3)

Solid State Sciences 2 (2000) 821– 831

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Properties of the perovskites, SrMn1 − x Fex O3 − l (x= 1/3, 1/2, 2/3) Ian D. Fawcett a, Gabriel M. Veith a, Martha Greenblatt a,*, Mark Croft b, Israel Nowik c a

Department of Chemistry, Rutgers, The State Uni6ersity of New Jersey, Piscataway, NJ 08854, USA b Department of Physics, The State Uni6ersity of New Jersey, Piscataway, NJ 08854 -8019, USA c Racah Institute of Physics, The Hebrew Uni6ersity, Jerusalem 91904, Israel Received 28 February 2000; accepted 12 April 2000 Dedicated to Professor J.M. Ho¨nig on the occasion of his 75th birthday

Abstract The SrMn1 − x Fex O3 − l (x=1/3, 1/2, 2/3) phases have been prepared and are shown by powder X-ray and neutron (for x =1/2) diffraction to adopt an ideal cubic perovskite structure with a disordered distribution of transition-metal cations over the six-coordinate B-site. Due to synthesis in air, the phases are oxygen deficient and formally contain both Fe3 + and Fe4 + . Magnetic susceptibility data show an antiferromagnetic transition at 180 and 140 K for x = 1/3 and 1/2, respectively and a spin-glass transition at 5, 25, 45 K for x =1/3, 1/2 and 2/3, respectively. The magnetic properties are explained in terms of super-exchange interactions between Mn4 + , Fe(4 + l) + and Fe(3 + m) + . The XAS results for the Mn-sites in these compounds indicate small Mn-valence changes, however, the Mn-pre-edge spectra indicate increased localization of the Mn-eg orbitals with Fe substitution. The Mo¨ssbauer results show the distinct two-site Fe(3 + m) + /Fe(4 + l) + disproportionation in the Mn- substituted materials with strong covalency effects at both sites. This disproportionation is a very concrete reflection of a localization of the Fe-d states due to the Mn-substitution. © 2000 E´ditions scientifiques et me´dicales Elsevier SAS. All rights reserved. Keywords: SrMn1 − x Fex O3 perovskite; Electrical; Magnetic; Mo¨ssbauer; XAS properties

1. Introduction Transition metal oxides have been studied extensively due to their fascinating structural, electronic and magnetic properties. In recent years, the mixedvalence perovskite manganates, Ln1 − x Ax MnO3 (where Ln is a rare-earth cation and A is a divalent cation) [1,2], have received considerable attention, for example, when x  0.2 – 0.4 so-called colossal * Correspondence and reprints. E-mail address: [email protected] (M. Greenblatt).

magnetoresistance (CMR) is found. The properties of the manganates are generally explained in terms of double exchange [3]. This mechanism considers the transfer of an electron between neighboring Mn3 + (3d4) and Mn4 + (3d3) ions through an MnOMn path, which is facilitated by the ferromagnetic exchange coupling of the local spins of the e1g and t32g Mn3 + and Mn4 + ions, respectively. We have prepared a series of compounds in which Fe4 + (3d4, isoelectronic with Mn3 + ) is introduced into a Mn4 + containing oxide, SrMnO3. The parent compound, SrMnO3 orders antiferromagnetically

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and possesses a hexagonal unit cell [4 – 6]. a-SrMnO3 is isostructural with BaNiO3 and is made up of four close-packed SrO3 layers in an ABAC stacking sequence along the c-axis with MnO6 octahedra sharing faces with close MnMn distances. At the other doping extreme, SrFeO3, which crystallizes in the ideal cubic perovskite structure, is an antiferromagnet with a helical spin structure [7]. The octahedrally coordinated Fe4 + ions are found in a high-spin configuration, however, there is an absence of cooperative Jahn – Teller distortion [8]. The motivation behind this work was to create an oxide with the perovskite structure containing both 3d3 and 3d4 ions, as found in the CMR manganates mentioned above. However, this simple approach can lead to additional complications; the presence of two or more transition-metal ions on the B-site can give rise to complex magnetic interactions. For longrange magnetic order to be established, an ordered array of the B-site transition metals is important [9]. For example, spontaneous magnetization has been observed in La2CoIrO6 [10], which contains an ordered array of cations over the six-coordinate site, whereas Sr2FeRuO6 [11], which contains a random distribution of transition-metal cations, shows spinglass behavior. It is often found that, due to cation disorder, long-range magnetic order is frustrated and the material behaves as a spin-glass. Mixtures of d3/d4 cations in the six-coordinate site have been studied in the perovskite LaMn1 − x Crx O3 [12] and the low dimensional n =3 Ruddlesden – Popper phase, A4Mn2FeO10 − l (ACa, Sr) [13]. In LaMn1 − x Crx O3, the ferromagnetic coupling was preserved, but the compounds were insulators and CMR behavior was lost. The A4Mn2FeO10 − l phases were found to be insulators that ordered antiferromagnetically below 100 K, with a spin glass transition at  11 K. In this paper, we report the synthesis and characterization, by powder X-ray diffraction (PXD), magnetic susceptibility (), electrical resistivity (z), X-ray absorption spectroscopy (XAS) and Mo¨ssbauer spectroscopy, of SrMn1 − x Fex O3 (x =1/3, 1/2 and 2/3). The series adopts a cubic perovskite structure in which three-dimensional (3-D) magnetic interactions are possible. However, the properties of the series are complicated by oxygen deficiency, which introduces Fe3 + into the lattice. The interaction between three disordered transition-metal ions (Mn4 + , Fe(4 + l) + , Fe(3 + m) + ) in the six-coordinate site leads to complex behavior.

2. Experimental

2.1. Synthesis Samples were prepared by heating stoichiometric quantities of SrCO3, Fe2O3 and MnO2. The reaction mixture was pressed into pellets and heated in air at 800, 1000 and 1200 °C for 1 day each and 1300 °C for 2 days. The mixture was reground frequently, and the reaction was monitored by powder X-ray diffraction (XRD). PXD data were collected at room temperature with a Scintag PAD V diffractometer employing Cu-Ka radiation over the range 2052q/ °5 120, with a step size of 0.02°. In order to derive accurate lattice parameters, silicon was added as an internal standard.

2.2. Time-of-flight neutron data These were collected for the x= 1/2 sample at the Intense Pulsed Neutron Source (IPNS), Argonne National Laboratory (ANL). The sample was sealed in a cylindrical vanadium can and diffraction experiments were carried out on the General Purpose Powder Diffractometer (GPPD). Data were collected at room temperature covering the range 0.505d/ A, 5 2.89. Rietveld refinement of the data was undertaken using the program GSAS [14].

2.3. Oxygen content The O2 content of the samples was determined indirectly by iodometric titration employing an amperometric dead-stop end-point detection using the technique described by Licci et al. [15]. This method essentially determines the average valence of Mn and Fe allowing one to compute the oxygen content of the samples. The MnFe site valence (assuming Sr2 + ), the Fe site valence (further assuming Mn4 + ), and the equivalent O-deficit (again assuming Mn4 + ) are shown in Table 1.

2.4. Magnetic susceptibilities These were obtained using a Quantum Design SQUID magnetometer over the temperature range 5–300 K in an applied field of 1000 G. Data were collected after cooling in zero field (ZFC) and in an applied field (FC).

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2.5. Resisti6ity measurements Resistivity measurements were made using a standard four-probe method in the range 120 –300 K. Gold wire contacts were attached to the sintered polycrystalline samples with silver paint.

the energy is about 9 0.05 eV. All spectra were normalized to unity step in the absorption coefficient from well below to well above the edge.

2.7. Mo¨ssbauer The Mo¨ssbauer studies were performed using a Co:Rh source (50 mCi) and a conventional constant acceleration Mo¨ssbauer drive. Spectra of the three samples were collected at 4.2 and 200 K. The spectra were analyzed and least square fitted by a computer program which allowed a Gaussian distribution of magnetic hyperfine fields in the 4.2 K spectra, and a quadruple interaction distribution in the 200 K spectra. 57

2.6. X-ray absorption spectroscopy (XANES) The Mn and Fe K-edge XAS measurements were performed on beam lines X-19A and X-18B at the Brookhaven national synchrotron light source using a double crystal Si (311) monochromator. Fluorescence mode and transmission mode measurements were made and checked for consistency. The relative energies between the various spectra were established by careful comparison of the simultaneously run standard spectra. Particular care was taken to use an identical standard sample, which was maintained, in a constant position to accurately calibrate the chemical shift results. In general, the relative accuracy of Table 1 Titration parameters for SrMn1−x Fex O3−l

Ž(Mn,Fe) valence ŽFe valence (Mn4+ assumed) l

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x= 1/3

x= 1/2

x=2/3

3.84 3.52 0.08

3.78 3.56 0.11

3.68 3.52 0.16

Fig. 1. Powder XRD patterns for SrMn1 − x Fex O3 − l. Insert variation of lattice parameter, a, with x.

3. Results Powder X-ray diffraction patterns for the SrMn1 − xFex O3 − l series are displayed in Fig. 1, and show that a single cubic perovskite phase is formed for all values of x studied. The peaks were indexed on a cubic space group (Pm3m) and the lattice parameters are given in Table 2. The insert in Fig. 1 shows the variation of the lattice parameter, a, with x. A Rietveld refinement was performed on room temperature powder neutron diffraction data for the x= 1/2 phase. Since the neutron scattering lengths [16] of Mn (− 3.73 fm) and Fe (9.45 fm) are sufficiently different compared with the corresponding XRD values, neutron diffraction is advantageous for the study of cation order in this system. Refinements in the space group Pm3m were successful (see Fig. 2) confirming the expected cubic perovskite structure. No additional peaks, which might signify cation ordering were found in the diffraction pattern. It is of note that no Jahn –Teller distortion is observed in these phases, as would be expected for high-spin tetravalent iron ions (3d4) in an octahedral site, from the room temperature diffraction data. This behavior is also observed in SrFeO3 [8]. The results of the idiometric titrations are given in Table 1 in terms of: average MnFe valence; an Fe average valence (assuming that all Mn is tetravalent); and the effective O-site deficit, l, (again assuming an oxidation state of + 4 for Mn). The mean iodometric oxidation state of Fe is ca. 3.5 in all the phases studied which compares very favorably with the mean Fe valence based on the average Mo¨ss-

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Table 2 Physical properties of SrMn1−x Fex O3−l Compound

SrMn2/3Fe1/3O2.92

SrMn1/2Fe1/2O2.89

a (A, ) V (A, 3) Tg (K) TN (K) q (K) C (emu K mol−1) veff (mB) zRT (V cm) Ea (eV)

3.8362(1) 56.453(5) 5 180 −80 2.43 4.41 210 0.20 (224BTB300 K)

3.8469(1) 56.928(6) 25 140 −82 2.56 4.53 107 0.23 (256BTB300 K)

bauer IS shift discussed later. In Table 2, and in the text, the O-site deficit, l, used in the chemical formulas is the iodometrically determined value. The temperature dependence of the magnetic susceptibility is shown in Fig. 3. When x = 1/3 and 1/2, a local maximum is found at 180 and 140 K, respectively. The ZFC and FC data diverge at lower temperatures, with a maximum in the ZFC data at 5, 25 and 45 K when x =1/3, 1/2 and 2/3, respectively. The high temperature region of the susceptibility data (250 –300 K) was fitted to a Curie –Weiss law (= C/T − q) and the parameters are given in Table 2. The resistivity as a function of temperature is shown in Fig. 4. Plots of ln z versus 1/T are non-linear; the activation energies, Ea were determined from linear region of the slope in the high temperature range, listed in Table 2. The MnK main edges for SrMn1 − x Fex O3 − l (x =1/3, 1/ 2 and 2/3) are shown in Fig. 5. The peaks of these MnFe spectra fall between those of LaMnO3 (Mn3 + ) and the CaMnO3 (Mn4 + ) standards but significantly closer to the latter. The MnK pre-edge feature structure for these compounds are shown in Fig. 6. These pre-edge features are related to transitions into final d-states. Consistent with the earlier work, three pre-edge features — a1, a2 and a3 are identified. Figs. 7 and 8 compare the FeK main-edge and pre-edge spectra, respectively, for FeO (Fe2 + ), Fe2O3 (Fe3 + ), and SrMn1 − xFex O3 − l (x = 1/3, 1/2, 2/3 and 1). The x= 1, SrFeO3 material is formally 4+. The chemical shift of the steeply rising portion of the main edge between the Fe2 + , Fe3 + , and Fe4 + spectra (Fig. 7) is quite apparent.

SrMn1/3Fe2/3O2.84 3.8558(2) 57.33(1) 45 −106 2.72 4.66 9 0.22 (262BTB300 K)

The Mo¨ssbauer spectra are shown in Fig. 9 (4.2 K) and Fig. 10 (200 K). The analysis of the spectra yields two inequivalent iron species corresponding to two different iron valence states, in terms of their isomer shift (IS) values, quadruple splitting (DEQ), and magnetic hyperfine field (Bhf); all of which are given in Table 3. Finally a comparison of the Mo¨ssbauer IS results to those in related Fe compounds is shown in Fig. 11.

4. Discussion

4.1. Structure The powder X-ray patterns (and powder neutron pattern for x= 1/2) reveal that the cubic perovskite

Fig. 2. Observed ( + ), calculated (solid line) and difference powder neutron diffraction profile of SrMn1/2Fe1/2O2.89. Reflection positions are represented by tic marks. Space group: Pm3m; a = 3.84724(6) A, ; atomic positions: Sr in 1b, Mn/Fe in 1a, O in 3d; Rp= 4.99 %; Rwp =6.79 %.

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the loss of O2 − as x increases (Table 1), the effect of the B cation size on the unit cell volume is clearly dominant. The increase in the average size of cations on the B-site also has the consequence that the compounds do not crystallize in the hexagonal aSrMnO3 structure. The stability of the perovskite structure is increased relative to the a-SrMnO3 structure, in which MO6 octahedra are grouped into face-sharing pairs, resulting in close MM interactions. Electrostatic repulsions between larger B-site cations force the perovskite structure to be adopted, where MO6 octahedra share vertices.

Fig. 4. Resistivity for SrMn1 − x Fex O3 − l as a function of temperature.

Fig. 3. Temperature dependence of the molar magnetic susceptibility for SrMn1 − x Fex O3 − l. Insert inverse susceptibility versus temperature.

structure, found in SrFeO3, is retained at least up to x 50.67. Due to the substitution of larger Fe4 + , Fe3 + ions (ionic radii=0.725, 0.785 A, , respectively) for the smaller Mn4 + ion (ionic radius= 0.67 A, ) [17,18], the lattice parameter increases linearly with x. Although a decrease in a might be expected with

Fig. 5. The MnK main edges of SrMn1 − x Fex O3 − l (x = 1/3, 1/2 and 2/3) compounds along with those of the CaMnO3 and LaMnO3 standards.

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4.2. Magnetic properties

Fig. 6. The MnK pre edges of SrMn1 − x Fex O3 − l (x =1/3, 1/2 and 2/3) compounds along with those of the CaMnO3 and LaMnO3 standards.

The maxima in the magnetic susceptibility plots for x=1/3 and 1/2 at 180 and 140 K, respectively, correspond to antiferromagnetic ordering temperatures (TN), which is evidenced by negative Weiss constants q listed in Table 2. Although no similar maximum is seen in the susceptibility plot for x=2/ 3, q is negative indicating that antiferromagnetic interactions predominate in this composition, but long-range magnetic interactions are frustrated. The rise in susceptibility after the antiferromagnetic transition, when x= 1/3 and 1/2, indicate that a proportion of magnetic moments are not antiferromagnetically coupled below TN, but remain disordered. In the related low dimensional n=3 Ruddlesden – Popper (RP) phases, CaMn2FeO9.75 and Sr4Mn2FeO9.80 [13], similar magnetic properties are observed. The Sr phase (with 2/3Mn:1/3Fe), orders antiferromagnetically at 90 K. The Ne´el temperature is lower than that in the 3-D perovskite SrMn2/3Fe1/ 3O3 (TN = 180 K), due to the reduced dimensionality and, therefore, weaker exchange interactions in the RP phase. At lower temperatures, a divergence of the FC and ZFC susceptibility curves is found across the

Fig. 7. The FeK main edges of SrMn1 − x Fex O3 − l (x =1/3, 1/2 and 2/3) compounds along with those of the FeO and a-Fe2O3.

The successful refinement of the powder neutron data in the space group Pm3m, in which all the B-sites are crystallographically equivalent, demonstrates that Mn and Fe are disordered in the x=1/2 sample. Considering that the size and charge of the Mn and Fe ions are very similar, disorder is to be expected.

Fig. 8. The FeK pre edges of SrMn1 − x Fex O3 − l (x = 1/3, 1/2 and 2/3) compounds along with those of the FeO and a-Fe2O3 materials.

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the disordered array of Mn4 + , Fe3 + and Fe4 + ions gives rise to spin-glass behavior. The increase in the spin glass transition temperature (Tg) with x and lack of long range magnetic order when x=2/3 is due to the change in the relative ratios of the three cations in the six-coordinate site. Other factors could influence the magnetic properties, for example, increase in oxygen vacancies would tend to de-couple the system and weaken the exchange interactions resulting in a lower TN and likewise, an increase in the lattice parameters (increase in MO bond length) would affect the ordering temperature. It is also likely that next-nearest neighbor interactions are important in these systems. For example, the helicalspin structure of SrFeO3 is due to the coexistence of ferromagnetic nearest-neighbor interactions and the strong antiferromagnetic second- and fourth-nearestneighbor interactions [21]. Further descriptions of the magnetic properties would require a detailed neutron scattering study.

Fig. 9. Mo¨ssbauer spectra of

57

Fe in SrMn1 − x Fex O3 − l at 4.2 K.

SrMn1 − x Fex O3 − l series. This behavior is reminiscent of a spin glass and has been observed in a variety of Fe-containing perovskite oxides. Examples include systems with competing exchange interactions, such as A4Mn2FeO10 − l [13] and Sr2FeRuO6 [19], as well as magnetically dilute materials, such as A2FeMO6 (ACa, Sr, Ba; MNb, Ta, Sb) [20]. The divergence between the FC and ZFC curves is likely to be due to short-range order among disordered spins attributed to competing interactions between 3d3 Mn4 + , 3d4 Fe4 + , and 3d5 Fe3 + on the B-site. Considering nearest neighbor interactions, the Mn4 + /Mn4 + and Fe3 + /Fe3 + interactions are antiferromagnetic, those between Mn4 + and Fe3 + are antiferromagnetic through y orbitals and ferromagnetic through s orbitals and Fe4 + /Fe4 + are antiferromagnetic or ferromagnetic depending on the oxygen environment [13]. The frustration caused by ferro- and antiferromagnetic interactions between

Fig. 10. Mo¨ssbauer spectra of K.

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Fe in SrMn1 − x FexO3 − l at 200

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Table 3 Mo¨ssbauer parameters for SrMn1−x Fex O3−l a

Site 1

Site 2

ŽIS

l T (K)

x=1/3 0.08 4.2 K

200 K

x = 1/2 0.11 4.2 K

200 K

x= 2/3 0.16 4.2 K

200 K

IS DEQ Bhf Area (%) IS DEQ Bhf Area (%) DIS DB

0.43 0.0 505 59 0.03 0.0 275 41 0.40 230 0.27

0.35 0.60 0.0 68 0.00 0.41 0.0 32 0.35 0.0 0.24

0.43 0.0 505 60 0.08 0.0 268 40 0.35 237 0.29

0.36 0.68 0.0 66 0.00 0.42 0.0 34 0.36 0.0 0.24

0.43 0.0 501 62 0.07 0.0 280 38 0.36 221 0.29

0.37 0.76 0.0 68 −0.01 0.44 0.0 32 0.38 0.0 0.25

a The IS and average-over-all-sites IS, ŽIS (9 0.02, relative to iron metal) and DEQ ( 90.02) have units of mm s−1; Bhf ( 910) has units of kOe.

We also note discrepancies between the antiferromagnetic ordering temperature and the Weiss constant with x (i.e. when x = 1/3, TN =180 K and q = − 80 K, however, when x = 2/3, we observe no antiferromagnetic transition, although q= −106 K,). Possible explanations for this involve increased frustrated magnetic interactions [22].

4.3. Electrical resisti6ity The resistivity of SrFeO3 is temperature-independent between 4 and 300 K, with z 10 − 3 V cm and has been attributed to a narrow s* conduction band [8]. The effect of oxygen deficiency and an increase of Mn doping on the B-site has a tendency to localize the d* electrons, leading to an increase in the resistivity and semiconducting behavior, Fig. 4.

small. Thus the prior assumption that the Mn valence remains essentially constant and independent of Fe content appears to be validated. However, the very small down shift in the MnK edge main peak as x increases from x = 1/3 to 1/2 to 2/3 is consistent with a small decrease in the average Mn-valence in these materials as x increases. The MnK peak pre-edge features in Fig. 6 are consistent with the earlier results on Mn4 + compounds with three pre-edge features a1, a2 and a3 being quite visible [24]. Simple crystal field and correlation arguments would suggest the association of the lowest feature, a1, with the eg-majority spin states, and the higher energy features with t2g- and

4.4. XAS In detailed studies of the La1 − x Cax MnO3 system (0.2B x B1.0), the main edge peak energy was shown to manifest a continuous chemical shift with increasing Mn valence [23]. The energies of the MnK main edge peaks for SrMn1 − x Fex O3 − l (x= 1/3, 1/2 and 2/3) in Fig. 5 fall close to (although at a somewhat lower energy than) the Mn4 + –CaMnO3 standard. Thus, at present, the XAS chemical shift supports a Mn valence close to 4+ (but slightly less than 4+ ). The Fe substitution dependence of the MnK edge chemical shift is, however, clearly quite

Fig. 11. Mo¨ssbauer isomer shift versus Fe-valence data for 57Fe in SrMn1 − x Fex O3 − l compared with other studies. Results from Takeda et al [32] on the homogeneous SrFeO3 system and the ‘disproportionated’ CaFeO3 system (300 K single site, and site I and II at 4 K). The nominal Fe-valance scale on the right is derived from the straight-line extrapolation (in the figure) between the Fe3 + and Fe5 + standards as noted.

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eg- minority spin states. Electronic structure calculations [25] and optical absorption [26] based electronic state distributions both clearly support the majority-spin-eg assignment for the a1-feature. The electronic structure calculations [25] indicate that the next higher states should be overlapping t2g and eg minority spin states, favoring the assignment of the a2 feature to such states. The states suggested by optical results [26] also place the t2g-minority states second highest in energy (again consistent with the a2-feature assignment), but it proposes that the minority spin eg states split up a few eV from the t2g states. It is, therefore, tempting to associate the a3-feature with the minority-spin-eg states split up in energy according to this last proposal. Clearly additional work is needed to clarify this issue. In contrast the to the main edge results, the Fesubstitution induced changes in the MnK pre-edge feature structure, shown in Fig. 6, are quite dramatic. Indeed, the sensitivity of these manganese pre-edge structures to Fe substitution far exceeds that observed in a host of other substitution studies. Specifically with the increasing Fe-content (x), the a1 feature dramatically gains intensity relative to the a2 feature. To address a potential cause for this strong effect, the Fe substitution studies, into (La1 − x Ax MnO3 compounds) of Jonker [27], Banks et al. [28], and Ahn et al. [29] can be recalled. Based on their observations, they have all argued that Fe substitutes as Fe3 + until the Mn has been transformed largely into Mn4 + . Indeed Ahn et al. [29] argued persuasively that the filled Fe-eg majority spin states lie substantially lower in energy than the Mn-eg states, (namely close to the bottom of the lower Mn-eg majority states). Within this framework, a Fe neighbor should reduce the resonant eg hopping pathways of the Mn-eg states by one. This effect could reasonably be expected to substantially increase the degree of localization of the Mn-eg orbitals with increasing Fe content. It is, therefore, suggested that the strong a1-feature enhancement is related to an increase in the transition matrix element with increasing eg-state (majority spin) localization. Increasing localization would also be expected to increase the final state core-hole Coulomb interaction energy with the eg electron, thereby, also contributing a shift in the a1-feature down in energy. As the t2g states are intrinsically more localized, their energy and matrix element would be less affected and hence, the a2-feature would be expected to be

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more immune to this effect. This effect would be consistent with the relative enhancement of the a1feature as the matrix element is increased and as eg-related spectral weight is shifted down in energy from beneath the a2-feature. It is worth noting that the above proposal would suggest that all perovskite-related manganite materials might show similarly dramatic Fe-substitution-induced pre-edge feature changes. Indeed XAS results in our lab have confirmed this Fe-modification trend in the Sr3Fe2 − x Mnx O7 − l [30] and Sr4Fe3 − xMnx O10 − l Ruddlesdon –Popper phases of the manganates [31]. Further, it should be pointed out that extension of the same argument to the Fe sites would imply an analogous Mn-substitution-induced localization of the Fe-d states. The Mo¨ssbauer results unambiguously favor such localization. Namely, SrFeO3 exhibits a homogeneous Fe valence, whereas, our Mn-substituted materials all exhibit a ‘disproportionated’ localization of the Fe-d states with differing d-counts at differing sites. Thus the mutual localization of the Mn- and Fe-d states (near the Fermi energy) in the mixed (Mn, Fe) materials is supported. Comparison of SrMn1 − x Fex O3 − l (x=1/3, 1/ 2, 2/3 and 1) FeK main edge spectra reveals only modest changes with Mn substitution (Fig. 7). Although we did not include a formally Fe5 + standard in Fig. 7, Buffat et al. [32] have demonstrated that the XAS edge energy of La2LiFeO6 (Fe5 + ) and that of SrFeO3 (Fe4 + ) do not differ significantly. There is a small shift of the peak in the absorption to lower energy with increasing Mn content (Fig. 7). This would be consistent with a decrease in the average Fe valence, however, the effect is quite small. Relative to SrFeO3, the Mn-substituted materials also exhibit a degradation in the intensity of the main peak and some additional weight on the low energy rising portion of the edge. Both of these effects are suggestive of a small enhancement of a lower valence component (i.e. Fe3 + ) in these Mn-substituted materials. In general, the FeK main edges have a less well-defined peak and more intensity in features, both below and above the main peak as compared with the MnK spectra in Fig. 5. This presumably reflects the greater importance of a configurational admixture for Fe in the materials relative to Mn. In contrast to the FeK main edge spectra, the FeK pre-edge spectra of SrMn1 − x Fex O3 − l (x=1/ 3, 1/2, 2/3 and 1) manifest a rather strong variation

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with increasing Mn content. The x = 1 spectrum exhibits a sharp pre-edge peak at a position identified as b1 in Fig. 8. With increasing Mn-content this b1-feature appears to remain, however, a second higher energy feature, at the position labeled b2, gains intensity. This trend is emphasized in the figure by the solid arrows both labeling the compositions and pointing to the peak of the pre-edge feature. As can be seen, the energy of the peak moves from b1 to b2 with increasing Mn-content. Association of the FeK pre-edge features with specific d-states, as in the Mn-case, is not presently possible. However, the rather distinct change from a sharp single feature, for the pure Fe (x = 1.0) compound, to distinctly bimodal features for the Mnsubstituted compounds is supportive of the Fe-site localization effects discussed in this paper.

4.5. Mo¨ssbauer In the 4.2 K spectra the first site, (I), is almost certainly ‘Fe3 + ’, and the second site, (II), corresponds probably to ‘Fe5 + ’, since the spectra look extremely similar to those observed in Sr3Fe2O7 [33], in CaFeO3 [34] and in LaSr2Fe3O9 [35] at low temperatures, where the spectra were analyzed as corresponding to disproportionated Fe3 + and Fe5 + . The 4.2 K Mo¨ssbauer spectra exhibit magnetic hyperfine field distributions and zero quadrupole interactions, while the 200 K (well above the magnetic ordering temperature) data shows only large quadrupole interactions (Figs. 9 and 10). This indicates that the samples are in a spin-glass-ordered state at the low temperatures. The association of the ‘Fe3 + ’ and ‘Fe5 + ’ Mo¨ssbauer sites with pure valence states leads to clear inconsistencies with our titration results (hence the quotation marks). Using the Fe-IS values of Fe3 + (IS :0.47) [32] and Fe5 + (IS : −0.34) [36] one can form a linear Fe-IS versus Fe-valence relation illustrated in Fig. 11. Within this nominal IS versus valence scale, all the sites in our MnFe compounds can be viewed as covalently-mixed Fe configurations. Earlier Mo¨ssbauer studies indicate that SrFeO3 is a homogeneous, Fe4 + compound [7], while CaFeO3 − l [33] and Sr3Fe2O7 − l [32] manifest a ‘Fe3 + /Fe5 + charge disproportionation’ at low temperatures. In fact those Mo¨ssbauer discussions emphasized that the charge-site separation in these materials is into temperature dependent Fe[(3 + m) + ]

and Fe[(4 + l) + ] states, the fractional valences depend on the temperature and oxygen content of the sample. Indeed the continuous variation of the charge separation in the Ca1 − x Srx FeO3 system further emphasizes the non-integral valence character of the states in such materials [21]. Thus the relative intensities of our Mo¨ssbauer sub-spectra, Table 3, which do not agree with the oxygen content, for integral Fe valences, may be associated with the incomplete disproportionation of the Fe4 + . The incomplete disproportionation is partially confirmed by the differences in the hyperfine fields and the isomer shifts between pure Fe3 + and Fe5 + , and those in our samples. This low temperature disproportionation provides evidence into the nature of the complicated magnetic interactions that result when 2Fe4 + disproportionate into dependent Fe(3 + m) + and Fe(4 + l) + states. If one calculates the Fe-IS averaged-over-all-sites, ŽIS (see Table 3 and Fig. 11), one obtains values in the 0.25 –0.29 mm s − 1 range for the MnFe compounds. This average IS is in deceptively good agreement with the titration estimate of a Fe valence of about 3.5 in these materials. Although this agreement is fortuitously good, it does support the IS-valence correlation and the mixed configuration character of the Fe sites observed here. This average IS value is somewhat larger than that for SrFeO3 [about 0.07 (at 4 K) and 0.2 (at 298) K mm s − 1] or that for CaFeO3 [0.07 mm s − 1 at 300 K]. The magnitude of the ISs at the two sites in our MnFe compounds is compatible with that at low temperature in CaFeO3 [about 0.0 –0.34]. Thus the effect of the Mn substitution appears to pull the overall average Fe-valence somewhat toward the Fe3 + valence and to stabilize the disproportionated (localized) state.

5. Conclusion In conclusion, we have determined that the SrMn1 − x Fex O3 − l (x=1/3, 1/2 and 2/3) phases adopt a cubic perovskite structure with a disordered arrangement of transition-metal cations in the six-coordinate site. Antiferromagnetic ordering is observed at 180 and 140 K when x=1/3 and 1/2, respectively, and spin-glass behavior is noted across the series. The magnetic properties have been explained in terms of interactions between a disordered array of

I.D. Fawcett et al. / Solid State Sciences 2 (2000) 821–831

formally Mn4 + , Fe3 + , Fe4 + (or Mn4 + , Fe(3 + m) + and Fe(4 + l) + ) ions on the B-site. Instead of introducing d3/d4 ferromagnetic double-exchange interactions found in the CMR manganates, super-exchange interactions are favored. The XAS results for the Mn-sites in these compounds show little Mn-valence change. However, we have interpreted the Mn-pre-edge spectra in terms of a localization of the Mn-eg orbitals as Fe substitution reduces resonant inter-site hopping. The XAS for the Fe sites indicates a small Mn-substitution induced departure in the average Fe-valence toward Fe3 + , which is born out by the Mo¨ssbauer results. However, the most dramatic result from the Mo¨ssbauer result is the distinct two-site Fe disproportionation observed in the Mn substituted materials. This disproportionation is a very concrete reflection of a localization of the Fe-d states due to the Mn-substitution. Thus, there is evidence of d-localization (suppression of inter-site resonant hopping) at both Mn and Fe sites due to the energetic mismatch of their respective d-orbitals.

Acknowledgements This work was supported by the NSF-Solid State Chemistry Grant DMR 96-13106. We thank Dr J. Richardson and Dr C. Murphy at the IPNS-Argonne National Laboratories for their help with the neutron data, and Professor W. H. McCarroll for his critical reading of the manuscript.

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