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A Raman spectroscopy study of cerium oxide in a cerium–5 wt.% lanthanum alloy
Vibrational Spectroscopy 70 (2014) 200–206
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A Raman spectroscopy study of cerium oxide in a cerium–5 wt.% lanthanum alloy D.W. Wheeler ∗ , I. Khan AWE, Aldermaston, Reading, Berkshire RG7 4PR, United Kingdom
a r t i c l e
i n f o
Article history: Received 16 September 2013 Received in revised form 21 November 2013 Accepted 12 December 2013 Available online 20 December 2013 Keywords: Cerium Cerium oxide Inclusions Oxidation Raman spectroscopy
a b s t r a c t This paper describes a study of a cerium–5 wt.% lanthanum (Ce–5 wt.% La) alloy using Raman spectroscopy and X-ray diffraction (XRD). Examination of the alloy microstructure by optical microscopy and Raman spectroscopy revealed the presence of inclusions which were identified as cerium oxide (CeO2 ). The study also highlighted the need to avoid excessive laser power during acquisition of the Raman spectra as this appeared to cause the oxidation of the region being analysed where previously no cerium oxide peak had been detected. The propensity of cerium to oxidise in air results in the formation of a CeO2 layer on the surface of the alloy. Raman spectroscopy of the oxide layer formed on the alloy after exposure to air for 21 days found that the Raman peak denoting cerium oxide was seen at between 5 and 7 cm−1 lower than the value for CeO2 (465 cm−1 ). This is attributed to a combination of a sub-stoichiometric oxide layer and the presence of La in the alloy. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Cerium (Ce), which is one of the lanthanide (or “rare earth”) elements, is a highly complex metal that can assume multiple allotropic forms depending on pressure and temperature. At ambient temperature and pressure its crystal structure is the facecentred cubic (fcc) ␥-Ce phase. However, at atmospheric pressure it exhibits no fewer than four allotropic crystal structures between absolute zero and its melting temperature (1071 K) [1]. Furthermore, when subjected to pressure ␥-Ce will transform to ␣-Ce (the so-called “collapsed fcc” phase): at 22 ◦ C this occurs at 8 kbar and the transformation is accompanied by a volume change of 18% [2]. In recent years, there has been an upsurge of interest in cerium, lanthanum (La) and other rare earth elements for use in electronic components and “green” technologies [3]. Ce and La are the main components of a ‘mischmetal’ mixture of rare earth elements that makes up the negative electrode in nickel metal hydride batteries [4]. However, before they are used in new applications their properties must be thoroughly understood, including their microstructure, mechanical properties and corrosion behaviour. The mechanical properties of Ce and a Ce–5 wt.% La alloy were the subject of a recent study [5] which used nanoindentation and ultrasonic velocity measurements to measure their hardness and elastic modulus.
∗ Corresponding author. Tel.: +44 118 982 4891. E-mail address: [email protected] (D.W. Wheeler).
One of the difficulties in handling Ce, and lanthanide metals in general, is their rapid oxidation, especially in the presence of water. Of the lanthanide elements, only europium (Eu) has been observed to corrode more rapidly than cerium in air at 25 ◦ C [6]. During the oxidation process spalling of the oxide layer may occur, exposing fresh metal, resulting in a continuation of the corrosion process. Therefore, Ce must be stored in an air-free environment, for example under argon or mineral oil. However, even when stored in a vacuum desiccator, Sheldon et al. [7] found that the surface tarnished within a day. Although Ce can, with care, be studied safely and reliable data obtained in an open laboratory, the rapid oxidation can present challenges for the metallographic preparation. In the present study the Ce–5 wt.% La alloy was observed to tarnish within 10 min of the specimen being etched. This observation concurred with the recommendations made by Zukas et al. [2] who proposed that in the metallographic preparation the final polish should be completed in less than 2 min, no more than 2 min should elapse between polishing and etching and photography should be completed in 6–8 min. The present study describes the use of Raman spectroscopy to study cerium oxide (CeO2 ) in a Ce–5 wt.% La alloy. The CeO2 observed is in the form of inclusions in the microstructure as well as an oxide layer formed on the surface of the alloy. Although Xray diffraction can identify the oxide layers on a metal, the low spatial resolution prevents the identification of microstructural features such as individual inclusions. Furthermore, although techniques such as energy dispersive spectroscopy (EDS) can identify the chemical constituents in such inclusions, it cannot provide
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definitive information on the chemical structure. The advantage of Raman spectroscopy is the combination of high spatial resolution, which allows the analysis of individual inclusions, and the ability to identify the chemical structure of microstructural features from the vibrational spectrum. Crystals with the fluorite structure (space group Fm3m), of which CeO2 is one example, have an exceptionally simple vibrational structure with one infrared active phonon of T1 symmetry and one Raman active phonon of T2g symmetry at k = 0 [8]. In Raman spectra this is manifested in the form of a single sharp peak which is seen at 465 cm−1 [8,9].
2. Experimental details The Ce-La alloy was produced by arc-melting at the University of Birmingham using starting materials supplied by Goodfellow Metals (Huntingdon, UK). The nominal composition was Ce–5 wt.% La; for brevity this alloy will hereafter be referred to as “Ce–5La”. The alloy was then subjected to characterisation, which is described below. In all cases characterisation was carried out in an open laboratory atmosphere (mean relative humidity 40%; mean temperature 22 ◦ C). Specimens cut from the alloy were cold mounted in epoxy resin and then subjected to metallographic preparation by grinding using progressively finer SiC abrasive in lapping oil (Buehler AutoMet). The final stage was 4000 grit SiC. Owing to the low hardness of the Ce-La alloy, which was measured to be 48 HV, polishing by finer grades of diamond was considered undesirable in order to avoid embedding of the diamond abrasive in the metal. Instead, the specimens were subjected to a chemical polish using a solution of 60 vol.% nitric acid/40 vol.% isopropanol before being washed in ethanol. Examination by optical microscopy was performed immediately after metallographic preparation. X-ray diffraction (XRD) was performed on free-standing Ce–5La specimens using a Bruker D8 diffractometer. Prior to analysis the specimens were prepared using the same procedure as the metallographic specimens, the final stage being 4000 grit SiC; the chemical polish was not performed in this case. A CuK␣ X-ray source was used and the voltage and current were 40 kV and 40 mA, respectively. The sample was scanned through the range 2Â = 20–90◦ at a rate of 0.025◦ s−1 , each scan taking approximately 50 min to complete. EDS was performed using a Jeol JSM7000F scanning electron microscope (SEM). Prior to examination, the specimens were sputter coated with a thin layer of carbon (approximate thickness 10 nm) to render them electrically conducting.
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The Raman analysis was performed on metallography specimens using a Renishaw InVia spectrometer which was equipped with a helium–neon (He–Ne) laser (wavelength 633 nm). The calibration of the instrument was verified using a Si standard. The laser spot diameter was approximately 2 m and the laser power was varied between 0.1 and 10 mW. Prior to Raman analysis the Ce specimens were ground to 4000 grit SiC in oil before being rinsed in ethanol and dried. For the purposes of comparison, Raman spectra were also acquired on samples of CeO2 powder (Sigma–Aldrich, UK) using the same conditions as those employed for the Ce–5La alloy. In addition to the metallographic specimens, a free-standing Ce–5La specimen that had been exposed to the open laboratory atmosphere for 21 days was also examined by both XRD and Raman spectroscopy. Prior to exposure to the air it was prepared in the same way as the XRD samples (described above). The conditions employed in the analyses were the same as those described above. 3. Results and discussion 3.1. Microstructure Fig. 1 shows a typical optical micrograph of the Ce–5La alloy. The chemical polish appeared to etch the surface revealing the grain structure, which rendered a separate etching stage unnecessary. Using the three-circle method (ASTM E112), G numbers of between 6.79 and 7.25 were obtained, which correspond to nominal grain diameters of between 30 and 35 m. EDS indicated a high degree of homogeneity with the La evenly distributed throughout the microstructure. Quantitative EDS across individual grains revealed that the La content varied over the range 4.0–6.2 wt.% with mean contents between 4.8 and 5.4 wt.%. An X-ray diffraction pattern for the Ce–5La alloy in the “as prepared” condition is shown in Fig. 2. The pattern consists of metallic Ce (fcc ␥ phase) and Ce oxide. The results are consistent with the X-ray powder file numbers 04-001-0124 (Ce) and 04-0054553 (CeO2 ). The specimen was also given an extended scan from 20–150◦ to determine the lattice parameter. From the reflections in the pattern (not shown) a lattice parameter of 5.1720 A˚ was determined, which compares with the literature value of 5.1615 A˚ for unalloyed Ce [10]. Therefore, the lattice parameter of the alloy was 0.2% larger than that of the unalloyed metal. Fig. 2 also shows an XRD pattern of the Ce–5La alloy after 21 days in air. It shows cerium oxide to be present, both as CeO2 and Ce2 O3 ; this is discussed in more detail in Section 3.3. 3.2. Examination of microstructural inclusions
Fig. 1. Optical micrograph showing the microstructure of the Ce–5La alloy.
As can be seen in Fig. 1, a prominent feature in the microstructure of the Ce–5La alloy was the presence of inclusions in the form of small brown precipitates. These inclusions were extensively distributed through the microstructure and were often acicular or dendritic in appearance, such as can be seen in transverse section in Fig. 1. Overall, the mean inclusion size was 2.9 ± 1.5 m, although a small number of inclusions, such as that shown in Fig. 1, were considerably larger. It should also be noted that depending on the orientation of individual inclusions the metallographic images, which are planar sections, may not provide a true indication of their size and/or morphology. The inclusion content in the Ce–5La alloy was determined by image analysis and the volume fraction was found to be between 1.8 and 5.1%; the mean oxide volume fraction was 3.5%. Fig. 3 shows a secondary electron image of an inclusion, together with two locations from which EDS spectra were acquired. The oxygen peak in Spectrum 1 is significantly more prominent than that
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1600 1500 1400 1300 1200
Intensity (Counts)
1100 1000 900 800 700 600 500 400 300 200 100 0 20
30
40
60
50
70
80
90
2 Theta (°) Fig. 2. X-ray diffraction pattern of the Ce–5La alloy in the “as-prepared” condition (black) and after 21 days in air (red). The positions of the Ce reflections are marked in blue, those of CeO2 are in red and those of Ce2 O3 are in green. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in Spectrum 2. The carbon peaks in the EDS spectra are from the coating applied to the specimen surface to reduce the charging of the specimen under the electron beam. Fig. 4 shows a backscattered electron image of a region of the microstructure containing some inclusions. This area was mapped to evaluate the cerium and oxygen distributions, the results of which are also shown in Fig. 4. They indicate elevated levels of oxygen relative to regions of the microstructure where the inclusions were not present. Although Figs. 3 and 4 suggest the inclusions are Ce oxide, they do not provide an unequivocal identification of the structure of these features. However, by using Raman spectroscopy these inclusions were definitively identified. A representative Raman spectrum from an inclusion is shown in Fig. 5 and compared with a
spectrum from CeO2 powder. Both spectra show the intense Raman active T2g mode at 463 cm−1 (inclusion) and 464 cm−1 (CeO2 powder) assigned to the symmetric Ce O vibration of the CeO2 lattice [11]. The Ce oxide inclusions are thought to have been formed during the arc-melting process by the reaction between molten Ce and residual oxygen in the furnace. The hardness of CeO2 is approximately 6 GPa, making it more than twenty times that of a nominally oxide-free Ce–5La alloy, which was measured at 0.25 GPa [5]. As a result, the hardness of the alloy in the present study – in which the mean oxide volume fraction was 3.5% – was approximately 90% higher than that of the nominally oxide-free alloy of the same composition. The presence of the oxide inclusions also results in elevated elastic
O
1 Ce
C
CeCe
Ce Ce
Ce 1
0
1
2
3
4
5
Ce Ce 6
7
8
9
10
Energy (keV) Ce 2
Ce
C Ce O Ce
Ce
Ce
60 µm 0
1
2
2
3
4
5
Ce Ce 6
7
8
9
10
Energy (keV) Fig. 3. Secondary electron image of an inclusion in the microstructure of the Ce–5La alloy, together with two EDS spectra, the locations of which are indicated.
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Fig. 4. Backscattered electron image of a region of the Ce–5La microstructure (a) together with EDS maps showing the distribution of the (b) Ce and (c) O content.
modulus values. The measured elastic modulus values for Ce–5La were between 32 and 39 GPa [5]; in contrast, the modulus of Ce has been reported to be between 30 and 33.5 GPa [12,13]. The elevated modulus values were seen using both a surface technique (nanoindentation) as well as a bulk technique (ultrasonic velocity measurement). This supports the conclusion that the bulk oxide inclusions and not the surface oxide layer were responsible for the elevated hardness and elastic modulus values recorded [5]. The Raman spectrum shown in Fig. 5 was acquired using a laser power of 1 mW. Spectra acquired under the same conditions from apparently oxide-free regions of the Ce–5La alloy showed only fluorescence with no CeO2 peaks detected. However, when the laser power was increased to 10 mW, the heat generated appeared to be sufficient to oxidise the surface. This process is illustrated in Fig. 6 where a set of three spectra taken from the same location are presented. The first spectrum was acquired using a laser power of 1 mW. The power was then increased to 10 mW and another spectrum acquired, which shows a strong peak at 457 cm−1 . The power was then reduced once more to 1 mW and a third spectrum acquired: the peak at 457 cm−1 remains, albeit at a lower intensity. The significance of the lower wave number of this peak (which is up to 8 cm−1 lower than the value for CeO2 powder) is discussed below. The apparent laser-induced oxidation seen in Fig. 6 highlights the need to avoid high laser power, the heat from which can
alter the spectrum and possibly lead to misleading observations. A Raman study of CeO2-y nanocrystals by Popovic et al. [14] found that heating caused a decrease in the Raman mode frequency and a change in peak shape, which became less asymmetric with increasing temperature. Another study by Graham et al. [15] found that thermal broadening of the Raman peak could be induced by laser heating. However, after investigating the effect of laser power on line width, they reported that at power levels of 1–2 mW the effect appeared to be negligible. Similar observations were reported by McBride et al. [16] in a study of sintered CeO2 specimens doped with various rare earth elements. They observed that small changes in temperature could produce changes to the frequency and width of the Raman line. They also noted that at a laser power of ∼2 mW laser heating did not affect the spectra of pure CeO2 or CeO2 doped with La, Eu, gadolinium (Gd) or neodymium (Nd). However, the samples doped with praseodymium (Pr) or terbium (Tb) were affected with the Raman peak position shifting by up to 2 cm−1 . In the present study, all subsequent spectra discussed were acquired using a laser power of 1 mW. 3.3. Examination of the surface oxide layer In addition to the presence of the inclusions in the microstructure of the Ce–5La alloy, CeO2 will also be present as a surface oxide
Raman Intensity (a.u.)
457
Spectrum 1 (1mW) Spectrum 2 (10 mW) Spectrum 3 (1 mW)
700
650
600
550
500
450
400
350
300
Wavenumber (cm-1) Fig. 5. Raman spectrum of a CeO2 inclusion in the Ce–5La alloy together with that of CeO2 powder.
Fig. 6. Three Raman spectra taken from the same region of the metallic matrix in the Ce–5La alloy showing the effect of laser power on the observed spectra.
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Fig. 7. Optical micrograph of the Ce–5La alloy after 21 days in air.
layer owing to the propensity of Ce to oxidise rapidly in air. Oxidation studies of Ce using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) have indicated that the oxide initially formed on the metal is Ce2 O3 , which is subsequently overlaid with CeO2 [17,18]. This might suggest that the peak shown in Fig. 6 could be that of Ce2 O3 . However, while Raman studies of CeO2 have been the subject of many published studies [8,9,11,15] no Raman spectra of Ce2 O3 have been found. The oxidation of Ce–5La was investigated by exposing a specimen of the alloy to the open laboratory atmosphere for 21 days. Over this period, the alloy was analysed daily by XRD in order to study the changes as a function of time. The XRD pattern of the alloy acquired after 21 days in open air is also shown in Fig. 2. When compared with the pattern for the alloy in the “as-prepared” condition (also in Fig. 2) it can be seen that the CeO2 peaks, previously barely above the baseline, have increased in intensity to the extent that they have eclipsed the prominence of the Ce metal peaks. Fig. 7 shows an optical micrograph of the oxide surface of the Ce–5La alloy after 21 days in air. The oxide layer was grey in colour and exhibited a nodular morphology with the nodules ranging in size from 2 to 30 m. Fig. 8 shows two Raman spectra, one taken from a nodular region and the other from a more fine-grained region. The spectra show no significant differences between the two regions. The appearance of the oxide layer, together with the Raman spectra, suggests that the oxide has grown via a Stranski–Krastanov (SK) – sometimes referred to as “layer plus island” – mode. In
Raman Intensity (a.u.)
458
459
Nodular region Fine-grained region
700
650
600
550
500
450
400
350
300
Wavenumber (cm-1)
Fig. 8. Raman spectra taken from two regions of the oxide layer formed on the Ce–5La alloy.
SK growth the film initially formed is as a layer that completely covers the metal; however, after a critical thickness is reached (which depends on the chemical potential of the film and the amount of strain present) island growth starts to predominate. In a strained film, the formation of islands is energetically favourable, as it reduces the strain energy in the crystal [19]. In the present study, the lattice parameter of the oxide on Ce–5La was measured ˚ giving a lattice mismatch strain between the oxide to be 5.4283 A, film and the metal substrate of 4.9%. The spectra in Fig. 8 show the Raman peaks to be between 458 and 459 cm−1 ; overall, the range of peak positions were between 457 and 460 cm−1 , depending on the location from which the spectra were acquired. When these spectra were compared with those obtained from the CeO2 powder specimens it was observed that the peaks from the oxide layer appeared to be both broader as well as being shifted to lower wave numbers; for example, in the spectra shown in Fig. 8 the peak positions are approximately 6 cm−1 lower than that for the CeO2 powder shown in Fig. 5. The Raman peak positions and peak widths measured from the Ce–5La oxide layer and the CeO2 powder are summarised in Table 1. There are a number of possible explanations for these discrepancies. They are: (i) residual stress; (ii) the presence of the La alloying element; and (iii) non-stoichiometry of the oxide layer. Each of these will now be considered in turn. In coated systems residual stresses can arise from differences in the properties between coating and substrate, for example density or coefficient of thermal expansion. These residual stresses can be evaluated by Raman spectroscopy. Typically, tensile strain in thin films causes a shift in the position of the Raman bands to lower wave numbers with respect to a strain-free reference material [20]; in contrast, compressive strain leads to an increase in wave number of the Raman peak. An example of this is seen in diamond coatings produced by chemical vapour deposition (CVD) [21,22]. For coatings deposited on most substrates the overall residual stress is generally compressive, the most prominent exception being silicon nitride (Si3 N4 ) substrates [21]. Tensile stresses cause a downward shift in the wave number from its stress-free value of 1332 cm−1 while an upward shift indicates the presence of a compressive residual stress [22]. Such findings have also been noted by Yang et al. [23] for epitaxial aluminium nitride (AlN) films on sapphire. In order to ascertain whether the displacement of the Raman peak to lower wave numbers in Ce–5La was due to residual stress, Raman spectra were acquired from free-standing pieces of the oxide film that had been detached from the Ce–5La specimen. A typical spectrum taken from the free-standing oxide layer is shown in Fig. 9; a spectrum taken from the oxide layer adhering to the Ce–5La metal is also included in the figure. The lack of any significant differences between the two spectra indicates that the effect of residual stress cannot explain the peak shifts. The Raman peak positions and peak widths recorded from the free-standing oxide are also summarised in Table 1. An alternative explanation to residual stress as the cause of the peak shift is the presence of La. Popovic et al. [24] used Raman spectroscopy to study CeO2 nano-powders, some of which were doped with barium (Ba), gadolinium (Gd) and neodymium (Nd). They found large shifts in the first-order Raman-active modes, an example being Ce0.75 Nd0.25 O2 , which was approximately 450 cm−1 . More recently, Popovic et al. [25] doped CeO2 nanocrystals with iron (Fe) and found both softening (i.e. shifting to lower wavenumbers) and broadening of the Raman T2g mode, which was attributed to changing the valence state of the dopant, as a consequence of the electron-molecular vibration coupling [25]. In another study McBride et al. [16] doped CeO2 samples with La, the concentration of which ranged from 0 to 50 at. %. They found that the presence of La caused the Raman peak to shift to lower wavenumbers with
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Table 1 Summary of the Raman peak measurements from the oxide layer of a Ce–5La alloy exposed to air for 21 days and from free-standing oxide that was detached from the Ce–5La alloy. Also included are the measurements from the oxide layer of unalloyed Ce that has also been exposed to air for 21 days. Specimen
Mean Raman peak position (cm−1 )
Oxide layer on Ce–5La Free-standing oxide from Ce–5La Oxide layer on Ce CeO2 powder
459.7 459.5 462.1 464.1
Range of peak positions (cm−1 ) 457.8–460.9 458.8–460.1 460.9–462.5 463.5–465.9
increasing doping level. For the CeO2 specimen with a La content of 20 at.% La the magnitude of the shift of the Raman peak was approximately 6 cm−1 ; however, for the specimen with a La content of 5 at.% (the closest to the alloy composition in the present study) the shift was only 1 cm−1 . This suggests that the shift of the Raman peak position cannot be attributed solely to the La. In order to investigate further the effect of the La content a specimen of unalloyed Ce (supplied by Goodfellow) that had also been exposed to air for 21 days was examined. XRD of this specimen (not shown) exhibited no significant differences from that of the Ce–5La alloy with the dominant peaks being those of CeO2 . Raman spectra were acquired from the oxide layer under the same conditions as those for the Ce–5La alloy and a typical spectrum is included in Fig. 9. It shows the peak position (462 cm−1 ) is slightly higher than it was in the alloy, although still lower than that of the CeO2 powder. The positions and widths of the Raman peaks in the unalloyed Ce are also summarised in Table 1. The position of the Raman peak in unalloyed Ce, which is 2–3 cm−1 lower than that of CeO2 powder suggests that La may not be the sole cause of the shift to lower wave numbers. Another possible cause could be the sub-stoichiometric nature of the oxide layer. Indeed, Popovic et al. [25] have reported the T2g mode of CeO2-y nanocrystals (<5 nm in size) to be centred at approximately 457 cm−1 , a finding attributed to the sub-stoichiometric nature of the oxide [25]. In CeO2 the triply degenerate stretching vibration of the CeO8 vibrational unit at ∼465 cm−1 is due to the movement of the oxygen atoms. This renders the vibration very sensitive to microstructural changes such as oxygen sublattice disorder and non-stoichiometry [26]. As the oxygen vacancy concentration increases, the 465 cm−1 T2g mode of the fluorite structure shifts to lower frequency, broadens and becomes asymmetric. These changes may also be accompanied by the appearance of a broad feature at approximately 570 cm−1 , which has also been attributed to the O vacancies [27].
Raman Intensity (a.u.)
Free-standing Ce-5La oxide
459
Unalloyed Ce oxide layer
462
700
650
600
550
500
450
400
9.4 9.6 7.4 4.3
Range of peak widths (cm−1 ) 9.0–9.7 9.3–10.1 6.3–10.3 4.0–4.7
In the present study, in addition to the shifts of the Raman peak to lower wave numbers (discussed above), Table 1 shows that the mean peak half width (at half maximum) of the oxide film on Ce–5La was 9.4 cm−1 , which is more than twice that of the CeO2 powder specimen (4.3 cm−1 ). Moreover, close inspection of the spectra reveals the presence of some peak asymmetry with greater broadening on the lower wavenumber side of the peaks. Furthermore, some spectra did exhibit very weak features in the range 520–570 cm−1 , although in many cases they were barely distinguishable from the baseline. Although the presence of La does not appear to account for the entire shift in the T2g peak it can exert an additional influence on the spectra arising from the substitution of the tetravalent Ce by the trivalent La. In the oxide layer, for every two La3+ ions that replace the Ce4+ ions, one O vacancy is needed to balance the charge [16]. The data in Table 1 appear to support this with both a greater shift of the Raman peak position and mean peak half width observed on the Ce–5La oxide layer compared with that of the unalloyed Ce. The oxygen content in the oxide layer of the Ce–5La alloy was quantified by EDS and a mean value of 65 at.% (the stoichiometric value is 67 at.%). The oxygen content was also measured on the oxide layer of unalloyed Ce which had also been exposed to air for 21 days; a mean value of 63 at.% was determined. These values are accurate to approximately ±1%. Although the oxygen contents as measured by EDS should only be regarded as an approximate indication, these values, together with the shifts and broadening of the Raman peaks, suggest that the oxide layers in both Ce–5La and unalloyed Ce are slightly sub-stoichiometric. The greater shifts in the Raman peak in the Ce–5La oxide layer are likely to be due to a combination of a higher O vacancy concentration and, to a lesser extent, the presence of La. 4. Conclusions The high spatial resolution of Raman spectroscopy has enabled microstructural inclusions in a Ce–5La alloy to be identified as CeO2 . In conducting these analyses, it has been shown that care must be taken to avoid excessive laser power which causes oxidation of the Ce–5La alloy to form CeO2 and introduce extrinsic features to the observed spectra. However, this study also demonstrates that when used carefully with low laser power (∼1 mW) Raman spectroscopy can distinguish between the oxide inclusions and inclusion-free regions of the microstructure. The oxidation of the Ce–5La alloy specimens in air was also shown to result in the growth of a CeO2 surface layer, as confirmed by XRD, EDS and Raman spectroscopy. A comparison of the Raman spectra acquired from the oxide layer formed on Ce–5La with that on unalloyed Ce has shown a greater shift and broadening of the Raman peak on the former. The causes of this are attributed to a combination of a higher O vacancy concentration and, to a lesser extent, the presence of La in the alloy.
459
Ce-5La oxide layer
Mean peak half width (cm−1 )
350
300
Wavenumber (cm-1) Fig. 9. Raman spectra taken from the oxide layer formed on the unalloyed Ce and Ce–5La alloy after 21 days in air, together with a spectrum taken from a free-standing oxide layer formerly adhered to the Ce–5La alloy.
Acknowledgments The authors would like to thank Mr. M. Matthews and Dr. S. Ennaceur for their help in the EDS analysis and Dr. P. Roussel,
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Dr. C. Puxley and Dr. R. Harker for their useful suggestions during the preparation of this manuscript. References [1] K.A. Gschneidner, V.K. Pecharsky, J. Cho, S.W. Martin, Scr. Mater. 34 (1996) 1717–1722. [2] E.G. Zukas, R.A. Pereyra, J.O. Willis, Metall. Trans. A 18 (1987) 35–42. [3] M. Ingebretsen, Adv. Mater. Proc. 168 (2010) 24–25. [4] J.M. Crow, New Scientist 210 (2011) 36–41. [5] D.W. Wheeler, J. Zekonyte, R.J.K. Wood, Mater. Sci. Eng. A 578 (2013) 294–302. [6] L. Lee, N.D. Greene, Corrosion 20 (1964) 145t–149t. [7] R.I. Sheldon, G.H. Rinehart, S. Krishnan, P.C. Nordine, Mater. Sci. Eng. B 79 (2001) 113–122. [8] V.G. Keramidas, W.B. White, J. Chem. Phys. 59 (1973) 1561–1562. [9] W.H. Weber, K.C. Hass, J.R. McBride, Phys. Rev. B 48 (1993) 178–185. [10] K.J. Pascoe, An Introduction to the Properties of Engineering Materials, Blackie and Son, London, 1961. [11] M.J. Sarsfield, R.J. Taylor, C. Puxley, H.M. Steele, J. Nucl. Mater. 427 (2012) 333–342. [12] K.A. Gschneidner, B.J. Beaudry, ASM Metals Handbook Non-Ferrous Alloys and Pure Metals, vol. 2, 9th ed., 1982, pp. 722–723.
[13] E.A. Brandes, G.B. Brook, Smithells Metals Reference Book, 7th ed., Butterworth Heinemann, London, 1992. [14] Z.V. Popovic, Z. Dohcevic-Mitrovic, A. Cros, A. Cantarero, J. Phys.: Condens. Matter 19 (2007) 496209, http://dx.doi.org/10.1088/0953-8984/19/49/496209. [15] G.W. Graham, W.H. Weber, C.R. Peters, R. Usmen, J. Catal. 130 (1991) 310–313. [16] J.R. McBride, K.C. Hass, B.D. Poindexter, W.H. Weber, J. Appl. Phys. 76 (1994) 2435–2441. [17] C.R. Helms, W.E. Spicer, Appl. Phys. Lett. 21 (5) (1972) 237–239. [18] A. Platau, L.I. Johansson, A.L. Hagstrom, S.E. Karlsson, S.B.M. Hagstrom, Surf. Sci. 63 (1977) 153–161. [19] A. Baskaran, P. Smereka, J. Appl. Phys. 111 (2012) 044321. [20] J. Kreisel, M.C. Weber, N. Dix, F. Sanchez, P.A. Thomas, J. Fontcuberta, Adv. Funct. Mater. (2013). [21] R.P. Martinho, F.J.G. Silva, A.P.M. Baptista, Wear 263 (2007) 1417–1422. [22] D.S. Knight, W.B. White, J. Mater. Res. 4 (1989) 383–393. [23] S. Yang, R. Miyagawa, H. Miyake, K. Hiramatsu, H. Harima, Appl. Phys. Exp. 4 (2011) 010310. [24] Z.V. Popovic, Z. Dohcevic-Mitrovic, M.J. Konstantinovic, M. Scepanovic, J. Raman Spectrosc. 38 (2007) 750–755. [25] Z.V. Popovic, Z. Dohcevic-Mitrovic, N. Paunovic, M. Radovic, Phys. Rev. B 85 (2012) 014302. [26] G. Gouadec, P. Colomban, Prog. Cryst. Growth Charact. Mater. 53 (2007) 1–56. [27] W.H. Weber, in: W.H. Weber, R. Merlin (Eds.), Raman Scattering in Materials Science, Springer, Berlin, 2000, p. p250.
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A Raman spectroscopy study of cerium oxide in a cerium–5 wt.% lanthanum alloy D.W. Wheeler ∗ , I. Khan AWE, Aldermaston, Reading, Berkshire RG7 4PR, United Kingdom
a r t i c l e
i n f o
Article history: Received 16 September 2013 Received in revised form 21 November 2013 Accepted 12 December 2013 Available online 20 December 2013 Keywords: Cerium Cerium oxide Inclusions Oxidation Raman spectroscopy
a b s t r a c t This paper describes a study of a cerium–5 wt.% lanthanum (Ce–5 wt.% La) alloy using Raman spectroscopy and X-ray diffraction (XRD). Examination of the alloy microstructure by optical microscopy and Raman spectroscopy revealed the presence of inclusions which were identified as cerium oxide (CeO2 ). The study also highlighted the need to avoid excessive laser power during acquisition of the Raman spectra as this appeared to cause the oxidation of the region being analysed where previously no cerium oxide peak had been detected. The propensity of cerium to oxidise in air results in the formation of a CeO2 layer on the surface of the alloy. Raman spectroscopy of the oxide layer formed on the alloy after exposure to air for 21 days found that the Raman peak denoting cerium oxide was seen at between 5 and 7 cm−1 lower than the value for CeO2 (465 cm−1 ). This is attributed to a combination of a sub-stoichiometric oxide layer and the presence of La in the alloy. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Cerium (Ce), which is one of the lanthanide (or “rare earth”) elements, is a highly complex metal that can assume multiple allotropic forms depending on pressure and temperature. At ambient temperature and pressure its crystal structure is the facecentred cubic (fcc) ␥-Ce phase. However, at atmospheric pressure it exhibits no fewer than four allotropic crystal structures between absolute zero and its melting temperature (1071 K) [1]. Furthermore, when subjected to pressure ␥-Ce will transform to ␣-Ce (the so-called “collapsed fcc” phase): at 22 ◦ C this occurs at 8 kbar and the transformation is accompanied by a volume change of 18% [2]. In recent years, there has been an upsurge of interest in cerium, lanthanum (La) and other rare earth elements for use in electronic components and “green” technologies [3]. Ce and La are the main components of a ‘mischmetal’ mixture of rare earth elements that makes up the negative electrode in nickel metal hydride batteries [4]. However, before they are used in new applications their properties must be thoroughly understood, including their microstructure, mechanical properties and corrosion behaviour. The mechanical properties of Ce and a Ce–5 wt.% La alloy were the subject of a recent study [5] which used nanoindentation and ultrasonic velocity measurements to measure their hardness and elastic modulus.
∗ Corresponding author. Tel.: +44 118 982 4891. E-mail address: [email protected] (D.W. Wheeler).
One of the difficulties in handling Ce, and lanthanide metals in general, is their rapid oxidation, especially in the presence of water. Of the lanthanide elements, only europium (Eu) has been observed to corrode more rapidly than cerium in air at 25 ◦ C [6]. During the oxidation process spalling of the oxide layer may occur, exposing fresh metal, resulting in a continuation of the corrosion process. Therefore, Ce must be stored in an air-free environment, for example under argon or mineral oil. However, even when stored in a vacuum desiccator, Sheldon et al. [7] found that the surface tarnished within a day. Although Ce can, with care, be studied safely and reliable data obtained in an open laboratory, the rapid oxidation can present challenges for the metallographic preparation. In the present study the Ce–5 wt.% La alloy was observed to tarnish within 10 min of the specimen being etched. This observation concurred with the recommendations made by Zukas et al. [2] who proposed that in the metallographic preparation the final polish should be completed in less than 2 min, no more than 2 min should elapse between polishing and etching and photography should be completed in 6–8 min. The present study describes the use of Raman spectroscopy to study cerium oxide (CeO2 ) in a Ce–5 wt.% La alloy. The CeO2 observed is in the form of inclusions in the microstructure as well as an oxide layer formed on the surface of the alloy. Although Xray diffraction can identify the oxide layers on a metal, the low spatial resolution prevents the identification of microstructural features such as individual inclusions. Furthermore, although techniques such as energy dispersive spectroscopy (EDS) can identify the chemical constituents in such inclusions, it cannot provide
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definitive information on the chemical structure. The advantage of Raman spectroscopy is the combination of high spatial resolution, which allows the analysis of individual inclusions, and the ability to identify the chemical structure of microstructural features from the vibrational spectrum. Crystals with the fluorite structure (space group Fm3m), of which CeO2 is one example, have an exceptionally simple vibrational structure with one infrared active phonon of T1 symmetry and one Raman active phonon of T2g symmetry at k = 0 [8]. In Raman spectra this is manifested in the form of a single sharp peak which is seen at 465 cm−1 [8,9].
2. Experimental details The Ce-La alloy was produced by arc-melting at the University of Birmingham using starting materials supplied by Goodfellow Metals (Huntingdon, UK). The nominal composition was Ce–5 wt.% La; for brevity this alloy will hereafter be referred to as “Ce–5La”. The alloy was then subjected to characterisation, which is described below. In all cases characterisation was carried out in an open laboratory atmosphere (mean relative humidity 40%; mean temperature 22 ◦ C). Specimens cut from the alloy were cold mounted in epoxy resin and then subjected to metallographic preparation by grinding using progressively finer SiC abrasive in lapping oil (Buehler AutoMet). The final stage was 4000 grit SiC. Owing to the low hardness of the Ce-La alloy, which was measured to be 48 HV, polishing by finer grades of diamond was considered undesirable in order to avoid embedding of the diamond abrasive in the metal. Instead, the specimens were subjected to a chemical polish using a solution of 60 vol.% nitric acid/40 vol.% isopropanol before being washed in ethanol. Examination by optical microscopy was performed immediately after metallographic preparation. X-ray diffraction (XRD) was performed on free-standing Ce–5La specimens using a Bruker D8 diffractometer. Prior to analysis the specimens were prepared using the same procedure as the metallographic specimens, the final stage being 4000 grit SiC; the chemical polish was not performed in this case. A CuK␣ X-ray source was used and the voltage and current were 40 kV and 40 mA, respectively. The sample was scanned through the range 2Â = 20–90◦ at a rate of 0.025◦ s−1 , each scan taking approximately 50 min to complete. EDS was performed using a Jeol JSM7000F scanning electron microscope (SEM). Prior to examination, the specimens were sputter coated with a thin layer of carbon (approximate thickness 10 nm) to render them electrically conducting.
201
The Raman analysis was performed on metallography specimens using a Renishaw InVia spectrometer which was equipped with a helium–neon (He–Ne) laser (wavelength 633 nm). The calibration of the instrument was verified using a Si standard. The laser spot diameter was approximately 2 m and the laser power was varied between 0.1 and 10 mW. Prior to Raman analysis the Ce specimens were ground to 4000 grit SiC in oil before being rinsed in ethanol and dried. For the purposes of comparison, Raman spectra were also acquired on samples of CeO2 powder (Sigma–Aldrich, UK) using the same conditions as those employed for the Ce–5La alloy. In addition to the metallographic specimens, a free-standing Ce–5La specimen that had been exposed to the open laboratory atmosphere for 21 days was also examined by both XRD and Raman spectroscopy. Prior to exposure to the air it was prepared in the same way as the XRD samples (described above). The conditions employed in the analyses were the same as those described above. 3. Results and discussion 3.1. Microstructure Fig. 1 shows a typical optical micrograph of the Ce–5La alloy. The chemical polish appeared to etch the surface revealing the grain structure, which rendered a separate etching stage unnecessary. Using the three-circle method (ASTM E112), G numbers of between 6.79 and 7.25 were obtained, which correspond to nominal grain diameters of between 30 and 35 m. EDS indicated a high degree of homogeneity with the La evenly distributed throughout the microstructure. Quantitative EDS across individual grains revealed that the La content varied over the range 4.0–6.2 wt.% with mean contents between 4.8 and 5.4 wt.%. An X-ray diffraction pattern for the Ce–5La alloy in the “as prepared” condition is shown in Fig. 2. The pattern consists of metallic Ce (fcc ␥ phase) and Ce oxide. The results are consistent with the X-ray powder file numbers 04-001-0124 (Ce) and 04-0054553 (CeO2 ). The specimen was also given an extended scan from 20–150◦ to determine the lattice parameter. From the reflections in the pattern (not shown) a lattice parameter of 5.1720 A˚ was determined, which compares with the literature value of 5.1615 A˚ for unalloyed Ce [10]. Therefore, the lattice parameter of the alloy was 0.2% larger than that of the unalloyed metal. Fig. 2 also shows an XRD pattern of the Ce–5La alloy after 21 days in air. It shows cerium oxide to be present, both as CeO2 and Ce2 O3 ; this is discussed in more detail in Section 3.3. 3.2. Examination of microstructural inclusions
Fig. 1. Optical micrograph showing the microstructure of the Ce–5La alloy.
As can be seen in Fig. 1, a prominent feature in the microstructure of the Ce–5La alloy was the presence of inclusions in the form of small brown precipitates. These inclusions were extensively distributed through the microstructure and were often acicular or dendritic in appearance, such as can be seen in transverse section in Fig. 1. Overall, the mean inclusion size was 2.9 ± 1.5 m, although a small number of inclusions, such as that shown in Fig. 1, were considerably larger. It should also be noted that depending on the orientation of individual inclusions the metallographic images, which are planar sections, may not provide a true indication of their size and/or morphology. The inclusion content in the Ce–5La alloy was determined by image analysis and the volume fraction was found to be between 1.8 and 5.1%; the mean oxide volume fraction was 3.5%. Fig. 3 shows a secondary electron image of an inclusion, together with two locations from which EDS spectra were acquired. The oxygen peak in Spectrum 1 is significantly more prominent than that
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1600 1500 1400 1300 1200
Intensity (Counts)
1100 1000 900 800 700 600 500 400 300 200 100 0 20
30
40
60
50
70
80
90
2 Theta (°) Fig. 2. X-ray diffraction pattern of the Ce–5La alloy in the “as-prepared” condition (black) and after 21 days in air (red). The positions of the Ce reflections are marked in blue, those of CeO2 are in red and those of Ce2 O3 are in green. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in Spectrum 2. The carbon peaks in the EDS spectra are from the coating applied to the specimen surface to reduce the charging of the specimen under the electron beam. Fig. 4 shows a backscattered electron image of a region of the microstructure containing some inclusions. This area was mapped to evaluate the cerium and oxygen distributions, the results of which are also shown in Fig. 4. They indicate elevated levels of oxygen relative to regions of the microstructure where the inclusions were not present. Although Figs. 3 and 4 suggest the inclusions are Ce oxide, they do not provide an unequivocal identification of the structure of these features. However, by using Raman spectroscopy these inclusions were definitively identified. A representative Raman spectrum from an inclusion is shown in Fig. 5 and compared with a
spectrum from CeO2 powder. Both spectra show the intense Raman active T2g mode at 463 cm−1 (inclusion) and 464 cm−1 (CeO2 powder) assigned to the symmetric Ce O vibration of the CeO2 lattice [11]. The Ce oxide inclusions are thought to have been formed during the arc-melting process by the reaction between molten Ce and residual oxygen in the furnace. The hardness of CeO2 is approximately 6 GPa, making it more than twenty times that of a nominally oxide-free Ce–5La alloy, which was measured at 0.25 GPa [5]. As a result, the hardness of the alloy in the present study – in which the mean oxide volume fraction was 3.5% – was approximately 90% higher than that of the nominally oxide-free alloy of the same composition. The presence of the oxide inclusions also results in elevated elastic
O
1 Ce
C
CeCe
Ce Ce
Ce 1
0
1
2
3
4
5
Ce Ce 6
7
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Energy (keV) Ce 2
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C Ce O Ce
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60 µm 0
1
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2
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Ce Ce 6
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Energy (keV) Fig. 3. Secondary electron image of an inclusion in the microstructure of the Ce–5La alloy, together with two EDS spectra, the locations of which are indicated.
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Fig. 4. Backscattered electron image of a region of the Ce–5La microstructure (a) together with EDS maps showing the distribution of the (b) Ce and (c) O content.
modulus values. The measured elastic modulus values for Ce–5La were between 32 and 39 GPa [5]; in contrast, the modulus of Ce has been reported to be between 30 and 33.5 GPa [12,13]. The elevated modulus values were seen using both a surface technique (nanoindentation) as well as a bulk technique (ultrasonic velocity measurement). This supports the conclusion that the bulk oxide inclusions and not the surface oxide layer were responsible for the elevated hardness and elastic modulus values recorded [5]. The Raman spectrum shown in Fig. 5 was acquired using a laser power of 1 mW. Spectra acquired under the same conditions from apparently oxide-free regions of the Ce–5La alloy showed only fluorescence with no CeO2 peaks detected. However, when the laser power was increased to 10 mW, the heat generated appeared to be sufficient to oxidise the surface. This process is illustrated in Fig. 6 where a set of three spectra taken from the same location are presented. The first spectrum was acquired using a laser power of 1 mW. The power was then increased to 10 mW and another spectrum acquired, which shows a strong peak at 457 cm−1 . The power was then reduced once more to 1 mW and a third spectrum acquired: the peak at 457 cm−1 remains, albeit at a lower intensity. The significance of the lower wave number of this peak (which is up to 8 cm−1 lower than the value for CeO2 powder) is discussed below. The apparent laser-induced oxidation seen in Fig. 6 highlights the need to avoid high laser power, the heat from which can
alter the spectrum and possibly lead to misleading observations. A Raman study of CeO2-y nanocrystals by Popovic et al. [14] found that heating caused a decrease in the Raman mode frequency and a change in peak shape, which became less asymmetric with increasing temperature. Another study by Graham et al. [15] found that thermal broadening of the Raman peak could be induced by laser heating. However, after investigating the effect of laser power on line width, they reported that at power levels of 1–2 mW the effect appeared to be negligible. Similar observations were reported by McBride et al. [16] in a study of sintered CeO2 specimens doped with various rare earth elements. They observed that small changes in temperature could produce changes to the frequency and width of the Raman line. They also noted that at a laser power of ∼2 mW laser heating did not affect the spectra of pure CeO2 or CeO2 doped with La, Eu, gadolinium (Gd) or neodymium (Nd). However, the samples doped with praseodymium (Pr) or terbium (Tb) were affected with the Raman peak position shifting by up to 2 cm−1 . In the present study, all subsequent spectra discussed were acquired using a laser power of 1 mW. 3.3. Examination of the surface oxide layer In addition to the presence of the inclusions in the microstructure of the Ce–5La alloy, CeO2 will also be present as a surface oxide
Raman Intensity (a.u.)
457
Spectrum 1 (1mW) Spectrum 2 (10 mW) Spectrum 3 (1 mW)
700
650
600
550
500
450
400
350
300
Wavenumber (cm-1) Fig. 5. Raman spectrum of a CeO2 inclusion in the Ce–5La alloy together with that of CeO2 powder.
Fig. 6. Three Raman spectra taken from the same region of the metallic matrix in the Ce–5La alloy showing the effect of laser power on the observed spectra.
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Fig. 7. Optical micrograph of the Ce–5La alloy after 21 days in air.
layer owing to the propensity of Ce to oxidise rapidly in air. Oxidation studies of Ce using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) have indicated that the oxide initially formed on the metal is Ce2 O3 , which is subsequently overlaid with CeO2 [17,18]. This might suggest that the peak shown in Fig. 6 could be that of Ce2 O3 . However, while Raman studies of CeO2 have been the subject of many published studies [8,9,11,15] no Raman spectra of Ce2 O3 have been found. The oxidation of Ce–5La was investigated by exposing a specimen of the alloy to the open laboratory atmosphere for 21 days. Over this period, the alloy was analysed daily by XRD in order to study the changes as a function of time. The XRD pattern of the alloy acquired after 21 days in open air is also shown in Fig. 2. When compared with the pattern for the alloy in the “as-prepared” condition (also in Fig. 2) it can be seen that the CeO2 peaks, previously barely above the baseline, have increased in intensity to the extent that they have eclipsed the prominence of the Ce metal peaks. Fig. 7 shows an optical micrograph of the oxide surface of the Ce–5La alloy after 21 days in air. The oxide layer was grey in colour and exhibited a nodular morphology with the nodules ranging in size from 2 to 30 m. Fig. 8 shows two Raman spectra, one taken from a nodular region and the other from a more fine-grained region. The spectra show no significant differences between the two regions. The appearance of the oxide layer, together with the Raman spectra, suggests that the oxide has grown via a Stranski–Krastanov (SK) – sometimes referred to as “layer plus island” – mode. In
Raman Intensity (a.u.)
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Nodular region Fine-grained region
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Wavenumber (cm-1)
Fig. 8. Raman spectra taken from two regions of the oxide layer formed on the Ce–5La alloy.
SK growth the film initially formed is as a layer that completely covers the metal; however, after a critical thickness is reached (which depends on the chemical potential of the film and the amount of strain present) island growth starts to predominate. In a strained film, the formation of islands is energetically favourable, as it reduces the strain energy in the crystal [19]. In the present study, the lattice parameter of the oxide on Ce–5La was measured ˚ giving a lattice mismatch strain between the oxide to be 5.4283 A, film and the metal substrate of 4.9%. The spectra in Fig. 8 show the Raman peaks to be between 458 and 459 cm−1 ; overall, the range of peak positions were between 457 and 460 cm−1 , depending on the location from which the spectra were acquired. When these spectra were compared with those obtained from the CeO2 powder specimens it was observed that the peaks from the oxide layer appeared to be both broader as well as being shifted to lower wave numbers; for example, in the spectra shown in Fig. 8 the peak positions are approximately 6 cm−1 lower than that for the CeO2 powder shown in Fig. 5. The Raman peak positions and peak widths measured from the Ce–5La oxide layer and the CeO2 powder are summarised in Table 1. There are a number of possible explanations for these discrepancies. They are: (i) residual stress; (ii) the presence of the La alloying element; and (iii) non-stoichiometry of the oxide layer. Each of these will now be considered in turn. In coated systems residual stresses can arise from differences in the properties between coating and substrate, for example density or coefficient of thermal expansion. These residual stresses can be evaluated by Raman spectroscopy. Typically, tensile strain in thin films causes a shift in the position of the Raman bands to lower wave numbers with respect to a strain-free reference material [20]; in contrast, compressive strain leads to an increase in wave number of the Raman peak. An example of this is seen in diamond coatings produced by chemical vapour deposition (CVD) [21,22]. For coatings deposited on most substrates the overall residual stress is generally compressive, the most prominent exception being silicon nitride (Si3 N4 ) substrates [21]. Tensile stresses cause a downward shift in the wave number from its stress-free value of 1332 cm−1 while an upward shift indicates the presence of a compressive residual stress [22]. Such findings have also been noted by Yang et al. [23] for epitaxial aluminium nitride (AlN) films on sapphire. In order to ascertain whether the displacement of the Raman peak to lower wave numbers in Ce–5La was due to residual stress, Raman spectra were acquired from free-standing pieces of the oxide film that had been detached from the Ce–5La specimen. A typical spectrum taken from the free-standing oxide layer is shown in Fig. 9; a spectrum taken from the oxide layer adhering to the Ce–5La metal is also included in the figure. The lack of any significant differences between the two spectra indicates that the effect of residual stress cannot explain the peak shifts. The Raman peak positions and peak widths recorded from the free-standing oxide are also summarised in Table 1. An alternative explanation to residual stress as the cause of the peak shift is the presence of La. Popovic et al. [24] used Raman spectroscopy to study CeO2 nano-powders, some of which were doped with barium (Ba), gadolinium (Gd) and neodymium (Nd). They found large shifts in the first-order Raman-active modes, an example being Ce0.75 Nd0.25 O2 , which was approximately 450 cm−1 . More recently, Popovic et al. [25] doped CeO2 nanocrystals with iron (Fe) and found both softening (i.e. shifting to lower wavenumbers) and broadening of the Raman T2g mode, which was attributed to changing the valence state of the dopant, as a consequence of the electron-molecular vibration coupling [25]. In another study McBride et al. [16] doped CeO2 samples with La, the concentration of which ranged from 0 to 50 at. %. They found that the presence of La caused the Raman peak to shift to lower wavenumbers with
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Table 1 Summary of the Raman peak measurements from the oxide layer of a Ce–5La alloy exposed to air for 21 days and from free-standing oxide that was detached from the Ce–5La alloy. Also included are the measurements from the oxide layer of unalloyed Ce that has also been exposed to air for 21 days. Specimen
Mean Raman peak position (cm−1 )
Oxide layer on Ce–5La Free-standing oxide from Ce–5La Oxide layer on Ce CeO2 powder
459.7 459.5 462.1 464.1
Range of peak positions (cm−1 ) 457.8–460.9 458.8–460.1 460.9–462.5 463.5–465.9
increasing doping level. For the CeO2 specimen with a La content of 20 at.% La the magnitude of the shift of the Raman peak was approximately 6 cm−1 ; however, for the specimen with a La content of 5 at.% (the closest to the alloy composition in the present study) the shift was only 1 cm−1 . This suggests that the shift of the Raman peak position cannot be attributed solely to the La. In order to investigate further the effect of the La content a specimen of unalloyed Ce (supplied by Goodfellow) that had also been exposed to air for 21 days was examined. XRD of this specimen (not shown) exhibited no significant differences from that of the Ce–5La alloy with the dominant peaks being those of CeO2 . Raman spectra were acquired from the oxide layer under the same conditions as those for the Ce–5La alloy and a typical spectrum is included in Fig. 9. It shows the peak position (462 cm−1 ) is slightly higher than it was in the alloy, although still lower than that of the CeO2 powder. The positions and widths of the Raman peaks in the unalloyed Ce are also summarised in Table 1. The position of the Raman peak in unalloyed Ce, which is 2–3 cm−1 lower than that of CeO2 powder suggests that La may not be the sole cause of the shift to lower wave numbers. Another possible cause could be the sub-stoichiometric nature of the oxide layer. Indeed, Popovic et al. [25] have reported the T2g mode of CeO2-y nanocrystals (<5 nm in size) to be centred at approximately 457 cm−1 , a finding attributed to the sub-stoichiometric nature of the oxide [25]. In CeO2 the triply degenerate stretching vibration of the CeO8 vibrational unit at ∼465 cm−1 is due to the movement of the oxygen atoms. This renders the vibration very sensitive to microstructural changes such as oxygen sublattice disorder and non-stoichiometry [26]. As the oxygen vacancy concentration increases, the 465 cm−1 T2g mode of the fluorite structure shifts to lower frequency, broadens and becomes asymmetric. These changes may also be accompanied by the appearance of a broad feature at approximately 570 cm−1 , which has also been attributed to the O vacancies [27].
Raman Intensity (a.u.)
Free-standing Ce-5La oxide
459
Unalloyed Ce oxide layer
462
700
650
600
550
500
450
400
9.4 9.6 7.4 4.3
Range of peak widths (cm−1 ) 9.0–9.7 9.3–10.1 6.3–10.3 4.0–4.7
In the present study, in addition to the shifts of the Raman peak to lower wave numbers (discussed above), Table 1 shows that the mean peak half width (at half maximum) of the oxide film on Ce–5La was 9.4 cm−1 , which is more than twice that of the CeO2 powder specimen (4.3 cm−1 ). Moreover, close inspection of the spectra reveals the presence of some peak asymmetry with greater broadening on the lower wavenumber side of the peaks. Furthermore, some spectra did exhibit very weak features in the range 520–570 cm−1 , although in many cases they were barely distinguishable from the baseline. Although the presence of La does not appear to account for the entire shift in the T2g peak it can exert an additional influence on the spectra arising from the substitution of the tetravalent Ce by the trivalent La. In the oxide layer, for every two La3+ ions that replace the Ce4+ ions, one O vacancy is needed to balance the charge [16]. The data in Table 1 appear to support this with both a greater shift of the Raman peak position and mean peak half width observed on the Ce–5La oxide layer compared with that of the unalloyed Ce. The oxygen content in the oxide layer of the Ce–5La alloy was quantified by EDS and a mean value of 65 at.% (the stoichiometric value is 67 at.%). The oxygen content was also measured on the oxide layer of unalloyed Ce which had also been exposed to air for 21 days; a mean value of 63 at.% was determined. These values are accurate to approximately ±1%. Although the oxygen contents as measured by EDS should only be regarded as an approximate indication, these values, together with the shifts and broadening of the Raman peaks, suggest that the oxide layers in both Ce–5La and unalloyed Ce are slightly sub-stoichiometric. The greater shifts in the Raman peak in the Ce–5La oxide layer are likely to be due to a combination of a higher O vacancy concentration and, to a lesser extent, the presence of La. 4. Conclusions The high spatial resolution of Raman spectroscopy has enabled microstructural inclusions in a Ce–5La alloy to be identified as CeO2 . In conducting these analyses, it has been shown that care must be taken to avoid excessive laser power which causes oxidation of the Ce–5La alloy to form CeO2 and introduce extrinsic features to the observed spectra. However, this study also demonstrates that when used carefully with low laser power (∼1 mW) Raman spectroscopy can distinguish between the oxide inclusions and inclusion-free regions of the microstructure. The oxidation of the Ce–5La alloy specimens in air was also shown to result in the growth of a CeO2 surface layer, as confirmed by XRD, EDS and Raman spectroscopy. A comparison of the Raman spectra acquired from the oxide layer formed on Ce–5La with that on unalloyed Ce has shown a greater shift and broadening of the Raman peak on the former. The causes of this are attributed to a combination of a higher O vacancy concentration and, to a lesser extent, the presence of La in the alloy.
459
Ce-5La oxide layer
Mean peak half width (cm−1 )
350
300
Wavenumber (cm-1) Fig. 9. Raman spectra taken from the oxide layer formed on the unalloyed Ce and Ce–5La alloy after 21 days in air, together with a spectrum taken from a free-standing oxide layer formerly adhered to the Ce–5La alloy.
Acknowledgments The authors would like to thank Mr. M. Matthews and Dr. S. Ennaceur for their help in the EDS analysis and Dr. P. Roussel,
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