Polarized x-ray absorption spectra of La1−xSr1+xMnO4: Electronic state of Mn atoms

Polarized x-ray absorption spectra of La1−xSr1+xMnO4: Electronic state of Mn atoms

Javier Herrero-Martín, Joaquín García,* Gloria Subías, Javier Blasco, and María Concepción SánchezInstituto de Ciencia de Materiales de Aragón, Departamento de Física de la Materia Condensada, CSIC-Universidad de Zaragoza,

Pedro Cerbuna 12, 50009 Zaragoza, Spain�Received 1 March 2005; published 2 August 2005�

We have measured polarized x-ray-absorption near-edge structure and extended x-ray-absorption fine struc-ture spectra at the Mn K edge on La1−xSr1+xMnO4 samples with x=0, 0.3, and 0.5 at different temperaturesfrom 50 K up to 300 K. Both fluorescence and total electron yield modes were used for polarized measure-ments, whereas nonpolarized absorption spectra have also been recorded in transmission mode on powdersamples for x=0, 0.3, 0.45, 0.5, and 0.55. The experiment discards the presence of distinct Mn3+/Mn4+ integervalence states for any of the mixed-valence samples �x=0.3,0.5� in the whole temperature range. The localstructure �geometrical and electronic� around the Mn atom is anisotropic in all samples. Moreover, there is nosignificant change in the Mn local geometric and electronic structure of compounds with 0.45�x�0.55crossing the charge-ordering transition �TCO=230 K�. The anisotropy in the absorption spectra is then ex-plained in terms of the tetragonal distortion of the MnO6 octahedron. It decreases as x increases, the oxygenenvironment being nearly isotropic for La0.5Sr1.5MnO4. Anisotropy is also observed at the prepeaks. Theintensity of the in-plane structures strongly increases with the Sr content, indicating that the doped holesmainly go into the ab plane. We conclude that the electronic state of Mn atoms can be only described in termsof an intermediate valence, even for anisotropic samples such as two-dimensional La-Sr manganites.

DOI: 10.1103/PhysRevB.72.085106 PACS number�s�: 71.28.�d, 61.10.Ht, 75.47.Lx

I. INTRODUCTION

Perovskite manganites and their related compounds havebeen extensively investigated since the discovery of the co-lossal magnetoresistance.1–3 The magnetoresistive propertiesof these materials are found to sensitively depend on thedimensionality of the manganese oxygen lattice. Compoundswith the layered K2NiF4 structure, such as La2−xSrxCuO4cuprates4,5 and La2−xSrxNiO4 nickelates,6 have alreadystimulated considerable interest due to the low dimensional-ity of the transition-metal sublattice �two dimensional �2D��and on the observed inhomogeneity of the charge distribu-tion.

La1−xSr1+xMnO4 belongs to the n=1Lan�1−x�Srnx+1MnnO3n+1 member of the Ruddlesden-Popperfamily of manganese oxides. These compounds have ahighly symmetric body-centered tetragonal structure �spacegroup I4/mmm� composed of perovskites-type layers�LaSr�MnO3 separated by a rock-salt-type layer �La,Sr�2O2

along the c axis �see Fig. 1�. The perovskite layers are shiftedto each other by �1/2�a+ �1/2�b, resulting in a pseudo-2Dcharacter and strongly anisotropic transport properties.7 Con-trary to the related La1−xSrxMnO3 perovskite �hereafter de-noted as 3D�, La1−xSr1+xMnO4 does not exhibit colossalmagnetoresistance.8,9 Magnetism is also different from 3Dperovskites. LaSrMnO4 is an antiferromagnetic insulator10

below TN=130 K with the moments aligned along the �001�direction. Upon increasing the Sr content, the electrical re-sistivity decreases but the samples keep on being semicon-ducting. Three regions have been established as a function ofhole doping.11,12 The samples are antiferromagnetic insula-tors for x�0.10. Between x=0.10 and 0.45, there is neithersuperstructural order nor magnetic order, some authors de-scribing this region as a spin-glass phase. For 0.45�x

�0.7, the samples develop a structural phase transition atabout TCO=230 K giving rise to an antiferromagnetic orderof CE type. This new phase is referred in literature as acharge-ordered �CO� phase because the electrical resistivitysimultaneously presents a discontinuity passing from an in-sulator to a semiconductor or bad-metallic state below TCO.Similar behavior has been also observed in other 2D manga-nites such as �Nd,Sr�2MnO4 and �PrSr�2MnO4.13–15

The extended frame of discussion for the manganites isbased on the ionic model originally proposed byGoodenough,16 where the low-temperature insulating phaseconsists of two sublattices resulting from the charge orderingof Mn3+ and Mn4+ ions. Several manganites with a similarformal Mn3+/Mn4+=1 ratio have closely related low-temperature phases with two different manganese crystallo-graphic sites.17–21 Six oxygen atoms exhibit a tetragonal-distortion coordinate-1 site, and the other site is nearlyundistorted. The two crystallographic sites have been gener-ally ascribed to Mn3+ and Mn4+ ions, respectively. It is note-worthy that the difference between both crystallographicsites is very small, the local structure of each site being dif-ferent from the correspondent to Mn3+ or Mn4+ referencecompounds �LaMnO3 and CaMnO3, respectively�. For in-stance, based on the bond valence sums method, the degreeof charge disproportionation between the two Mn sitesranges between 0 and 0.4 electrons for different half-dopedmanganites. The same can be said for the tetragonal distor-tion of the so-called Mn3+ ions in CO systems. The measureddistortion in these materials is 4 or 5 times lower than thedistortion found in LaMnO3. Despite these facts, the implicitacceptance of the ionic model is very extended and the low-temperature phase is described as a charge-orbital-ordered�COO� phase.

The charge and orbital ordering phenomena have beenrecently studied by x-ray resonant scattering �XRS�. The first

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XRS experiment on manganites was performed onLa0.5Sr1.5MnO4.22 The observation of strong resonances atthe Mn K edge in some superlattice reflections of the low-temperature phase was interpreted as a confirmation of theCOO phase. Further experiments on other half-doped 3Dmanganites were also interpreted in terms of this COOmodel.23–26 However, this interpretation has been recentlyshown to be derived from a very imprecise analysis.27,28 Arigorous analysis of the XRS experiments confirms the pres-ence of two different crystallographic sites for Mn atomsbelow TCO but with similar valence state.27,29,30 The observedXRS is then easily understood as due to the anisotropy in-duced by the local distortion at the Mn sites. The currentquestion is whether the COO model is still valid for 2Dmanganites.

The use of x-ray-absorption spectroscopy �XAS� iscomplementary to diffraction techniques. XAS has the ad-vantage that it does not require long-range order in theatomic arrangement; i.e., no condition for diffraction isneeded. On the other hand, the interaction time for the pho-toabsorption process is extremely short, t�10−15 s, lowerthan either the required time to observe charge fluctuationsor the characteristic time for vibrational motion. These char-acteristics make this technique unique to distinguish �i� thepresence of different ionic states by analysis of the x-ray-absorption near-edge structure �XANES� spectra and �ii� dif-ferences in the Mn local structure depending on the dopingratio or temperature by analysis of the extended x-ray-absorption fine structure �EXAFS� spectra.

XAS experiments on 3D manganites have been alreadycarried out by several groups �including ours� with contro-versial results.31–38 In the present work, we extend our re-search to 2D manganites, with the aim of confirming ourprevious findings.31–34 The advantage of studying these an-isotropic materials lies in the fact that polarization-dependentspectra can separate different contributions—i.e., the in-plane �equatorial bonds� and the out-of-plane �apical bonds�ones. Anisotropy in the XANES spectra would give us infor-

mation on the anisotropy of the Mn p-empty states and indi-rectly, on the density of d-projected states �pre-edge struc-tures�. Polarized EXAFS can independently determine thein-plane and out-of-plane Mn-O interatomic distances over-coming the limitation of the nonpolarized EXAFS spectros-copy mainly done on polycrystalline samples.

We present a XAS study �nonpolarized and polarized� ofselected 2D manganites.

�i� LaSrMnO4 is the counterpart of the 3D manganitesmother compound LaMnO3. Both compounds only containformal Mn3+ ions.

�ii� La0.7Sr1.3MnO4 corresponds to a formal Mn3.3+ va-lence state, which develops colossal magnetoresistance in 3Dmanganites.

�iii� La0.5Sr1.5MnO4, with a formal Mn3.5+ valence stateshows a similar COO transition as for related 3D mangan-ites.

We will pay attention to the evolution of the valence stateand local structure of the Mn atom with both the Sr compo-sition and the temperature, in particular across the COO tran-sition in La0.5Sr1.5MnO4. We will show that the different be-havior for different compositions originates from solid-stateeffects, discarding the presence of individual Mn3+ and Mn4+

ions. In this sense, we also propose that the tetragonal dis-tortion in LaSrMnO4 is not caused by the single-ion Jahn-Teller effect, but by phonon coupling with extended elec-tronic states.

II. EXPERIMENT

Powder samples of the La1−xSr1+xMnO4 series �x=0,0.3,0.45,0.5,0.55� were prepared by the ceramicmethod. Stoichiometric amounts of La2O3, SrCO3, andMnCO3 were mixed and calcined 3 times at 1000–1500 °Cfor 72 h in different atmospheres according to the valencestate of the manganese atom. The powders resulted in singlephases as determined by powder x-ray diffraction. The pow-ders were then pressed into rods and sintered at 1500 °C.

FIG. 1. Right: crystal structure of theLa1−xSr1+xMnO4 series. Thin gray lines indicatethe basic unit cell. Left top: detail of the incidentbeam configurations used for polarized x-raysmeasurements. Left bottom: detail of the MnO6

octahedron. Mn is located at the center while ver-tices are occupied by apical and equatorialoxygens.

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Single crystals were grown from the rods in a two-elliptical-mirror optical image furnace �floating zone method�. Thegrowth atmosphere varied from pure argon for theLaSrMnO4 sample, air for x=0.3 composition, and 2 bars ofoxygen pressure for 0.45�x�0.55. The feeding speed wasbetween 10 and 20 mm/h. Crystals were cut normal to the�001� direction. Both powder samples and single crystalswere carefully characterized by x-ray powder diffraction andmagnetic measurements. We will mainly focus here on thex=0, 0.3, and 0.5 compositions as the macroscopic proper-ties and the obtained results for the compositions 0.45, 0.5,and 0.55 are identical.

X-ray-absorption measurements were carried out at thebeam line BM29 �Ref. 39� at the ESRF in Grenoble �France�.The storage ring operated at electron beam energy of 6 GeVwith a maximum stored current of about 200 mA. A fixed-exit Si �111� monochromator was used, the estimated resolu-tion being �E /E�7.5�10−5 at the Mn K edge. Harmonicrejection was achieved by 50% detuning of the two crystalsfrom the parallel alignment. Pellets of powder samples wereprepared by dilution with cellulose to optimize the signal-to-noise ratio. Ionization chambers were used as detectors, andthe energy scale was calibrated by the simultaneous measure-ment of a Mn foil �absorption edge E0=6537.7 eV� for mea-surements in transmission mode.

Polarized spectra were measured using two methods.�i� Fluorescence yield detection at room temperature by

means of a Camberra 13-element Ge solid-state detector. Ex-periments with the crystal surface �ab plane� perpendicular tothe incoming x-ray beam—i.e., the electric field vector E iscontained in this plane—are called in-plane configurations�see Fig. 1�. Measurements were also collected by rotatingthe crystal 75° with respect to the beam direction. Here theangle between the electric field vector E and the c-axis crys-tal is about 15°. This is denoted the out-of-plane configura-tion. Polarized spectra in fluorescence mode were sequen-tially measured after recording a LaMnO3 pellet data �E0=6552.6 eV� to minimize the experimental error in the en-ergy scale.

�ii� Polarized spectra in the in-plane configuration werealso measured as a function of temperature by detecting thetotal electron yield �TEY� signal in a helium atmosphere.

XANES spectra were normalized to the high-energy partof the spectrum �around 100 eV above the absorption edge�after a linear background subtraction.40 EXAFS spectra��k�� were obtained by removing the smooth atomic absorp-tion coefficient ��0� by means of a cubic spline fit. The struc-tural analysis was performed in the R-space fitting mode us-ing the FEFF 8.10 and FEFFIT packages.41

III. RESULTS

A. XANES spectra

Figure 2 shows the normalized XANES spectra of powdersamples of the La1−xSr1+xMnO4 series �x=0, 0.3, and 0.5�and the reference compounds LaMnO3 and CaMnO3. Themeasurements were carried out in transmission mode atroom temperature. All the spectra are characterized by a

main resonance at about 6555 eV, a prepeak structure �cor-responding to transitions to the mixed O 2p and Mn 3dbands�, and extended structures above the main edge relatedto multiple-scattering contributions. A broadening of themain peak is clearly observed for the LaSrMnO4 spectrum.Apart from this fact, the main difference among theLa1−xSr1+xMnO4 spectra lies in the position of the absorptionedge that shifts to higher energies as x increases. We note asimilar energy position for the absorption edges of LaMnO3and LaSrMnO4 �both samples containing formal Mn3+ ions�.

XANES spectra have been also measured on these pow-der samples from 50 K up to room temperature �not shownhere�. The spectra did not show noticeable changes with tem-perature, indicating similar geometrical and electronic localstructures in the whole temperature range for each sample.The relative variation between spectra at successive tempera-tures was found to be lower than 2% without any noticeableshift of the absorption edge with the temperature.

Figure 3 displays the polarized XANES spectra of thesingle crystals, measured in fluorescence mode as indicatedin Sec. II, at room temperature. The spectra show a largeanisotropy between in-plane and out-of-plane componentsregarding either the spectral shape or the absorption edge’sposition. The differences are more pronounced forLaSrMnO4 with a stronger geometrical anisotropy, which de-creases as x increases. For instance, the anisotropic shift,defined as the energy difference in the position of the absorp-tion edge between both polarizations, is 3.3, 1.7, and 0.7 eVfor x=0, 0.3, and 0.5 samples, respectively. We note that theanisotropic shift is very low for the 0.5 sample. The compari-son among several samples shows that the average chemicalshift �chemical shift of nonpolarized spectra� mainly comesfrom the out-of-plane component. The energy position of ab-sorption edge for the in-plane component is nearly identicalfor the three samples whereas a strong chemical shift is ob-served for the out-of-plane component. Finally, the powderspectra �nonpolarized� recorded in fluorescence mode arewell reproduced by the weighted sum �2/3 :1 /3� of the po-larized in-plane and out-of-plane spectra as is shown in theupper part of Fig. 3. We would like to note here that the

FIG. 2. Normalized XANES spectra of La1−xSr1+xMnO4 for x=0 �solid line�, x=0.3 �dashed line�, and x=0.5 �dotted line � pow-der samples �nonpolarized spectra� compared to those of LaMnO3

�white circles� and CaMnO3 �black circles� reference compounds.

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nonpolarized fluorescence spectra were found to be identicalto the transmission ones. The strong absorption of the fluo-rescence photons by heavy elements �La, Sr� in these man-ganites makes them to behave as diluted samples, as has alsobeen previously reported for the cuprates.42

In order to check the temperature evolution of theXANES spectra, we have collected the in-plane spectra be-tween 50 and 300 K by using the TEY method. TEY spectrawere found to be identical to fluorescence ones. As occurredfor the nonpolarized spectra, the in-plane spectra for thethree studied samples do not significantly change with tem-perature �not shown here�.

Figure 4 shows a detail of the pre-edge structures �labeledA�, which give information on the O 2p–Mn 3d mixing.Figure 4�a� shows an expanded view of the pre-edge featuresfrom the Mn K edge nonpolarized spectra for powderedsamples. The dashed lines are background approaches byusing a Lorentzian fit to the edge. The intensity of these Afeatures increases upon increasing the Sr content �see inset ofFig. 4�a��, suggesting a correlation with the increase in theMn 3d hole count. This increase in the pre-edge feature in-tensity with Sr content has previously been reported in re-lated compounds such as La2−xSrxNiO4 �Ref. 43� where pdhybridization effects were also pointed out. The pre-edgestructures also show noticeable anisotropy as it is clearly

observed in Fig. 4�b�. Closer inspection of the polarizedspectra indicates that the intensity of the prepeaks for thein-plane polarization is higher than for the out-of-plane po-larization for all samples. Moreover, the overall increase ofthe A-feature intensity with the Sr content is mainly arisingfrom the in-plane component. This indicates that the holesinduced by Sr substitution are mainly located in the ab plane.

B. EXAFS spectra

EXAFS spectra of powder La1−xSr1+xMnO4 �x=0, 0.3,0.45, 0.5, and 0.55� samples have been measured at differenttemperatures ranging from 60 K up to 280 K. Figure 5shows the k-weighted EXAFS spectra taken at room tem-perature. Upon increasing x, significant changes are notice-able in these spectra according to the changes of the localstructure, except for the last three spectra, which are verysimilar to each other as expected from their close composi-tion. The modulus of the Fourier transform �FT� for x=0,0.3, and 0.5 compounds at different temperatures, extracted

FIG. 3. XANES spectra of the La1−xSr1+xMnO4 series at roomtemperature. The three upper spectra show the comparison betweenthe nonpolarized spectra �solid line� and the weighted sum of thein-plane and out-of-plane polarized spectra �black circles�. Threelower ones: the polarized spectra with the electric field vector E ofthe incident beam nearly perpendicular �out-of-plane configuration,dashed line� and parallel �in-plane configuration, dotted line� to theab plane.

FIG. 4. �a� Expanded view of the pre-edge from Mn K-edgenonpolarized XANES spectra for powdered La1−xSr1+xMnO4

samples. The dashed lines are background approximations by alorentzian fit. Inset: background-subtracted pre-edge features vs xfor La1−xSr1+xMnO4 series. �b� Pre-edge structures of the polarizedXANES spectra of La1−xSr1+xMnO4 in the in-plane �solid lines� andout-of-plane �dashed lines� configurations at room temperature.


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in the range 2.5�k�11.5 Å−1, is displayed in Fig. 6. Weobserve a first peak corresponding to the first oxygen coor-dination shell and two other peaks at about 2.6 Å and 3.4 Åcorresponding to the near neighbors La �Sr� and Mn atoms,respectively. The intensity of the peaks slightly decreases asthe temperature increases due to the higher atomic thermalmotion. We note that this effect is very small regarding thefirst oxygen shell for x=0.3 and 0.5 samples. This resultindicates nearly a same spread of Mn-O distances between60 K and 280 K for those samples.

In order to separate contributions arising from apical�Mn-Oap� and equatorial �Mn-Oeq� oxygen atoms in theMnO6 octahedron, we have measured polarized EXAFSspectra in the oriented single crystals at room temperature.EXAFS spectra also show a strong anisotropy between in-plane �Mn-Oeq� and out-of-plane �Mn-Oap� configurations.

The FT of the polarized spectra for the three samples �x=0,0.3, and 0.5� are shown in Fig. 7. The three in-plane polar-ization spectra are very alike as expected from the similardistribution of Mn-Oeq distances along the La1−xSr1+xMnO4series. The out-of-plane polarization spectra, instead, areunique for each sample, indicating a strong variation of theMn-Oap distances with increasing x. Finally, we note that thenonpolarized spectra are well reproduced by the weightedsum �not shown here� of the two polarized spectra as it wasfound for XANES spectra in Sec. III A.

Both polarized and nonpolarized EXAFS spectra havebeen analyzed using the FEFF 8.10 package. Fits were per-formed in real space by refining the structural parameters bynonlinear least-squares minimization. Theoretical amplitudesand phases were calculated from the crystallographic data.The energy E0 and the overall reduction factor S0

2 were fixedto −9.0 eV �related to the absorption edge� and 0.7, respec-tively. These values were obtained from the LaSrMnO4sample. Coordination numbers were set to 6 �4 in the in-plane and 2 in the out-of-plane contributions�. Only inter-atomic distances and Debye-Waller ��2� factors were thenfitted. No simultaneous fittings of the different componentswere performed, but we have checked the self-consistency ofthe results for the three spectra of each sample �nonpolarizedand the two in-plane and out-of-plane polarizations�. Thefitting range is �0.8–2.3� Å for both nonpolarized and eitherin-plane or out-of-plane polarized spectra. As a matter ofillustration, we display in Fig. 8 the fits obtained for theLaSrMnO4 sample. The structural values resulting from thefits are compared to crystallographic data in Table I. Overall,there is a good agreement between both techniques. Con-cerning the crystal structure, the axial c parameter shrinksand the in-plane a parameter slightly expands upon introduc-tion of Sr.44 As expected from this trend, the in-planeMn-Oeq distances slightly increase though they are verysimilar for the three samples whereas the Mn-Oap onesstrongly shrink as x increases, resulting in a slight differencebetween Mn-Oeq and Mn-Oap distances for x=0.5. On theother hand, LaSrMnO4 presents a strong tetragonal elonga-tion of the MnO6 octahedron according to the biggest value

FIG. 5. From the top to the bottom, nonpolarized k-weightedEXAFS spectra of the La1−xSr1+xMnO4 series for x=0, 0.3, 0.45,0.5, and 0.55 measured in transmission mode at room temperature.The three last spectra are nearly identical.

FIG. 6. k-weighted Fourier transform curves of the EXAFSspectra for the La1−xSr1+xMnO4 powder samples at different tem-peratures: T=60 K �black solid line�, 150 K �dashed line�, 200 K�dotted line�, 230 K �circles�, 245 K �squares�, 260 K �triangles�,and 280 K �grey solid line�.

FIG. 7. k-weighted Fourier transform curves of the EXAFS po-larized spectra of La1−xSr1+xMnO4 in the in-plane configuration�solid lines� and out-of-plane �dashed lines� configurations at roomtemperature.


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of the c axis. Thus, the reminiscent elongation of the MnO6octahedron along the c direction for intermediate x can beunderstood in terms of purely structural effects—i.e., the in-fluence of the planar K2NiF4 crystal structure. The increaseof the Sr content also leads to shorter average Mn bond dis-tances, which couples to a higher oxidation state of the Mnatom as deduced from the nonpolarized XANES spectra �seeFig. 2�. The �2 factors for the Mn-Oap distances are higherthan the ones for Mn-Oeq, indicating a larger vibrational

disorder for the former distances. Moreover, the �2 factors ofthe Mn-Oap distances for x=0 and 0.3 samples are also muchhigher than the correspondent for the x=0.5 compound. It isworth mentioning that the structural disorder of the La/Srsublattice could give rise to an extra source of spreading inthe Mn-O distances, above all in the apical ones. In fact, thedisorder within the La/Sr sublattice decreases with furtherincreasing the Sr content, the maximum disorder being foundfor LaSrMnO4.

The temperature evolution of the Mn local structurehas also been studied. We have measured the nonpolarizedEXAFS, spectra of powdered samples in transmission modefrom 50 K up to room temperature. The results are shown inFig. 9. We point out that in LaSrMnO4 the tetragonal distor-tion of the MnO6 octahedron is even stronger than inLaMnO3, so Mn-Oap and Mn-Oeq bond distances are wellresolved by EXAFS, the single average Mn-O bond lengthmodel being worse from the statistical point of view. For therest of the samples, the splitting in the Mn-O bond distancesis at the limit that it can be resolved by EXAFS, so we haveused a single average Mn-O bond length model to analyzethe local structure. There are no significant changes in theMn-O distances with decreasing the temperature for any ofthe samples.

For LaSrMnO4, the Debye-Waller factor for the Mn-Oapdistance shows a strong decrease as a function of tempera-ture whereas the Mn-Oeq distances show a very low �2 fac-tor over the whole temperature range. This suggests a strongstability of the tetragonal MnO6 distortion �static cooperativedistortion� as occurs in related Mn3+ compounds.45 On theother hand, the �2 factors of the average Mn-O distances forx=0.3 and 0.5 samples at low temperatures are higher thanfor x=0 and remains almost constant with temperature. Fo-cusing on x=0.5, we have observed no change either in theMn-O distance or the �2 factor at TCO�230 K.

IV. DISCUSSION

We have shown here the existence of a strong anisotropyat the Mn site in the XANES spectra of the La1−xSr1+xMnO4series. This anisotropy manifests as �i� a splitting betweenthe in-plane and out-of-plane components of the Mn absorp-tion coefficient by an energy , which is called the aniso-tropic shift, and �ii� a stronger intensity of the pre-edge struc-tures for the in-plane component. The highest anisotropicshift belongs to LaSrMnO4 �formal Mn3+�, being about3.3 eV, and decreases with increasing x, being very small�0.7 eV� for La0.5Sr1.5MnO4 �formal Mn3.5+�. Polarization-dependent EXAFS measurements reveal that the magnitudeof this anisotropic shift depends monotically on the magni-tude of the local distortion of the MnO6 octahedron �see Fig.10, top�. These facts confirm a structural origin for the an-isotropy of the absorption coefficient as already pointed outby previous XRS experiments.29 These results also show astrong correlation between the formal valence state and theaverage Mn-O interatomic distance, as is shown in Fig. 10,bottom. In this sense, the chemical shift among the nonpo-larized XANES spectra originates from the decrease of theaverage Mn-O interatomic distance with increasing x,

FIG. 8. Experimental versus best-fit simulations for LaSrMnO4

EXAFS spectra. Top: modulus of the experimental FT �lines� com-pared to the first-shell best fit �circles�. Bottom: comparison of theimaginary parts of FT, same meaning for symbols and lines asabove.

TABLE I. Best-fit structural parameters of the first-oxygen-shellEXAFS analysis of in-plane and out-of-plane polarized spectracompared to the crystallographic single-crystal data �Ref. 44� atroom temperature. Estimated errors in the last significant digit areindicated by the number in parentheses.

SampleX-ray diffraction

r �Mn-O� �Å�

X-ray absorption

r �Mn-O� �Å� �2 �Å2�

LaSrMnO4 1.89335�1� 1.90�1� 0.002�1�2.2668�4� 2.27�3� 0.007�5�

La0.7Sr1.3MnO4a 1.9250�1� 1.92�1� 0.004�1�

2.083�2� 2.05�2� 0.008�3�La0.5Sr1.5MnO4 1.93164�3� 1.92�1� 0.004�1�

2.0005�9� 1.98�1� 0.005�2�aNo crystallographic data are reported for the Sr content x=0.3. Thereported Mn-O bond lengths are obtained by interpolation betweenthe powder-crystal data of x=0.25 and x=0.4 samples.


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mainly reflecting the decrease of the apical Mn-O bondlength. Needless to say, this correlation makes the determi-nation of the valence state of the Mn atom, either by thebond-valence-sum �BVS� method46 or by the chemical shiftof the absorption edge, comparable. On the other hand, thepre-edge feature area increases uniformly as the Sr contentdoes �inset of Fig. 4�a��, and for x=0.5 it becomes 2 timeslarger than for the x=0 compound. Comparing in-plane andout-of-plane components, the largest differences in the inten-sity of the pre-peaks are observed for x=0.5 and decreasewith decreasing x down to a nearly isotropic pre-edge struc-ture for the x=0 compound. These results indicate that theextra holes induced by Sr doping mainly go into empty statesof mixed O 2p–Mn 3d character in the 2D �MnO2� planes.

O K-edge measurements in related La1−xSrxMnO3 series alsoshow similar changes, which confirm a strong O 2p–Mn 3dmixing.47 Now, we are going to focus on the results for eachsample.

A. LaSrMnO4

As is shown in Fig. 2, the position of the absorption edgecoincides with the 3D LaMnO3 showing the same formalvalence state for the two samples �Mn3+�. Polarization-dependent XANES spectra of LaSrMnO4 show a large an-isotropy with an anisotropic shift of about 3.5 eV. Severalauthors44,48 attribute this anisotropy to the antiferro-orbitalordering of the occupied eg orbitals through the Jahn-Tellereffect of the Mn3+. However, other works indicate that thisanisotropy exclusively originates from the tetragonaldistortion.27–29,49,50 Our results also point out the purelystructural effects due to the strong correlation that exists be-tween the tetragonal distortion and the anisotropic shift alongthe series.

We also observe anisotropy at the pre-edge structures giv-ing information on the Mn 3d empty density of states

FIG. 9. Temperature dependence of the best-fit average Mn-Obond lengths �solid symbols� and Debye-Waller factors �open sym-bols connected with an eye-guide line� obtained from the combinedanalysis of powder and polarized spectra for La0.5Sr1.5MnO4 �bot-tom� and La0.7Sr1.3MnO4 �middle�. For LaSrMnO4 �top�, in-plane�circles� and out-of-plane �triangles� Mn-O bond lengths and �2

factors are shown.

FIG. 10. Top: correlation between the tetragonal distortion �Å�and the measured anisotropic shift �eV� derived from Fig. 3 for thesame samples. Bottom: correlation between the average Mn-Obond-length �Å� and the chemical shift �eV� measured at the Mn Kedge in the nonpolarized XANES �derived from Fig. 2� ofLa1−xSr1+xMnO4 �with respect to LaMnO3�.


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�through hybridization with O 2p�. This anisotropy shouldgive indication on the so-called OO: namely, as differencesin the directional density of empty states with d symmetry.The fact that LaSrMnO4 presents a small difference in thepre-edge features between the two polarizations with the firstpeak of this pre-edge structure �see Fig. 4�b�� present in theboth polarizations with similar intensity indicates that thefirst empty 3d states are nearly isotropic. Thus, these resultsalso throw doubts on an OO model for this sample.

B. La0.7Sr1.3MnO4

The formal valence state of this sample corresponds to theoptimal value of 3D manganites showing colossal magne-toresistance. The description of the mixed-valence mangan-ites has been traditionally seen as a heterogeneous mixing ofMn3+ and Mn4+ ions. The ferromagnetic ground state of 3Dmanganites has been then ascribed to the so-called double-exchange interaction between Mn3+ and Mn4+ ions. Despiteit having been demonstrated that the Mn atoms in 3D man-ganites are in an intermediate valence state,31,32 this single-ion picture is still used to interpret the experiments in 2Dmanganites.12,44 However, in the present case, the magneticstate of this material can be described as a spin glass; i.e., nolong-range magnetic ordered state has been found.7,12

This study shows that EXAFS and XANES spectra at theMn K edge are perfectly described considering the existenceof only one kind of Mn atom in La0.7Sr1.3MnO4. If we as-sume a bimodal distribution of Mn3+ and Mn4+ ions, theXANES spectrum is expected to be reproduced by theweighted addition of two spectra with ionization energiesrelated to the 3+ and 4+ states. As a matter of illustration, wehave simulated the polarized XANES spectrum ofLa0.7Sr1.3MnO4 in the out-of-plane configuration where themaximum differences are observed along the series due tothe elongation of the MnO6 octahedron along that direction�Fig. 11�b��. We have used the non polarized XANES spec-trum of CaMnO3 for the isotropic Mn4+O6 reference and theout-of-plane polarized spectrum of LaSrMnO4 for the aniso-tropic Mn3+O6 reference. We note here that CaMnO3 is avalid reference even in this case as effects arising from adifferent local structure beyond the first shell around the pho-toabsorbing atom locate at energies above the absorptionedge.32 Although the simulation reproduces the position ofthe absorption edge, differences are clearly observed in theslope and intensity of the main edge resonances. This agreeswith the presence of an intermediate valence state for the Mnatom in this sample. The polarization-dependent XANESspectra of this sample indicate a strong anisotropy too, being about 1.5 eV. As for the LaSrMnO4, this anisotropicshift originates from the tetragonal distortion. Significant an-isotropy is observed at the pre-edge features for this samplein contrast to LaSrMnO4 �Fig. 4�b��. The strong increase inthe intensity of these pre-peaks for the in-plane componentcompared to the out-of-plane one indicates that the dopedholes go mainly into the MnO2 sheets. This highly aniso-tropic hole doping correlates well with the fact that the re-sistivity is considerable reduced for the in-plane component,being c�103 times as large as ab.

7

Polarized EXAFS analysis at room temperature showsthat the coordination of the Mn atom in La0.7Sr1.3MnO4 is atetragonal-distorted octahedron with the magnitude of thedistortion ��0.13 Å� much lower than that of LaSrMnO4. Interms of the simple picture of Mn3+-Mn4+ dilution, MnK-edge EXAFS spectra should be described as composed bya bimodal distribution of highly tetragonal distorted Mn3+

and nondistorted Mn4+ octahedra with a Mn3+/Mn4+ ratio of0.7:0.3. However, the magnitude of the tetragonal distortionfor La0.7Sr1.3MnO4 decreases more rapidly than that expectedfor the Mn3+/Mn4+ ratio. In particular, it is one-third of thatobserved for LaSrMnO4 �formally Mn3+�. Moreover, this te-tragonal distortion is compatible with the Debye-Waller�DW� factor obtained for the average Mn-O interatomic dis-tance from the nonpolarized EXAFS spectra in the wholetemperature range. Then, no significant changes occur in thelocal structure at the Mn site as a function of temperature.Our accurate study points out the fact that all Mn atoms arenearly identical from a structural point of view. This result isin agreement with crystallographic data which does not showsuperstructural order for this composition,12 and it is consis-tent with only one type of Mn site.

Once an intermediate valence state for the Mn atoms inthis compound is established, the tetragonal distortion cannotbe ascribed to the Jahn-Teller effect of an ionic d4 configu-rational state. This indicates that the tetragonal distortionoriginates from solid-state effects, and it is highly dependenton the band filling. This result agrees with recent XAFSexperiments on the LaMn1−xGaxO3 series �formally Mn3+ va-lence compounds� which show that the formal Mn3+ ion isnot tetragonal distorted for x�0.5.51

C. La0.5Sr1.5MnO4

This compound was the first example where the occur-rence of CO and OO was proposed to account for the metal-

FIG. 11. �a� Comparison of the out-of-plane polarized XANESspectrum of La0.5Sr1.5MnO4 sample �open circles� with the 0.5/0.5addition of the LaSrMnO4 out-of-plane polarized spectrum and theCaMnO3 nonpolarized spectrum �dashed line�. �b� Comparison ofthe out-of-plane polarized XANES spectrum of La0.7Sr1.3MnO4

�solid circles� with the 0.7/0.3 addition of the LaSrMnO4 out-of-plane polarized spectrum and CaMnO3 nonpolarized spectra �solidline�.


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insulator transition of manganites with formal Mn3.5+ va-lence. This transition has been interpreted as arising from areal-space ordering of Mn3+ and Mn4+ ions. This model wasfirst supported by XRS studies,22 but further analysis dem-onstrated that this interpretation is wrong.27 Moreover, accu-rate analyses of the XRS signal in 3D manganites such asPr0.6Ca0.4MnO3 and Nd0.5Sr0.5MnO3 have shown that thecharges associated with the two Mn atoms in these com-pounds are almost identical although they have different lo-cal environments.29,30

The results of the present EXAFS and XANES studies arealso difficult to reconcile with a mixing of Mn3+ and Mn4+

ions in the 2D half-doped manganite. Polarized and nonpo-larized XANES spectra of the La0.5Sr1.5MnO4 sample con-clude that if two Mn atoms with different valence state existfor this sample, the charge disproportionation between thesetwo Mn atoms would be very small and thus it cannot beconsidered as a mixture of Mn3+ and Mn4+ ions. The simu-lation by the weighted 1/2 :1 /2 addition of two components,as references for Mn3+ and Mn4+ ionic states, shows much astrong discrepancy with the experiment �Fig. 11�a�� than theone made for the x=0.3 sample. Finally, this compound pre-sents the largest difference in the intensity of the pre-edgefeatures comparing in-plane and out-of-plane components�Fig. 4�b��. Accordingly, the electrical resistivity is alsohighly anisotropic for x=0.5 and it rather decreases with in-creasing the doped holes. It is worth noting that a steep in-crease in the resistivity occurs at the so-called CO transitionfor this sample, the jump being bigger for ab.

7 Then, theyare likely the holes, which are mainly located on the MnO2sheets, the ones that localize at the CO transition.

EXAFS data fully support the XANES results. The MnO6octahedron is very symmetric in the whole temperaturerange. We observe a tetragonal distortion of about 0.06 Åthat is around 3 or 4 times lower than the typical tetragonaldistortion of Mn3+ compounds. Hence, XAS experimentsconclude that Mn atoms in La0.5Sr1.5MnO4 are also nearlyidentical from the local geometrical structure.

The apparent discrepancy arises from the fact that eithercrystallographic determinations or XRS measurements dem-onstrate the existence of two different crystallographic sitesbelow TCO. The task is to find out the actual difference be-tween these sites. We have measured EXAFS spectra for thein-plane polarization by TEY detection as a function of tem-perature across the COO transition. We could not determinethe distortion on this sample within the ab plane but a higher�2 factor for the in-plane Mn-O interatomic distance thanthat of LaSrMnO4 sample was found �Fig. 12, upper panel�.We notice that the DW factors for the in-plane Mn-O bondsdo not depend on the temperature, showing that the localdistortion remains unaltered through TCO. The difference be-tween in-plane EXAFS spectra at two temperatures, oneabove and the other below TCO, is reported in Fig. 12 �bot-tom panel�. No differences are observed between the spectraacross the COO transition. We can compare it with measureddistortions in other samples. For example, the average Mn-O distances in LaSr2Mn2O7 for the two sites are 1.939 and1.918 Å, respectively.20 Differences in distances of this mag-nitude cannot be resolved by EXAFS and are completelycompatible with the relatively large �2 factor found in our

study. Thus, our results agree with the existence of a mini-mum difference between the two crystallographic sites.

Therefore, the two Mn sites are very similar to each otherin terms of the local structure and electronic state. BelowTCO, these two sites are spatially distinguishable and orderedin a checkerboard pattern. Above TCO, they are temporallydistinguishable but disordered. The COO phase transition isthen described as a structural order-disorder transition.

V. CONCLUSIONS

In summary, we have carried out a polarization-dependentx-ray-absorption spectroscopic study at the Mn K edge of the2D La1−xSr1+xMnO4 series. The experiment shows that thelocal structure �geometrical and electronic� around the Mnatom is anisotropic, according to their nearly 2D structure.This anisotropy arises from the tetragonal distortion of theMnO6 octahedron. The structural anisotropy �tetragonal dis-tortion� is correlated with the electronic one �anisotropic en-ergy shift� and decreases with increasing x, the oxygen envi-ronment being nearly isotropic for the La0.5Sr1.5MnO4sample. Anisotropy is also observed between in-plane andout-of-plane components for the pre-edge features. Thestrong increase in the intensity of the in-plane pre-edge struc-

FIG. 12. Upper panel: the magnetic susceptibility forLa0.5Sr1.5MnO4 sample and the temperature evolution of the Debye-Waller factor for the in-plane Mn-O bonds in the neighborhood ofthe CO transition. Arrow indicates TCO. Lower panel: differencespectrum �solid line� between the in-plane polarized EXAFS ofLa0.5Sr1.5MnO4 below �150 K, dashed line� and above �290 K, dot-ted line� TCO.


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tures with increasing x �opposite trend to the anisotropic en-ergy shift� implies that the doped holes by substitution of Laby Sr locate predominantly in the ab plane. This result agreeswith the strong decrease reported for the in-plane resistivityof the x=0.3 and 0.5 samples by hole doping.7

The experiments are unable to distinguish different Mnatoms corresponding to different integer valence states in themixed-valence samples, indicating a nearly homogeneous in-termediate valence state. This also occurs for the low-temperature phase of La0.5Sr1.5MnO4 where the so-calledCOO phase takes place. Our results show that the Mn localgeometric and electronic structure remains unaltered crossingTCO, indicating that the transition must be described as astructural order-disorder one as proposed for similar metal-insulator transitions in other transition-metal oxides such asLaMnO3 �Ref. 45� and Fe3O4 �Refs. 52 and 53�. The result-ing “structural origin” for the so-called COO transitionsseems to be a general phenomena in correlated mixed-valence oxides.

As a final concern, these results cast doubts on two of themain ingredients generally used for the description of mixed-valence manganites. The first one is the lack of electronic dlocalization at the Mn atom. The Mn atom for x=0.3 and 0.5compositions is in an intermediate valence state between theformal Mn3+ and Mn4+ one, in agreement with our previousfindings in 3D manganites.31–33 The second one deals withthe Jahn-Teller coupling. We observe that the tetragonal dis-tortion decreases with x, but it operates for the intermediatevalence states, indicating that the electron-phonon couplingmechanism giving rise to this tetragonal distortion cannot beascribed to a single-ion effect as it was also concluded fromthe study of the LaMn1−xGaxO3 compounds.51

ACKNOWLEDGMENTS

We thank the financial support from the Spanish CICyTProject No. MAT2002-01221 and from D.G.A. We also ac-knowledge ESRF and BM29 beamline for granting beamtime and technical support.

*Corresponding author. FAX: �34-976-761225. Electronic address:[email protected]

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Documents

Polarized x-ray absorption spectra of La1−xSr1+xMnO4: Electronic state of Mn atoms