], we reported the analysis of negative secondary ion mass spectra obtained from as received samples of SrCe 0.95 Yb 0.05 O 2.975 . It was concluded that the signals at m /z516 and 18 were due to 16 O 2 and 18 O 2 respectively. The signal at m /z517 was attributed to 16 1 2 O H ions, which originated from the sample bulk. Unequivocal assignment of m /z519 was not possible, although it was concluded that 19 F was the most probable source, as we found no evidence for
Table 1 Possible mass interferences for negative secondary ions in the m /z range 16 to 19 m /z
Ions
16 17 18 19
16
O O 18 O 19 F 17
16
O 1H O 1H 18 1 O H 17
16 17
O 1H 2 O 1H 2
16
O 1H 3
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the existence of the exotic OH 2 3 ion proposed by Ohgi et al. [2]. The electrical conductivity of the alkaline earth cerates and zirconates contains contributions from oxygen vacancies, protons, electrons and electron holes, and consequently exhibits a complex dependence upon temperature and oxygen and water partial pressures. There have been attempts to extract the various partial conductivities by fitting the experimental data to simple defect models [4–6]. In two of these studies [4,5], measurements were only made as a function of PO 2 at several temperatures, and the total conductivity partitioned into three /4 /4 components: n-type (P 21 ), p-type (P 11 ), and O2 O2 ionic (PO 2 independent). In both cases the ionic contribution was ascribed to oxygen ions, although this method cannot differentiate between oxygen ion and proton transport as neither displays a dependence upon PO 2 . In contrast Kurita et al. measured the conductivity as a function of PO 2 , PH 2 O , and PD 2 O , and thus were able to distinguish between the two ionic transport processes [6]. The activation energies for oxygen ion transport quoted in the literature show variation both within one system, e.g. 0.5 to 0.8 eV for BaCeO 3 [4,7] and for the alkaline earth cerate and zirconate perovskite family, e.g. from 0.5 / 0.8 eV for BaCeO 3 to 2.5 eV for CaZrO 3 [6,8]. In contrast the activation energies for proton transport only vary from |0.5 eV for BaCeO 3 to |0.75 eV for CaZrO 3 . Moreover, 2.5 eV appears to be far too high to solely represent oxygen vacancy migration, yet no questions have been raised regarding this in the literature.
1.3. This work In their investigations Ohgi et al. [2] carried out anneals on samples of SrCe 0.95 Yb 0.05 O 2.975 either in nominally dry, oxidizing conditions (air, undefined PH 2 O ), or in wet, reducing conditions (‘humidified’ H 2 ). Consequently the reported variations in the secondary ion intensities may have resulted from the change in the water activity, or the change in the oxygen activity, or possibly some combination thereof. It was therefore decided to investigate the effects of varying PH 2 O and PO 2 separately. The behaviour
411
as a function of PH 2 O at constant (high) PO 2 is detailed here, and follows on from our previous publication on negative molecular ion analysis [3]. In the present work oxygen tracer diffusion data for CaZr 0.9 In 0.1 O 2.95 , SrCe 0.95 Yb 0.05 O 2.975 , and BaCe 0.9 La 0.1 O 2.95 are also presented, some of which were reported in a preliminary study [9]. The 18 O / 16 O IEDP SIMS technique was employed to investigate oxygen transport. This method has been successful in the unambiguous determination of oxygen tracer diffusion coefficients in mixed conducting oxides, in which sel 4 sion [10]. Here, it is applied to a different sort of mixed conductors: mixed ionic conductors. It is important to determine the oxygen transport behaviour of these compounds more thoroughly, not only in view of the differences in the reported activation energies, but also with the aim of establishing the ionic domains and of elucidating the relationship between the two ionic conduction processes.
2. Experimental Ceramic samples of CaZr 0.9 In 0.1 O 2.95 and SrCe 0.95 Yb 0.05 O 2.975 were gratefully received from Dr. T. Yajima (TYK Corp., Japan); samples of BaCe 0.9 La 0.1 O 2.95 , from Dr. K.-D. Kreuer (MPI, Stuttgart, Germany). Preparation for IEDP experiments consisted of polishing with successive grades of diamond polish (final polish ]14 mm), prior to an equilibration anneal in 16 O 2 . The isotope exchange was carried out in an atmosphere of 93% enriched 18 O 2 gas. The water content of the annealing gases was measured in situ using a moisture meter (Shaw Ltd.) and both were found to be less than 2 ppm. The diffusion profiles were determined on an Atomika 6500 SIMS instrument with a 10 keV Xe 1 or Cs 1 primary beam. A 2 keV electron beam was used to provide charge compensation. For profiles less than 10 mm the machine was operated in depth profile mode. The crater depth was measured post analysis using a surface profilometer. For penetration profiles greater than 100 mm the linescan technique was employed. Details of both are given elsewhere [10].
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412
The oxygen self diffusion and surface exchange coefficients are obtained by fitting C9(x), the experimental data corrected for the natural isotope concentration of 18 O (50.2%) and the concentration of the gas (593%), to
F
x C9(x) 5 erfc ]] ] Π2 D*t
G
D S F S œ DG
kx k 2t x 2 exp ] 1 ] 3 erfc ]] ] ΠD* D* 2 D*t ] t 1k ] D*
targets can be treated as the linear superposition of elemental targets.
3. Results and discussion
3.1. Wet and dry anneals
(1)
where D* is the tracer diffusion coefficient, k is the surface exchange coefficient and t is the time of the 18 O isotope anneal. Polished specimens of SrCe 0.95 Yb 0.05 O 2.975 were annealed at 6008C for 10 h, one in dry oxygen (PH 2 O ,2310 26 atm), another in wet oxygen (PH 2 O | 7310 22 atm). The water contents for humidified conditions were assumed to be determined by the water vapour pressure at the temperature of the water bath. To ensure saturation of the carrier gas, a dual Drecksel bottle arrangement was used, and the rig was heated to above the temperature of the water bath to prevent condensation. Acquisition of negative secondary ion mass spectra was accomplished with a 10 keV Cs 1 beam with a 2 keV electron beam for charge compensation. High primary ion current density conditions were used to minimise residual gas absorption [3]. The possibility of 19 F incorporation was investigated by means of a resonance method of prompt radiation analysis: Proton Induced Gamma-ray Emission (PIGE). The 872 keV resonance of the 19 F( 1 H,ga )16 O reaction of width 4.5 keV was used in analysis. The g ray yield was measured as a function of beam energy. The absolute concentration of fluorine was obtained by calibration against an AlF 3 thin film standard, which was fabricated by holding a piece of Al foil over HF for 2.5 min, and is considered to be a reasonably well defined fluoride. To establish the relationship between the beam energy and the depth of emission, it is necessary to know the stopping power of SrCe 0.95 Yb 0.05 O 2.975 for protons. This was calculated from the elemental stopping powers [11] assuming that compound
Negative secondary ion mass spectra were obtained from three specimens of SrCe 0.95 Yb 0.05 O 2.975 : received, dry annealed and wet annealed. Selected secondary ion ratios, calculated from the spectra, are given in Table 2. Unfortunately quantitative comparisons with the values determined by Ohgi et al. [2] are not possible, as signals were normalised, in their case, against the m /z532 ( 16 O 2 ) signal rather than the m /z516 ( 16 O 2) signal. First, the results for the dry annealed sample are compared to those for the as-received sample. It is seen that the S 17 / S 16 ratio fell almost to the natural isotopic level ( 17 O / 16 O50.04%), but that no change was detected for the other ratios. The decrease in S 17 / S 16 implies that the m /z517 signal is representative of bulk hydrogen (though not necessarily quantitative) and that annealing in dry conditions removes protons from the sample. Consequently the fact that no change was observed for either of the other ratios is consistent with neither the m /z518 signal nor the m /z519 signal being due to hydrogen containing species. Secondly, comparing the ratios for the wet annealed sample to those for the as received sample, it is found that S 17 / S 16 was lower for the wet annealed sample than for the as received sample, while S 19 / S 16 was higher for the wet annealed sample. Once again there was no change detected in S 18 / S 16 . Based on the conclusions reached in our previous work, the increase in the S 19 / S 16 ratio indicates that further fluorine was introduced during the wet oxyTable 2 Selected secondary ion ratios obtained from samples of SrCe 0.95 Yb 0.05 O 2.975 annealed in dry O 2 or wet O 2 at 6008C, or as-received
S 17 / S 16 S 18 / S 16 S 19 / S 16
As-received
Dry O 2 anneal
Wet O 2 anneal
0.20% 0.20% 0.04%
0.05% 0.20% 0.04%
0.14% 0.20% 0.12%
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413
gen anneal, while the decrease in the S 17 / S 16 ratio indicates that protons were removed from the sample during the wet anneal. These changes are identical to those seen by Ohgi et al. [2] and are discussed further in the following section. On the other hand, here the S 18 / S 16 ratio was found to be independent of the water partial pressure of the annealing gas, whereas they observed an increase in the m /z518 signal. If this is a real effect, it must be related to the use of H 2 as the annealing gas. However, the reason for this increase is not clear, given that m /z518 is due to 18 O 2 and that no sensible mass interferences can be found.
9 ] 5 2[V O?? ] 1 [OH O? ] 1 [F O? ] [Yb Ce
3.2. Fluorine contamination
while the dry annealed sample may be described by
The wet oxygen annealed sample of SrCe 0.95 Yb 0.05 O 2.975 sample was examined with PIGE. Analysis explicitly confirmed the presence of 19 F within this sample; the concentration of fluorine was calculated to maintain a constant level of 0.0860.01 at.% down to 0.7 mm. An approximate comparison with the SIMS data can be made, which yields a concentration of 0.07 at.%. The agreement between PIGE and SIMS analysis implies that the S 19 / S 16 ratio is a fair indication of the fluorine concentration. Thus, fluorine is present in the asreceived sample; it may have been introduced either during fabrication or during preparation for analysis (cutting and polishing). Although the amount of fluorine determined by PIGE analysis does not seem to be significant, the maximum vacancy concentration (dopant cations compensated entirely by anion vacancies) is only 0.5 at.%. Therefore 0.08 at.% F may have a significant effect on the defect chemistry of these perovskite oxides. For instance, fluorine contamination may explain some of the discrepancies concerning the concentration of protons within these materials. O 22 and F ions possess very similar ionic radii [12], and it is therefore considered that fluorine ¨ substitutes for oxygen, denoted F ?O in Kroger–Vink notation. Hence, the electroneutrality condition, commonly used for these materials [Yb 9Ce ] 5 2[V ??O ] 1 [OH O? ] now becomes,
(2)
(3)
where dopant cations are compensated by oxygen vacancies, hydroxide ions, and fluorine ions, and the concentration of electronic defects is assumed to be small, as before. The trends observed for the secondary ion ratios upon annealing may now be explained by making some assumptions concerning the dominant defects for each anneal state. In other words it is assumed that the as received and wet annealed samples may be described by the simplified expression [Yb 9Ce ] 5 [OH O ? ] 1 [F O ? ]
[Yb 9Ce ] 5 2[VO ? ] 1 [F O ? ].
(4)
(5)
Therefore upon annealing the as received sample in wet conditions, which increased the concentration of fluorine (S 19 / S 16 ), the concentration of protons (S 17 / S 16 ) fell as a consequence (see Eq. (4)). Annealing the as-received sample in dry conditions did not alter the fluorine concentration, but simply reduced concentration of protons, whilst increasing the concentration of oxygen vacancies (i.e. transition from Eq. (4) to Eq. (5)). Although the presence of fluorine may be regarded with some suspicion, many fluorides are prepared by the reaction of an oxide with HF, F 2 or NH 4 F [13]; moreover, the partial substitution of fluorine for oxygen in high T c superconducting oxides (perovskite related structures) has been demonstrated [14]. Clearly within perovskite structures, substitutions based on ionic radii criteria are possible on the anion sublattice (e.g. OH or F for O) as well as the cation sublattices (e.g. Yb for Ce).
3.3. Oxygen transport In view of the conclusions reached in the previous section, the fluorine concentration in each material is estimated from the S 19 / S 16 ratios obtained at background 18 O concentration (i.e. 0.2%), to avoid interference from 18 O 1 H 2 . Values were, as expected, not affected by the annealing temperature and are listed in Table 3. The materials clearly fall into two groups: BaCe 0.9 La 0.1 O 2.95 which displays high 19 F
R. A. De Souza et al. / Solid State Ionics 97 (1997) 409 – 419
414
Table 3 Estimated fluorine concentrations compared to the acceptor dopant level for the materials of interest 19
BaCe 0.9 La 0.1 O 2.95 SrCe 0.95 Yb 0.05 O 2.975 CaZr 0.9 In 0.1 O 2.95
F (at.%)
0.38 0.02 0.01
Dopant (at.%) 2 1 2
concentrations; and SrCe 0.95 Yb 0.05 O 2.975 and CaZr 0.9 In 0.1 O 2.95 , which display low 19 F concentrations. The former was supplied by Dr. K.-D. Kreuer; the latter by Dr. T. Yajima. Therefore, the difference in fluorine concentration between the two groups may be ascribed to the different fabrication routes. In the case of SrCe 0.95 Yb 0.05 O 2.975 and CaZr 0.9 In 0.1 O 2.95 , the fluorine concentrations are negligible in comparison with the acceptor dopant concentration. In contrast BaCe 0.9 La 0.1 O 2.95 contain significant amounts of fluorine. As a consequence the concentration of oxygen vacancies is no longer defined solely by the acceptor dopant level, but is reduced by the donor doping effect of the fluorine (Eq. (5)). A typical 18 O penetration plot is given in Fig. 1. It is seen from the good agreement between the experimental data and the fitted curve that high quality diffusion data can be obtained by this method. The oxygen tracer diffusion and surface exchange coefficients for each of the three compositions are
Fig. 1. 18 O penetration profile for SrCe 0.95 Yb 0.05 O 2.975 together with fitted curve. 18 O anneal for 500 s at 6078C.
now presented in turn. The activation energies stated on the graphs were determined by regression analysis; the errors given are the standard uncertainties associated with fitting the values of D* and k to Arrhenius type behaviour. Values of D* and k for CaZr 0.9 In 0.1 O 2.95 are shown in Fig. 2 as a function of inverse temperature. Two depth profiles were carried out on the same sample for each temperature investigated. While the high temperature values for D* and k are of extremely high precision, the low temperature data shows some scatter. This is attributed to the shallow 18 O penetration depths in these samples. The Arrhenius plot for the oxygen tracer diffusion and surface exchange coefficients obtained for SrCe 0.95 Yb 0.05 O 2.975 is given in Fig. 3. As for CaZr 0.9 In 0.1 O 2.95 , two measurements were made on each sample: depth profiling was employed for the two low temperature profiles and linescanning for the high temperature ones. Besides containing high concentrations of fluorine, the samples of BaCe 0.9 La 0.1 O 2.95 suffered from poor mechanical integrity, which meant that sample preparation was fraught with problems; many of the samples also had to be analysed by linescanning (on account of the high diffusivities), which thus entailed further cutting and polishing. All this is inevitably reflected in the quality of the data, which are shown in Fig. 4. It is seen that, in the case of
Fig. 2. Oxygen tracer diffusion and surface exchange coefficients for CaZr 0.9 In 0.1 O 2.95 as a function of inverse temperature at a nominal oxygen partial pressure of 1 atm.
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415
Table 4 Activation energies obtained for oxygen tracer diffusion and surface exchange
BaCe 0.9 La 0.1 O 2.95 SrCe 0.95 Yb 0.05 O 2.975 CaZr 0.9 In 0.1 O 2.95
DHD * (eV)
DHk (eV)
0.960.1 1.9160.07 2.3060.05
0.660.4 1.460.1 1.960.2
Errors are standard errors.
Fig. 3. Oxygen tracer diffusion and surface exchange coefficients as a function of inverse temperature for SrCe 0.95 Yb 0.05 O 2.975 at a nominal oxygen pressure of 1 atm.
Fig. 4. Oxygen tracer diffusion and surface exchange coefficients as a function of inverse temperature for BaCe 0.9 La 0.1 O 2.95 at a nominal oxygen pressure of 1 atm.
BaCe 0.9 La 0.1 O 2.95 , a dramatic increase in D* of around five orders of magnitude (and in k of two orders of magnitude) was found between 500 and 6008C. It should be noted though that there is a large error (factor of 5, at least) associated with the values of D* and k at 5008C, due to the extremely small 18 O profile length; for the anneal time used, a far higher penetration depth was clearly expected. Nevertheless, there is a considerable change in D* and k at low temperatures.
3.3.1. Oxygen tracer diffusion The activation energies for oxygen tracer diffusion are summarised in Table 4; they are, with the exception of BaCe 0.9 La 0.1 O 2.95 , really too high to represent migration enthalpies alone, cf. [15,16]. Assuming that the oxygen vacancy concentration is constant – this point will be returned to later – it is thought that the additional contribution arises from the trapping of oxygen vacancies, and this may be either by association between dopant cations and oxygen vacancies, or by superlattice ordering. Evidence of vacancy ordering in the alkaline earth cerate perovskites was first suggested by the work of Knight and Bonanos [17]. Neutron powder diffraction indicated that at room temperature the vacancies in Y-doped BaCeO 3 were confined exclusively to one of the two oxygen positions in the orthorhombic structure. Ranlov et al., in their neutron diffraction study of SrCe 0.85 Y 0.15 O 2.925 [18], similarly found that the oxygen vacancies were situated at the apical positions of the CeO 6 octahedra. They proposed that this is due to the longer Ce–O bond length (and therefore weaker electrostatic potential) associated with the apical position. For CaZr 0.9 In 0.1 O 2.95 and SrCe 0.95 Yb 0.05 O 2.975 it is therefore believed that the measured activation energy, DHD * , is the sum of DHM , the oxygen vacancy migration enthalpy, and DH O , a term which reflects the fact that the oxygen vacancies are ordered. In defect chemistry, such systems may be regarded either as oxygen deficient perovskites with defect clustering, or as vacancy ordered structures with pseudo oxygen Frenkel disorder. Both frameworks, though, lead to the conclusion that trapping reduces the number of vacancies that contribute to diffusion, that is there is a dynamic equilibrium between the free vacancies and those that are trapped by ordering. Superlattice ordering not only accounts for the high activation energies determined for
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CaZr 0.9 In 0.1 O 2.95 (2.30 eV) and SrCe 0.95 Yb 0.05 O 2.975 (1.91 eV) but also provides an explanation for the sudden increase in D* observed for BaCe 0.9 La 0.1 O 2.95 . The room temperature crystal structure of barium cerate is known to be orthorhombic, and hence contains two distinct oxygen sites. At higher temperatures, a transformation occurs to a rhombohedral structure, for which there is but a single oxygen site. Thus, the oxygen sublattice would be expected to disorder at the transition temperature, resulting in an increase in the number of free oxygen vacancies and a decrease in the activation energy. Knight [19] reported rhombohedral symmetry above 4008C for undoped barium cerate, but around 1008C higher for BaCe 0.9 Y 0.1 O 2.95 – the temperature at which the sharp increase in D* was observed for BaCe 0.9 La 0.1 O 2.95 . It is emphasised that possible contributions from defect associates have not been ruled out: it is merely proposed that ordering provides the dominant term. Smyth [20] states that the high polarizability of perovskite oxides favours the stabilisation of ordered structures and reduces the tendency for defect association. However, atomistic computer simulations of cubic BaCeO 3 by Hill and Cormack [21] have indicated favourable binding energies (0.3 to 0.6 eV) 9 –V O?? )? associates. for (M Ce The trapping of oxygen vacancies has important implications for ionic transport in these materials, specifically the question of oxygen ion versus proton conduction. The behaviour of BaCe 0.9 La 0.1 O 2.95 indicates that the disordering of the oxygen sublattice promotes oxygen ion conduction at the expense of proton conduction (the transition from proton to oxygen ion dominated transport starts at |6008C [22]). The converse is equally significant, if not more so: vacancy ordering ‘freezes’ the oxygen sublattice, thereby allowing proton conduction to dominate. This model would explain why compounds such as CeZr 0.9 In 0.1 O 2.95 and SrCe 0.95 Yb 0.05 O 2.975 , in comparison to BaCeO 3 materials, are predominantly proton conductors even at reasonably high temperatures (|10008C), as vacancy trapping is still effective. Indeed the study by Yajima et al. [23] on Ba 12x Ca x Ce 0.9 Nd 0.1 O 2.95 demonstrates that by increasing the degree of orthorhombic distortion (by raising the level of Ca substitution, x), oxygen vacancies are hindered from migrating, without greatly affecting proton conduction.
Central to the arguments detailed above concerning the high activation energies is the assumption of a constant total vacancy concentration. In the case of these AII B IV O 3 perovskites, it is expected that the total vacancy concentration is defined by the dopant concentration. Lattice oxidation, that is the reaction of oxygen vacancies with oxygen to give electron holes, does not result in large changes in oxygen vacancy concentration for these materials. The major concern, though, is the annihilation of vacancies by hydroxide ions (Eq. (2)). Based on numerical modelling of the defect structure of SrCe 0.95 Yb 0.05 O 2.975 [24], it is believed that, in the temperature range of interest, and due to the low water partial pressure in which the isotope anneals were carried out, protons were minority species and thus the oxygen vacancy concentration was, to a first approximation, unchanged (Fig. 5). Furthermore, the S 17 / S 16 ratios from the SIMS analyses (i.e. an indication of the OH 2 concentration) were all near the natural isotopic level of 17 O / 16 O of 0.04%. The oxygen diffusivities of the three compositions are compared in Fig. 6. Curiously the two cerate compositions exhibit very similar diffusivities compared to CaZr 0.9 In 0.1 O 2.95 . This is surprising given that BaCe 0.9 La 0.1 O 2.95 is characterised by a disordered oxygen sublattice, whereas in SrCe 0.95 Yb 0.05 O 2.975 , and CaZr 0.9 In 0.1 O 2.95 , the vacancies are thought to be trapped. It may have been expected from the trend seen for the
Fig. 5. Calculated concentrations of oxygen vacancies and protons in SrCe 0.95 Yb 0.05 O 2.975 as a function of inverse temperature at PH 2 O 510 24.5 . Data taken from Larring [24].
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417
studied in several AII B IV O 3 perovskites as a function of dopant cation and concentration.
3.4. Comparison with literature data The oxygen diffusion coefficients obtained in this work are compared with reported transport data by means of the Nernst–Einstein equation Nq 2 D* s 5 ]] kT
Fig. 6. Comparison of oxygen tracer diffusivities for the perovskite compositions investigated.
activation energies that BaCe 0.9 La 0.1 O 2.95 and CaZr 0.9 In 0.1 O 2.95 would represent the extremes, with SrCe 0.95 Yb 0.05 O 2.975 exhibiting intermediate behaviour. There are, however, several factors to be taken into account, from which it may be supposed that the diffusion coefficients obtained for BaCe 0.9 La 0.1 O 2.95 are lower than expected. Firstly, there is the widely differing fluorine concentrations between compositions; as already noted, this causes a reduction in the concentration of oxygen vacancies by |25%. Secondly, it is well known that electrical conductivity of the alkaline earth cerates and zirconates is dependent upon the size of the dopant cation relative to that of the host cation, e.g. see Refs. [25,26]. Yb 2 O 3 , and Y 2 O 3 to a lesser extent, are generally regarded as optimum dopants for SrCeO 3 . The same is not true of La 2 O 3 doped BaCeO 3 : Gd 2 O 3 , Nd 2 O 3 and Sm 2 O 3 are considered to be better. Thirdly, the ionic radius of ˚ in twelve-fold co-ordination (A site) La 31 is 1.36 A ˚ and 1.03 A in six-fold coordination (B site), in˚ and Ce 41 (0.87 A) ˚ between that of Ba 21 (1.61 A) [12]; therefore, the lanthanum ions may be distributed over the two sites. Such a possibility has been put forward by Kreuer et al. [7] for 10% Gd:BaCeO 3 , which they believed corresponded to a bulk composition of (Ba 0.965 Gd 0.035 )(Ce 0.935 Gd 0.065 )O 2.985 . The net acceptor concentration is thus far lower than the nominal one and the oxygen vacancy concentration is hence greatly diminished. To solve this problem satisfactorily, oxygen diffusion must be
(6)
where q, k and T have their usual meanings and correlation factors are ignored; N is the number of anion sites per unit volume. It is stressed that this equation exclusively defines the relationship between s and D* and does not require the concentration of vacancies to be included as this is contained within the transport parameters themselves. This allows the direct comparison of activation energies obtained from IEDP experiments with those from electrical measurements.
3.4.1. CaZr0.9 In0.1 O2.95 The oxygen ion conductivity, calculated from the pre-exponential term and activation energy derived, is plotted in Fig. 7 together with the data of Kurita et al. for CaZr 0.9 In 0.1 O 2.9 [6] and Wang et al. for various Y 2 O 3 -doped CaZrO 3 compositions [8]. Excellent agreement is found between the three sets of
Fig. 7. Comparison of oxygen ion conductivities for various acceptor-doped calcium zirconate compositions: (A) CaZr 0.9 In 0.1 O 2.95 , this work; (B) CaZr 0.9 In 0.1 O 2.95 , Kurita et al. [6]; (C) Y 2 O 3 -doped CaZrO 3 , Wang et al. [8].
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data, each determined by a different method, both in terms of the magnitude of the conductivity and the activation energy. The value of DHD * 52.3 eV determined here is close to Ea 52.560.5 eV calculated by Kurita et al., and well within the 2.0 to 2.7 eV range found by Wang et al.
3.4.2. SrCe0.95 Yb0.05 O2.975 In addition to the calculated oxygen ion conductivity, the results from two other groups are also shown in Fig. 8: the partial ionic conductivity calculated by Kosacki and Tuller [5], which they ascribed to oxygen ions; and the extrapolated (proton) conductivity data of Bonanos et al. [27] from measurements in an N 2 :5%H 2 atmosphere. It is evident from Fig. 8 that the data of Kosacki and Tuller cannot be attributed to oxygen ion conduction. It is proposed that the activation energy of 0.77 eV they determined refers to mixed oxygen ion and proton transport, being only slightly higher than the activation energy of 0.59 eV reported by Bonanos et al., and considerably less than DDH * 5 1.91 eV obtained here. Still, it is not clear why the mixed conductivity is lower than the calculated oxygen ion conductivity. Perhaps, secondary effects, e.g. microstructural, play a part. Comparing the
theoretical oxygen ion conductivity with the data of Bonanos et al., it appears that in an N 2 :5%H 2 atmosphere proton transport would dominate the conductivity at least up to temperatures of 10008C.
Fig. 8. Comparison of calculated oxygen ion conductivity A of SrCe 0.95 Yb 0.05 O 2.975 with the oxygen ion conductivity B determined by Kosacki and Tuller [5], and the extrapolated proton conductivity C from Bonanos et al. [27].
3.4.3. BaCe0.9 La0.2 O2.95 The theoretical oxygen ion conductivities are compared in Fig. 9 to the converted diffusivities obtained by Kreuer et al. [7] for BaCe 0.9 Gd 0.1 O 2.95 by means of 18 O / 16 O TGA, and the extracted ionic conductivity data of Bonanos for BaCe 0.9 Gd 0.1 O 2.95 [4], and of Paria and Maiti for BaCe 0.9 La 0.1 O 2.95 [28]. It is apparent that the values measured by Bonanos are significantly higher than the other data sets, which suggests that the PO 2 independent conductivity extracted by Bonanos, though free from electronic contributions, may nevertheless contain a proton component. This is supported by examination of the activation energies: 0.52 eV determined by Bonanos is significantly lower than 0.9 eV (this work), 0.8 eV (Kreuer et al.) and 0.75 eV (Paria and Maiti). It is also found that the Gd 2 O 3 -doped sample of Kreuer et al. exhibits higher conductivities than either of the La-doped samples: this is consistent with Gd 2 O 3 being the best dopant for BaCeO 3 .
Fig. 9. Comparison of oxygen ion conductivities for BaCe 0.9 M 0.1 O 2.95 : (A) BaCe 0.9 La 0.1 O 2.95 , this work; (B) BaCe 0.9 La 0.1 O 2.95 , Paria and Maiti [28]; (C) BaCe 0.9 Gd 0.1 O 2.95 , Kreuer et al. [7]; (D) BaCe 0.9 Gd 0.1 O 2.95 , Bonanos [4].
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4. Conclusions Analysis of negative secondary ion mass spectra (m /z516 to 19) of SrCe 0.95 Yb 0.05 O 2.975 ceramics, annealed in various conditions, provided a qualitative indication of the amount of hydrogen and fluorine within the bulk of the samples as a function of annealing atmosphere. A nuclear reaction technique (PIGE) was used to confirm the presence of fluorine; good agreement was seen between the 19 F concentrations obtained from SIMS and PIGE analyses. The oxygen diffusion data obtained for CaZr 0.9 In 0.1 O 2.95 , SrCe 0.95 Yb 0.05 O 2.975 , and BaCe 0.9 La 0.1 O 2.95 indicated that oxygen vacancy ordering plays a decisive role in the transport characteristics of these perovskite materials.
Acknowledgments Financial assistance for RADS was obtained from EPSRC and Siemens AG.
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