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Analysis of Metal Site Preference and Electronic Structure of Brownmillerite-Phase Oxides (A2B′xB2-xO5; A ) Ca, Sr; B′/B ) Al, Mn, Fe, Co) by X-ray Absorption Near-Edge Spectroscopy Andrew P. Grosvenor and John E. Greedan* Department of Chemistry and Brockhouse Institute for Materials Research (BIMR), McMaster UniVersity, Hamilton, Ontario L8S 4M1, Canada ReceiVed: February 17, 2009; ReVised Manuscript ReceiVed: April 29, 2009
X-ray absorption near-edge spectroscopic (XANES) measurements of the Fe and Co K-edges for a series of Brownmillerite-type transition-metal oxides (A2B′xB2-xO5; A ) Ca, Sr; B′/B ) Al, Mn, Fe, Co) have been made. In these compounds, the metal atoms are present in both octahedral and tetrahedral coordination environments. The spectra were interpreted with the aid of electronic structure calculations. Significant changes in intensity and energy were observed in the Fe K-edge spectra as the average Fe3+ coordination number (CN) was changed from 5.5 (Ca2AlFeO5) to 4.2 (Ca2FeMnO5). In Ca2CoxFe2-xO5, the Fe K- and Co K-edge XANES spectra indicated that Fe has a slight octahedral site preference in these series. This confirmed that size effects play a significant role in controlling the site preference of metal atoms in these compounds. The change in the average Fe CN with Co substitution was examined by analysis of a 3D plot, which compared all of the changes observed in the spectra. This type of analysis gives an accurate depiction of the change in Fe CN as compared to standard materials. The Fe K-edge peak intensities in Ca2-ySryFe2O5 decreased with higher values of y, a result of decreased Fe 4p(3d)-O 2p overlap with increasing Sr concentration. 1. Introduction Brownmillerite-type oxides (oxygen-deficient perovskites) are well-known and have received considerable attention, particularly because of the variation in space group with composition that can occur.1-4 The chemical formula of Brownmillerites can be written as A2B′xB2-xO5, where A ) alkaline-earth atoms (e.g., Ca, Sr) and B′/B ) group III or transition-metal atoms (e.g., Al, Ga, Cr, Mn, Co, Fe).3,5 In these compounds, the metal (B′/ B) charges are 3+ but can be a mixture of 2+, 3+, and 4+ when a rare-earth atom is substituted into the system (e.g., LaAMn2O5, A ) Ca, Sr).6,7 These G-type antiferromagnetic materials may have many applications (e.g., ion conductors in fuel cells, catalysts) and are an important component of Portland cement (e.g., Ca2AlxFe2-xO5).2,8-11 Because of their interesting properties, applications, and structure, these materials have been studied extensively by means of physical property measurements, Mo¨ssbauer spectroscopy, and X-ray, electron, and neutron diffraction.3,5,8,12-18 However, much less attention has been given to studying these materials by X-ray or electron spectroscopy.6,7,19-21 Such analyses, discussed herein, can provide valuable information on both the electronic structure and the crystal structure. The Brownmillerite structure is shown in Figure 1a. As compared to perovskite-type materials, the Brownmillerites contain two oxygen vacancies in every second layer, forming an orthorhombic structure consisting of layers of metal centered octahedra (Oh) and distorted tetrahedra (Td) stacked along the b axis.3 In the following discussion, B′ represents metal atoms in tetrahedral sites (e.g., Al in Ca2AlFeO5), and B represents octahedrally coordinated metal atoms (e.g., Fe in Ca2AlFeO5).4 The BO6 octahedra are corner sharing and tilted with respect to the b axis. Variations in the space group result from differences * Corresponding author. E-mail:
[email protected].
Figure 1. (a) Orthorhombic crystal structure of Brownmillerite-phase compounds, A2B′xB2-xO5. The BO6 octahedra are dark gray, the B′O4 tetrahedra are light gray, and the interstitial alkaline-earth, A, atoms are gray spheres. (b) Cooperative displacement of B′ (larger spheres) and O (smaller spheres) leading to either L- or R-handed Td chains. The B′-O-B′ bond angle is labeled (θ).
in ordering of zigzag chains of the B′O4 units (along the [110] direction) in the tetrahedral layers.3 These zigzag chains form by a cooperative displacement of B′ and coordinating O atoms leading to the chains having one of two distinct orientations (left (L)- or right (R)-handed; see Figure 1b).2,3 The L- and R-handed tetrahedral chains possess a dipole moment of equal magnitude but opposite direction.2,3,17 The magnitude of the dipole moment depends on the B′-O-B′ bond angle (θ, Figure 1b).3,17 The three most common space groups used to describe the Brownmillerite structure are Pnma, Icmm, and Ibm2, although the structures of some materials are best described by a super structure space group.3 These space groups are similar and differ only in inter- and intralayer tetrahedral chain ordering.3 The tetrahedral chain ordering is related to the values of θ and the b lattice constant, which describes the interlayer separation.3 Both θ and b can be altered through substitution of the metal atoms in the B′ site or by exchanging the alkaline-earth, A, in
10.1021/jp901467v CCC: $40.75 2009 American Chemical Society Published on Web 06/04/2009
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Figure 2. Change in lattice constants with substitution for (a) Ca2CoxFe2-xO5 and (b) Ca2-ySryFe2O5. (c) Powder XRD pattern for Ca2Fe2O5 and Ca2Co0.50Fe1.50O5. The main Pnma peaks are labeled and are common to the Ibm2 phase except for the (131) and (151) reflections.
the interstitial site.3 The chain ordering results from having to compensate the dipole moment formed along the tetrahedral chains.3,17 If θ and b are small (e.g., Ca in Ca2GaMnO5), then the dipole moment is large and a small interlayer separation is found (Pnma).3 This leads to interlayer ordering along the b direction with the following pattern observed: L-Oh-R-Oh-... (L or R represents Td layers containing chains of one orientation). If θ and b are large (e.g., Sr2AlMnO5), the dipole moment is small and no interlayer ordering is required.3 This results in a random distribution of R- and L-handed chains within and between the tetrahedral layers (Icmm). When θ is large and b is small (e.g., Ca2AlMnO5), a small dipole moment is present, and domains having Td layers containing all R- or L-handed chains are formed.3 The weak dipole moment is compensated by twin domains containing chains of the opposite order (Ibm2). The composition, thermal treatment used to synthesize these materials, and the temperature at which they are examined affect the tetrahedral chain ordering (and therefore the space group).14,15,22 It is easy to differentiate between the Pnma and Ibm2 (or Icmm) space groups by X-ray diffraction as the former contains (131) and (151) reflections that are not found in the latter.14 Differentiating between the Ibm2 and Icmm space groups requires electron diffraction, allowing for detection of diffuse scattering.15 However, examination of θ and b alone can be informative (e.g., b is larger for compounds that adopt the Icmm vs Ibm2 space group). Analysis of the B-O bond lengths in the octahedra reveals that the axial bonds (along the b axis) are longer than the equatorial bonds.2,3 Such an observation is expected for Ca2FeMnO5 where the Mn3+ atoms preferentially reside in the Oh site and undergo a Jahn-Teller distortion.8 However, bond lengthening along the b axis is also observed in materials like Ca2Fe2O5 and is a consequence of the octahedra having to connect to the much thinner tetrahedral layers.2 An interesting
aspect of these materials is the preference for metals to reside in Td or Oh sites. Ionic radius appears to be important for Fecontaining compounds, as high-spin Fe3+ has no crystal field stabilization energy-related site preference.23 For example, in Ca2AlxFe2-xO5, the larger Fe3+ atoms (ionic radius of 0.645 Å as compared to 0.535 Å for Al3+)24 preferentially occupy the Oh sites (long B-O bond lengths).4 X-ray absorption near-edge spectroscopy (XANES) is often applied to the study of changes in the electronic and crystal structure of materials by analysis of the absorption edge energy and line shape. Herein, transition-metal K-edge XANES spectra will be reported for a series of Brownmillerites (Ca2AlFeO5, Ca2Fe2O5, Ca2FeMnO5, Ca2CoxFe2-xO5, and Ca2-ySryFe2O5) with focus on how the spectra change with coordination number (CN). The variation in the absorption-edge energies and peak intensities will be reported as one metal is substituted for another into the Td or Oh sites, and attention will be given to Ca2CoxFe2-xO5 where B′ and B are similar. Ca2CoxFe2-xO5 has been investigated previously by Mo¨ssbauer spectroscopy for x ) 0.55, and it was observed that the Co atoms had a slight preference for residing in the tetrahedral site.5 Analysis of the Ne´el temperature (TN) revealed that high spin Co3+ is present as TN would decrease greatly as compared to Ca2Fe2O5 if low spin Co3+ were present, which is not the case.5 In this study, the limiting value of x has been extended, and Fe K- and Co K-edge XANES spectra have been collected to examine how this site preference changes for higher values of x. The change in the strength of the metal-O bond with substitution of the alkaline-earth (e.g., Ca2-ySryFe2O5) will also be discussed. 2. Experimental Section 2.1. Synthesis. Single-phase materials were formed by stoichiometric reaction of alkaline-earth carbonates (ACO3) and metal oxides ((B/B′)2O3, MnO, or CoO). The starting materials
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Figure 3. (a) Fe K-edge spectra from Ca2AlFeO5, Ca2Fe2O5, and Ca2FeMnO5. The three major peaks in the spectra, described in the text, are labeled as A, B, and C. The peaks observed at energies higher than C result from the excitation of 1s electrons into higher energy continuum states and/or EXAFS. The inset shows the changes in the pre-edge (A) peak intensity and energy of the inflection point of peak B (indicated by the thin solid or dashed lines) with Fe CN. (b) Similarity in absorption energy and line shape between Fe2O3 and Ca2Fe2O5 as compared to FeO. (c) Fitted pre-edge peaks (marked by the appropriate 3d symmetry label) from Ca2FeMnO5 and Ca2AlFeO5. The pre-edge background was removed by fitting the spectra to an arctangent function. The ratio of the peak areas used to fit the Ca2AlFeO5 Fe pre-edge is less than 3:2 (t2g:eg) because CN < 6. (d) Pre-edge peaks areas from Ca2AlFeO5, Ca2Fe2O5, Ca2FeMnO5, and Fe3+ and Fe2+ standard compounds versus average Fe CN. Data points from the Brownmillerites are the average of two separately synthesized samples. The pre-edge peak area of Fe2O3 was not included because of the presence of a low energy intersite hybrid making fitting of this peak difficult.32
Figure 4. (a) TB-LMTO ASA calculated Fe 3d/4p, O 2p, and Ca 3d conduction states for Ca2Fe2O5 (0 eV represents the Fermi edge). The main contributions have been labeled as A, B, or C in accordance with Figure 3a and the spectral peak identities described in section 3.2. Even though the calculation incorrectly predicts that this material is a metal, it does correctly predict the distribution of states. (b) Change in Fe 3d and 4p states depending on Fe coordination.
were CaCO3 (Cerac, 99.999%), SrCO3 (Alfa Aesar, 99.99%), Al2O3 (Alfa Aesar, 99.5%), Fe2O3 (Alfa Aesar, 99.998%), MnO (Alfa Aesar, 99.99%), and CoO (Cerac, 99.5%). The reactants were mixed and heated in alumina crucibles in air to ∼900 °C and held for >2 h to decompose the carbonates. The materials were pressed into pellets and heated in air at ∼1200-1300 °C for >2 days with intermediate grinding. All materials studied by XANES were quenched in air except for Sr2Fe2O5, which was quenched in N2(l) to produce the pure Brownmillerite phase, and Ca2FeMnO5, which was reacted at 1200 °C and cooled
under Ar(g) after being heated in air to restrict the oxidation of Mn.8,25,26 The materials produced were Ca2Fe2O5, CaAlFeO5, Ca2FeMnO5, Ca2CoxFe2-xO5 (x ) 0.25, 0.50, 0.75, 1.00), and Ca2-ySryFe2O5 (x ) 0.10, 0.25, 0.80, 2.00). The purity was checked by powder X-ray diffraction using a PANalytical X’Pert Pro MPD diffractometer with a linear X’Celerator detector and a monochromatic Cu KR1 X-ray source. 2.2. Transition-Metal (Fe, Co) K-Edge XANES. XANES spectra of the Fe and Co K-edges were collected using the Pacific Northwest Consortium/X-ray Operations and Research Collaborative Access Team (PNC/XOR-CAT, Sector 20) bending magnet beamline (20BM) at the Advanced Photon Source (APS), Argonne National Laboratory. A silicon (111) double crystal monochromator was used to provide a monochromatic photon flux of ∼1011 photons/s, with a resolution of 1.4 eV at 10 keV and a beam size of approximately 1 × 5 mm2. Finely ground samples were sandwiched between Kapton tape and positioned 45° to the X-ray beam. Transmission spectra were measured with N2-filled ionization chambers. Through the absorption edge, the X-ray energy was increased by 0.15 eV per step. For the K-edge spectra, a standard of the elemental metal was positioned behind the sample and analyzed concurrently in transmission mode with N2-filled ionization chambers, and the peak maximum of the first derivative was calibrated to the accepted values of 7112 eV (Fe) or 7709 eV (Co).27 The precision of the measurements was estimated to be (0.1 eV on the basis of multiple analyses of the compounds. All XANES
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Figure 5. TB-LMTO ASA calculated crystal orbital Hamilton population (COHP) curves for Fe-O and Fe-Ca interactions from Ca2Fe2O5. The main contributions have been labeled as A, B, or C in accordance with Figure 3a and the spectral peak identities described in section 3.2.
spectra were analyzed using the Athena software program.28 To compare the change in pre-edge peak area and absorption-edge energy of the Brownmillerites, standard spectra were collected for CuFeS2 (Alfa Aesar), FeCl3 · 6H2O (Fisher Scientific), FeO,29 FePO4 · xH2O (Alfa Aesar), and Fe2O3 (Alfa Aesar). 2.3. Band Structure Calculations. To help interpret the different features in the Fe K-edge XANES spectra, tight-binding linear muffin-tin orbital band structure calculations were performed within the atomic spheres approximation (TB-LMTO ASA).30 The Ca, Sr, Fe, and O conduction states from Ca2Fe2O5 and Sr2Fe2O5 were calculated. The calculations were performed with >144 k points. While LMTO ASA does not allow for correlation associated with the Fe sites, most of the states involved in the observed transitions are of np character, and LMTO is adequate for those purposes. 3. Results and Discussion 3.1. Affect of Metal and Alkaline-Earth Substitution on Structure. The lattice constants determined for Ca2Fe2O5, Ca2AlFeO5, Ca2MnFeO5, and Sr2Fe2O5 are close to those reported in the literature.4,8,15,25 For the Ca2CoxFe2-xO5 and Ca2-ySryFe2O5 series, the lattice constants interpolate smoothly from the end members (see Figure 2a,b and Table S1 in the Supporting Information). Both series experience a space-group change from Pnma to Ibm2 (Ca2CoxFe2-xO5) or Icmm (Ca2-ySryFe2O5), which was identified by analysis of the (131) and (151) reflections in the XRD patterns (Figure 2c). Although it is difficult to distinguish between Ibm2 and Icmm by XRD, the similar value of b in Ca2CoxFe2-xO5 (y > 0.25) and the larger value of b in Ca2-ySryFe2O5 (y > 0.1) as compared to Ca2Fe2O5 agrees with the selection rules discussed in section 1. Electron diffraction has confirmed that the bulk structure of Sr2Fe2O5 is best described by the Icmm space group.15 It was observed for Ca2Co0.25Fe1.75O5 that when quenched quickly in N2(l), the space group was Ibm2 (or possibly a mix of Pnma and Ibm2 based on the presence of a weak (131) reflection). No space group change was observed for Ca2Fe2O5 or Co2CoxFe2-xO5 (x > 0.25). 3.2. Fe K-Edge XANES of Ca2AlFeO5, Ca2Fe2O5, and Ca2FeMnO5. The normalized Fe K-edge XANES spectra of Ca2AlFeO5, Ca2Fe2O5, and Ca2FeMnO5 are presented in Figure 3a with the three main components labeled as A, B, and C. These compounds were investigated first as they have been well
J. Phys. Chem. C, Vol. 113, No. 26, 2009 11369 studied by X-ray and neutron diffraction, and the average Fe CN is known (∼5.5 in Ca2AlFeO5, 5.0 in Ca2Fe2O5, and ∼4.2 in Ca2FeMnO5).4,5,8 As observed in Figure 3a, all three components (A, B, C) change either in energy or in intensity, systematically, as the CN is varied. The spectral changes observed are not an effect of charge as it is fixed at 3+ (confirmed by comparison of the absorption edge energy to FeO and Fe2O3; see Figure 3b). Peak A is a pre-edge peak resulting, primarily, from a quadrupolar Fe 1sfFe 3d excitation.31-34 As inversion symmetry is lost and the coordination geometry is lowered from octahedral to tetrahedral, the 3d states are overlapped by Fe 4p states adding a dipolar contribution.31-33 (The Fe 3d/4p states are hybridized with O 2p states.) The change in the distribution of Fe 4p states with CN can be observed by analysis of the LMTO calculated conduction states of Ca2Fe2O5, which are presented in Figure 4a. Because dipolar excitations have a greater cross-section than do quadrupolar excitations, the preedge peak intensity increases substantially as the CN is lowered.31 Methods for fitting the pre-edge have been developed and used here to extract the pre-edge peak area.31,32 In octahedral Fe3+ environments, where the excitation is quadrupolar, the 1s electrons are excited into either Fe 3d t2g or eg states, and, in the case of Ca2AlFeO5 (Fe CN ) 5.5), the pre-edge was fitted by two Lorentzian peaks with a ratio of near 3:2 and a peak full width at half-maximum (fwhm) of g1.7 eV (Figure 3c).31,32 As the coordination geometry is lowered to tetrahedral, and 4p states overlap the 3d states, the pre-edge is fitted by two peaks; the lower energy peak is the 1sf3d e excitation, while the higher energy, more intense peak represents the 1sf3d t2/4p excitation (e.g., Ca2FeMnO5, Figure 3c).31,32 When the Fe atoms are equally distributed between the Oh and Td sites (e.g., Ca2Fe2O5), the pre-edge is fitted by two peaks having similar areas. Although the pre-edge peak of Ca2AlFeO5 (CN ) 5.5) is wider than that of Ca2FeMnO5 (CN ) 4.2), it is not as broad as expected, a result of the presence of some Td contributions. This makes analysis of the crystal field splitting difficult; however, as will be shown below, it is the pre-edge peak area that is most important. The pre-edge peak areas of Ca2AlFeO5, Ca2Fe2O5, and Ca2FeMnO5 are presented in Figure 3d along with those of Fe2+ and Fe3+ standard compounds. As the average Fe CN decreases in the Brownmillerite compounds, the preedge peak area increases. Considering numerous studies of transition-metal containing materials, peak B is best described as a dipolar Fe 1sfFe 4p excitation.35-37 However, upon examination of the Fe 3d and 4p partial density of states (Figure 4b) in the region from 4 to 8 eV, a dipolar intersite hybrid cannot be ruled out. This type of excitation is a result of 1s electrons being promoted into 3d states on adjacent metal atoms, strongly hybridized with 4p states on the absorbing atom through O 2p states.38,39 The absorption edge energy of peak B changes significantly (see inset of Figure 3a) because of the change in Fe CN. Similar changes in absorption energy with CN have been previously observed in transition-metal oxides.34 The shift to lower energy with decreasing CN is well-known for main group metals (e.g., Al) and has been linked to changes in core-hole screening of the final state.40,41 An octahedral coordination leads to greater final-state core-hole screening, and therefore a higher overall absorption energy is observed as compared to a tetrahedral environment.40,41 Ground-state effects are important to shifts in absorption energy but appear to be less important in this case, which can be identified by the observation that the pre-edge peak does not appear to shift as significantly in energy. The
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Figure 6. (a) Fe K-edge spectra from Ca2CoxFe2-xO5 with the inset showing the small intensity change observed for the pre-edge peak with Co substitution. (b) 3D plot of the change in the peak area of A versus the absorption energy of B versus the intensity of C for Brownmillerites having a known Fe CN (b: Ca2AlFeO5, CN ) 5.5; Ca2Fe2O5, CN ) 5; Ca2FeMnO5, CN ) 4.2) and Ca2CoxFe2-xO5 (2). The Brownmillerite data points represent the average of two separately synthesized samples. The inset of (b) shows the change in A versus B versus C for the Ca2CoxFe2-xO5 samples more clearly, proving the slight octahedral preference exhibited by the Fe atoms with increasing Co concentration (x).
Figure 7. Co K-edge spectra from Ca2CoxFe2-xO5. The peaks labeled as A, B, and C can be described by the processes explained in section 3.2 in terms of the Fe K-edge spectra. The inset shows the slight decrease in the pre-edge peak intensity (area) observed with increasing Co concentration.
Figure 8. Fe K-edge spectra from Ca2-ySryFe2O5. The identities of A, B, and C are explained in section 3.2. The insets show more clearly the changes in the intensities of peaks A and C observed with increasing Sr concentration.
reason peak B shifts in energy more than peak A may be described by the idea of hard and soft screeners where the tightly bound 3d states screen the core-hole better than the diffuse 4p states.34 The last prominent peak in the Fe K-edge spectra (C) can be described as a dipolar excitation of 1s electrons into 4p states that are hybridized with O 2p states (see Figure 4a).39 The decrease in peak intensity with CN can be related to the fewer
hybridized Fe 4p-O 2p states present when Fe is in a tetrahedral versus octahedral environment. This can be observed in Figure 4b by inspection of the change in the density of Fe 4p states depending on CN. The lowering of the intensity of peak C with decreasing CN has been observed in other compounds.20,42 Alternatively, peak C might be described as a multiscattering resonance (MSR) peak.43 These MSR peaks are a special case of the EXAFS (extended X-ray absorption fine-structure) phenomena whereby a photoelectron excited from an atom can interact with multiple atoms within the sample and be backscattered toward the atom from which it was initially excited, modifying the absorption coefficient (appears as oscillations in the spectrum).44 The MSR path lengths are long so the peaks appear at energies near the absorption edge (i.e., they overlap the XANES).44 The intensities of MSR peaks have been found to vary with CN and may explain the change in intensity of peak C.43,45 However, if peak C were a MSR, the peak should shift to lower energy with increasing Fe-O bond lengths, which is not observed in Figure 3a.45 In fact, the energy of peak C actually decreases slightly with decreasing CN in a fashion similar to that observed for peak B. The lack of a similar change in intensity of peak B as compared to peak C with CN might be related to the weakness of the interaction (see Figure 3a and Figure 4b). Another factor that diminishes the sensitivity of the intensity of peak B to changes in CN may be that it contains contributions from 1s electrons excited into Fe 4p states hybridized with O 2p or Ca 3d states. This is observed by analysis of the crystal orbital Hamilton populations (COHP, Figure 5). Although the Ca-Fe distances are long (>3 Å),4 it is not unusual for such an interaction to occur. For example, in rare-earth filled Skutterudites (e.g., REFe4Pn12; RE ) La, Ce, Pn ) P, Sb), the RE 4f states interact with Fe 3d states even though the RE atoms are present in a dodecahedral cage of Pn atoms with the Fe atoms present in the second coordination shell.46 3.3. Fe K- and Co K-Edge XANES of Ca2CoxFe2-xO5. As was shown in the previous section, not only do transition-metal K-edge XANES spectra provide valuable information on the electronic structure of materials, there are also multiple features in the spectra that can be used to examine changes in CN. Iron K-edge spectra from Ca2CoxFe2-xO5 (0.00 e x e 1.00) are presented in Figure 6a with the main spectral features outlined
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TABLE 1: Bond Lengths for Fe in an Octahedral (Oh) or Tetrahedral (Td) Coordination Environment in A2Fe2O5, A ) Ca, Sr Ca2Fe2O5 (ref 4) Fe(Oh)-O (Å) 1.96 × 2 (equatorial) 1.97 ×2 (equatorial) 2.12 × 2 (axial)
Sr2Fe2O5 (ref 49) Fe(Td)-O (Å) 1.84 × 2 1.91 × 2
in section 3.2 labeled as A, B, and C. The spectra are all very similar with only subtle differences being observed. Inspection of the pre-edge peak (see inset of Figure 6a) shows that its intensity decreases with greater x, implying that Fe has a minor octahedral preference. This is verified by small increases in the absorption energy of peak B and the intensity of peak C with increasing x. To show this more clearly, the change in the peak area of A versus the absorption energy of B versus the intensity of C for Ca2AlFeO5, Ca2Fe2O5, CaFeMnO5, and Ca2CoxFe2-xO5 has been plotted in Figure 6b. The Ca2CoxFe2-xO5 data points overlap those of Ca2Fe2O5 (average Fe CN ) 5.0) when x is small and shift toward Ca2AlFeO5 (average Fe CN ) 5.5)4,5 with increasing x. The above observations can be confirmed by analysis of the Co K-edge spectra, which are presented in Figure 7. Because of the presence of Fe K-edge EXAFS peaks near the Co K-edge, which can make normalization of the spectra difficult, only those spectra where x is large (g0.5) are presented. The spectra compare well to those previously reported for Sr2Co2O5 in terms of energy (confirming Co3+) and line shape and contain three features that are similar to those observed in the Fe K-edge spectra.20 All three Co K-edge spectra appear to overlap with only slight differences being observed. The pre-edge peak intensity (inset of Figure 7) decreases in intensity as x is increased, confirming that Co has a minor preference for the Td site, an observation that has also been made for Sr2CoFeO5.22 This result is interesting as high spin Co3+ has a small octahedral site preference (based on crystal field stabilization energy).23 The fact that Co3+ preferentially resides in the Td site in Ca2CoxFe2-xO5 confirms that ionic radius (rCo3+, 0.610 Å < rFe3+, 0.645 Å)24 is a factor to be considered when determining metal site preference in Brownmillerite-phase compounds. This appears to be a counter-example to the case of Ca2Fe1.5Cr0.5O5 where Cr3+ with rCr3+ ) 0.615 Å and a large octahedral crystal field stabilization energy does occupy the Oh site.23,24,47 Neutron diffraction can be used to further analyze the Ca2CoxFe2-xO5 system given the excellent contrast between Co and Fe.48 3.4. Fe K-Edge XANES of Ca2-ySryFe2O5. The Fe K-edge spectra from Ca2-ySryFe2O5 are presented in Figure 8. In these materials, the Fe3+ atoms are equally distributed between the Td and Oh sites so the changes observed are not related to the Fe CN. The largest change in the spectra occurs for peak C, which decreases in intensity with increasing Sr concentration (y). As was described in section 3.2, peak C is likely a dipolar excitation of 1s electrons into 4p states that are strongly overlapped by O 2p states. Although the intensity of this peak was shown to principally change with CN due to a change in the number of hybridized Fe 4p-O 2p states, one might suggest that this peak intensity will also be affected by the degree of overlap of the Fe 4p and O 2p states. As Sr is substituted into the system, the b lattice parameter increases dramatically (see Figure 2b). (The enhanced low temperature O mobility in Sr, versus Ca, containing Brownmillerites has been associated with this lengthening.26) Representative Fe-O bond lengths for Ca2Fe2O5 and Sr2Fe2O5 are presented in Table 1.4,49 The bond lengths are longer in Sr2Fe2O5 than in Ca2Fe2O5 with the largest difference observed between the axial Fe-O bonds in the
Fe(Oh)-O (Å) 1.98 × 4 (equatorial) 2.20 ×2 (axial)
Fe(Td)-O (Å) 1.85 × 2 1.88 1.90
Figure 9. Change in Fe 4p partial density of states depending on coordination and effect of increasing the unit cell as Ca is replaced with Sr. Because of the difficulty in calculating systems containing disorder, the Sr2Fe2O5 calculation was performed using the Imb2 space group, instead of Icmm.
octahedra (i.e., those directed along the b axis), which increase by ∼4% (the other bond lengths vary by
