Polaronic

Hyperfine Interact (2010) 196:385–394DOI 10.1007/s10751-010-0174-7

Polaronic and charge ordering effectsin Pr0.5Sr0.5Mn1−xFexO3

Sami H. Mahmood · Jumana Dawood ·Abdel-Fatah Lehlooh · Abdelwaheb Cheikhrouhou ·Abdallah Ammar

Published online: 19 March 2010© Springer Science+Business Media B.V. 2010

Abstract In this work we report the results of X-ray diffraction and Mössbauer spec-troscopy for the systems Pr0.5Sr0.5Mn1−xFexO3 (with x = 0.05, 0.10, 0.15, 0.20, 0.25,

0.30). XRD patterns indicated that all samples were single phase with slightlydistorted orthorhombic symmetry. Room temperature Mössbauer spectra are allquadrupole split, indicating paramagnetic relaxation of the Fe moment for all valuesof Fe concentrations. The spectra are fitted with two doublet components associatedwith Fe3+ ions in octahedral sites with different distortions. Mössbauer spectrarecorded at liquid nitrogen temperature for this system also indicate paramagneticrelaxation of the Fe moments down to liquid nitrogen temperature (LNT). Inthese spectra a third quadrupole component with quadrupole splitting close to zerodevelops. This component is associated with the delocalization of the charge carriersand the consequent disappearance of lattice distortions produced by the polaroniceffect at room temperature. The component with the high quadrupole splitting(0.81 to 1.07 mm/s) results from Jahn–Teller distortion as a consequence of chargeordering transition at low temperature.

Keywords Mössbauer spectroscopy · Manganites · Polarons · Charge order ·Jahn–Teller distortion · Delocalization

1 Introduction

Heterogeneous ferromagnetic materials exhibiting giant magnetoresistance (GMR)have been of great interest due to their potential use in magnetic recording [1].

S. H. Mahmood (B) · J. Dawood · A.-F. LehloohPhysics Department, Yarmouk University, Irbid, Jordane-mail: [email protected]

A. Cheikhrouhou · A. AmmarFaculté des Sciences de Sfax, Laboratoire de Physique des Matériaux, 3018, Sfax, Tunisia

386 S.H. Mahmood et al.

More recently, some 3d transition-metal oxides possessing large room-temperaturemagnetoresistance associated with a paramagnetic-ferromagnetic phase transitionhave been recognized. Specifically, the magnetic perovskites T1−x DxMnO3 whereT is a trivalent lanthanide cation and D is a divalent cation have been the focus ofstudies due to their structural, magnetic, transport and electronic structure proper-ties [2].

A number of models and theories have been proposed to explain the observedlarge (colossal) magnetoresistance (CMR) in manganites systems. Such models are:the paramagnetic-ferromagnetic phase transition [3, 4], the double exchange (DE)interaction between Mn3+ and Mn4+ ions [5], electron–phonon coupling arising fromthe Jahn–Teller distortion of the Mn3+O6 octahedra and the formation of latticepolarons induced by the electron-phonon coupling in the paramagnetic phase [6, 7],and the charge ordering (CO) transition [8, 9].

Fe57 MS of the manganite system La1−xCaxMn0.99Fe0.01O3 (x = 0.5, 0.6) [10]have shown that at T = 150 K a resolved doublet with quadrupole splitting �Q =0.426 mm/s was observed. At room temperature the Mössbauer spectrum has aquadrupole splitting �Q = 0.200 mm/s. The large increase in quadrupole splitting atlow temperatures is attributed to Jahn–Teller (J-T) distortion due to charge ordering(CO) as the temperature is reduced.

Simopoulos et al. [11] have used XRD, neutron diffraction, and Mössbauerspectroscopy to investigate the structural characteristics and hyperfine interactionsfor the systems La0.67Ca0.33MnO3 doped with 1% and 2% Fe57. At T = 300 KMössbauer spectrum for each sample was fitted with one doublet. The isomer shift(δ = 0.368 mm/s) and quadrupole splitting (�Q = 0.184 mm/s) were typical for Fe3+in slightly distorted octahedral coordination.

Moutis et al. [12] used XRD to illustrate that the system La0.67(BaxCa1−x)0..33

MnO3 with x ≤ 0.4 displayed a single orthorhombic phase, while those with x ≥ 0.50displayed a single rhombohedral phase.

Mössbauer measurements of the system La0.75Ca0.25Mn0.98Fe0.02O3 [13] haveshown that at T = 300 K the spectrum was fitted with one doublet, with hyperfineparameters δ = 0.364 mm/s, and �Q = 0.348 mm/s. The small value of �Q wasassociated with orthorhombic distortion.

The system La0.75Ca0.25Mn1−xFexO3 (x = 0.042, 0.045) was studied in the temper-ature range 4–300 K [14]. Mössbauer spectra from 300 K down to 190 K were fullyparamagnetic, and could be fitted with two doublets, the hyperfine parameters forthe first doublet are δI = 0.35 mm/s, �QI = 0.12 mm/s; the hyperfine parameters forthe second doublet are δII = 0.37 mm/s, �QII = 1.42 mm/s.

The crystal structure and the magnetic properties of the system Pr0.5Sr0.5Mn1−x

FexO3 under investigation have been previously reported [15]. XRD shows thatall samples are single phase of the orthorhombic structural type. In this work weintroduce an evidence of polaronic effects and electron localization–delocalizationtransition, and charge ordering in Fe doped manganites as the temperature isreduced from room temperature down to liquid nitrogen temperature. This work ismotivated by the interesting structural, magnetoresistive, and transport properties ofthe perovskite manganites, the various models proposed to explain the experimentalresults, and the variability of the measured hyperfine parameters associated withthe sensitivity of the local structural symmetries to the preparation techniques andconcentrations of the various elements in the compounds.

Polaronic and charge ordering effects in Pr0.5Sr0.5Mn1−xFexO3 387

2 Experimental

The system Pr0.5Sr0.5Mn1−xFexO3 (0.00 ≤ x ≤ 0.30) was prepared using the standardsolid-state reaction method. Appropriate amounts of high purity (at least of 99.9%)Pr6O11, MnO2, SrCO3 and Fe2O3 were thoroughly mixed in an agate mortar, andthen fired in air at about 1,000◦C for 48 h. After pulverizing another time, the powderwas pressed into pellets (of about 1 mm thickness) and sintered at 1,400◦C in air for72 h.

X-ray diffraction patterns of the samples were obtained using a Philips PW 1729diffractometer with Co-Kα radiation (λ = 1.79 Å).

The Mn4+ ratio in the samples was checked by redox titration using standardpotassium permanganate and ferrous sulfate solution. This ratio was found to bebetween 51.8% and 47.3% for the concentration range between 0.05 and 0.30,indicating that Fe3+ ions substitute Mn3+ ions. The magnetic data were obtainedusing a vibrating sample magnetometer in the temperature range 10–300 K, in anapplied field of 500 Oe. Further details are found in [15].

Samples for Mössbauer studies were prepared by sprinkling a thin layer of thepowdered compound on one face of a piece of a double-sided scotch tape withdiameter ∼= 2 cm. Mössbauer spectra were collected using a standard constantacceleration Mössbauer Spectrometer with a 25 mCi source of Co57 in palladium.The spectrum of α-iron at room temperature was used to calibrate the system. Thespectra were fitted using routines based on least squares analysis.

3 Results and discussion

3.1 X-ray diffraction

The structure of the system Pr0.5Sr0.5Mn1−xFexO3 was examined by X-ray powderdiffraction, and the patterns in Fig. 1 show that the system is single phase for all valuesof x. This result is consistent with the previous structural analysis of the system [15],where it was found that all samples crystallize in the orthorhombic structure withImma space group. The lattice parameters are listed in Table 1. As the concentrationof Fe increases, the unit cell parameters change slightly due to the fact that the Fe3+ions replacing Mn3+ have a similar ionic radius of 0.645 Å. However, this smallchange in lattice parameters is indicative of small lattice distortions caused by thesubstitution.

3.2 Magnetic

The magnetization measurements versus temperature in the range of 10–300 Krevealed that all samples exhibit paramagnetic to ferromagnetic (FM) transitionupon cooling, with a great weakness of the magnetization and a decrease of the fer-romagnetic transition temperature (Tc) for x ≥ 0.05. This behavior was interpretedas due to the interruption of the double exchange interaction between Mn3+ andMn4+ ions. The FM transition temperature decreases linearly from 256 K for x = 0,to 209 K for x = 0.25 [15].


Fig. 1 Powder X-raydiffraction patterns forPr0.5Sr0.5Mn1−xFexO3(0.05 ≤ x ≤ 0.3)

20 40 60 80 100

(131

)

(113

) (004

)

(132

)(0

24)

(222

)(1

14)

(220

)

(022

)

(121

)

(112

)

(002

)

x=0.30

x=0.25

x=0.20

x=0.15

x=0.10

x=0.05

Inte

nsity

(ar

bitr

ary

units

)

2θ (deg)

Table 1 Crystallographic dataof Pr0.5Sr0.5Mn1−xFexO3samples (0.00 ≤ x ≤ 0.30)

X a(Å) b(Å) c(Å)

0.00 5.391 5.434 7.7330.05 5.387 5.434 7.7290.10 5.380 5.435 7.7150.15 5.374 5.437 7.7080.20 5.371 5.438 7.7020.25 5.367 5.441 7.6990.30 5.355 5.444 7.696

3.3 Mössbauer

Mössbauer spectra of the system Pr0.5Sr0.5Mn1−xFexO3 (with x = 0.05, 0.10, 0.15,

0.20, 0.25, 0.30) were recorded at room temperature and at liquid nitrogen temper-ature. The spectra are all quadrupole split with no magnetic splitting, indicatingparamagnetic relaxation for the Fe moment at both temperatures for all values ofFe concentrations.

The spectra at room temperature are fitted each with two doublet components ofalmost the same isomer shift. Figure 2 shows the fitted spectra for the compoundswith x = 0.05 and x = 0.30, and the fitting parameters for the spectra of all samplesare listed in Table 2.

The isomer shift (∼0.4 mm/s) for the two doublets is characteristic of Fe3+ ions.This indicates that Fe3+ ions replace Mn3+ ions, which is due to their compatibleionic sizes [10]. This is also consistent with the ratio of Mn4+determined by theredox titration method. The quadrupole splitting for the first component rangesfrom 0.30 to 0.46 mm/s, while that for the second component ranges from 0.68–0.89 mm/s. Due to the absence of vacancies in our fully oxidized compounds, thepossibility of the presence of different coordination numbers around the Fe3+ ionsis excluded. Thus, these two components are associated with Fe3+ ions in sites withdifferent distortions of the Fe3+ O6 octahedra. This conclusion is consistent with theobserved small changes in lattice parameters for our samples, and in agreement withprevious studies [10, 11, 14, 16]. The component with the lower quadrupole splitting


Fig. 2 Room temperatureMössbauer spectra ofPr0.5Sr0.5Mn1−xFexO3(x = 0.05, 0.30)

-4 -3 -2 -1 0 1 2 3 4

Pr0.5Sr0.5Mn0.95Fe0.05O3

Tra

nsm

issi

on

(ar

b. u

nit

s)

Velocity (mm/s)

-4 -3 -2 -1 0 1 2 3 4

Pr0.5Sr0.5Mn0.70Fe0.30O3

Tra

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issi

on

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nit

s)

Velocity (mm/s)

can be associated with charge carrier localization at room temperature and theconsequent development of small magnetic polarons producing lattice distortions ofthe octahedral sites. This assignment is consistent with the reported lattice distortionassociated with the gradual localization of the charge carriers above Tc due to strongelectron-phonon coupling in (La-Y/Tb)2/3Ca1/3MnO3 [17]. The second componentwith larger quadrupole splitting is associated with larger distortions of the latticeresulting in a rhombohedral symmetry around the Fe3+ ions. The temperaturedependence of the spectra (to be discussed below) shows that this component isintrinsically different in nature from that arising from polaronic effects. The smallmonotonic increase in quadrupole splitting with increasing the Fe concentration isassociated with a gradual increase in lattice distortion resulting from the depletionof the Mn3+/Mn4+ ratio as a consequence of Fe3+ replacing Mn3+. This increase inlattice distortion could be a consequence of the Fe3+–Mn4+ DE interaction beingweaker than the Mn3+–Mn4+ DE interaction. This behavior is also consistent withour structural data.


Table 2 Fitting parameters forthe spectra of the systemPr0.5Sr0.5Mn1−xFexO3 at RT

x Comp. # δ (mm/s) W (mm/s) I (%) �Q (mm/s)

0.05 I 0.39 0.32 75 0.30II 0.33 0.32 25 0.68

0.10 I 0.40 0.32 70 0.37II 0.40 0.32 30 0.76

0.15 I 0.40 0.35 89 0.38II 0.38 0.35 11 0.79

0.20 I 0.40 0.37 91 0.40II 0.37 0.37 9 0.84

0.25 I 0.40 0.35 77 0.43II 0.40 0.35 23 0.86

0.30 I 0.41 0.37 73 0.46II 0.40 0.37 27 0.89

Mössbauer spectra for the system Pr0.5Sr0.5Mn1−xFexO3 were also collected atliquid nitrogen temperature (LNT) for all values of x. The spectra for the sampleswith x = 0.05 and x = 0.30 are shown in Fig. 3. Since the spectra of the investigatedsamples do not show any magnetic splitting, we conclude that the Fe ions experienceparamagnetic relaxations down to LNT. This behavior will be discussed below.

It was not possible to fit the Mössbauer spectra at LNT with two doublets as inthe RT spectra. It is obvious that a third component at the center of the spectrum ispresent. Thus, the best fit of the spectrum for each value of x is obtained using threedoublets, and the fitting parameters are listed in Table 3.

The isomer shifts of the spectral components are similar, and are characteristicof the Fe3+ valence state. The small increase in isomer shift in comparison with theroom temperature spectra is due to second order Doppler shift. The line widths of theLorentzians used to fit all the spectra of the system (0.30–0.43 mm/s) are considerablylarger than the typical width for the Lorentzian associated with the spectrum of Feexisting in one distinct environment (W = 0.25 mm/s). This broadening suggests themultiplicity of Fe-sites with slightly different distortions.

The development of the third component with a relatively small quadrupole split-ting at low temperature can be associated with the charge carrier delocalization andthe consequent disappearance of lattice distortion produced by the polaronic effectat room temperature. This is the same effect responsible for the observed anomaliesin volume expansion, small angle neutron scattering intensity, resistivity, and mag-netostriction in magnetoresistive perovskites in the temperature range below 1.8 Tc

[17]. The intensity of this component (20–40%) suggests that an appreciable fractionof the Fe3+ ions at LNT exists in sites which are almost undistorted due to chargedelocalization. The relatively low intensity (20%) of this component at x = 0.05 isdue to the fact that at this concentration the Fe–Fe separation is compatible with thatreported for Fe-doped La0.75Ca0.25MnO3 at the threshold Fe concentration of 0.045at which anomalous behavior is observed in the transport properties of the manganite[14, 18]. At this concentration, the Fe–Fe separation is reported to be about threelattice parameters (12 Å), where polarons of this size develop, and the observedtransition in resistivity behavior is associated with the localization–delocalizationtransition for the charge carrier–polaron complex. At higher Fe concentrations, thenumber of small polarons increases due to the obstruction of the Mn3+–Mn4+ DEinteraction by Fe3+ ions. This result is in agreement with the reported significant


Fig. 3 The fitted Mössbauerspectra of Pr0.5Sr0.5Mn1−xFexO3 with x = 0.05, 0.30 atliquid nitrogen temperature

-4 -3 -2 -1 0 1 2 3 4

Pr0.5Sr0.5Mn0.95Fe0.05O3

Tra

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issi

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Velocity (mm/s)

Tra

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-4 -3 -2 -1 0 1 2 3 4

Pr0.5Sr0.5Mn0.70Fe0.30O3

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increase in resistivity, and enhancement of the magnetoresistance as x increases inthe Pr2/3Sr1/3Mn1−xO3 manganite system [19]. The relatively high intensity (40%)for the low quadrupole component at x = 0.15 is consistent with the observeddramatic enhancement of the colossal magnetoresistance at 12% Fe doping inLa2/3Ca1/3MnO3 [20].

The persistence of the smaller room temperature quadrupole splitting component(∼0.45 mm/s) with ∼30–50% relative spectral intensity is a signature that carrierlocalization and the consequent onset of magnetic polarons still exists at LNT.Recently, the presence of polarons both above and below the transition temperaturein La0.7Ca0.3MnO3 has been reported [21]. Thus, the production of materials withhigher degree of charge delocalization below Tc should improve the metallicity ofthe material, and consequently result in a sharper insulator–metal transition at Tc.Also, this remaining component could be arising in part from other effects that


Table 3 Fitting parameters forthe spectra of the systemPr0.5Sr0.5Mn1−xFexO3 at LNT

x Comp. # δ (mm/s) W (mm/s) I (%) �Q (mm/s)

0.05 I 0.43 0.42 40 0.42II 0.44 0.42 40 0.97III 0.44 0.42 20 0.08

0.10 I 0.46 0.43 36 0.43II 0.47 0.43 34 1.04III 0.45 0.43 30 0.16

0.15 I 0.43 0.36 28 0.46II 0.44 0.36 32 0.87III 0.46 0.36 40 0.10

0.20 I 0.41 0.30 50 0.45II 0.41 0.30 24 0.81III 0.42 0.30 26 0.14

0.25 I 0.40 0.38 34 0.44II 0.42 0.38 36 1.07III 0.41 0.38 30 0.16

0.30 I 0.43 0.40 38 0.45II 0.45 0.40 36 1.07III 0.44 0.40 26 0.04

cannot be explained by the simple polaronic effects. Recently, it was stated that thedisregard of some of the essential ingredients of a comprehensive microscopic modelfor manganites, such as electron–electron interactions and disorder effects, is notjustified [22].

The obvious increase in the splitting of the high quadrupole component (∼0.81–1.07 mm/s) as the temperature is lowered down to LNT for almost all samples iscontrary to what is expected from localization–delocalization effects, and indicatesthat the low and high quadrupole components observed in the room temperaturemeasurements are different. While the first is arising from lattice distortions pro-duced by polaronic effects above Tc, and is expected to diminish as we approach Tc

from above, the second component is not. Thus, the observed increase in quadrupolesplitting is associated with Jahn–Teller (J-T) distortions due to CO transition as thetemperature is lowered. The intensity of this component indicates that it is arisingpartially from the original room temperature high splitting component (temperatureindependent distortions), and partially from J-T distortions due to CO around sitesoccupied by Fe3+ ions participating in the low splitting component. This chargeordering as the temperature is reduced results in doubling (or more) the value of thequadrupole splitting. Such CO transitions have been reported at low concentrationsof Fe in Pr1/2Sr1/2MnO3 below 140 K [9]. A similar doubling of the quadrupole split-ting had been reported in the system La0.5Ca0.5MnO3 below the CO temperature,and was associated with the J-T distortion [10]. The suggestion of the presence ofcharge ordering in our compounds is consistent with the reports that the electrondoped (Mn4+ rich) region favors CO mechanism, however, is contrary to the reportedabsence of CO in Pr0.5Sr0.5MnO3 based on ED results [22]. Another basic differencebetween our results and those of [23] is that the latter reports a ferromagnetic-metalto antiferromagnetic-insulator transition at around 150 K, which is not observed inour samples. This could be due to the Fe doping in our samples, resulting in theincrease in the Mn4+/Mn3+ ratio.


Finally, the apparent contradiction between the results of the magnetic measure-ments [15] which show ferromagnetic-like transition as the temperature is lowered,and those of the Mössbauer measurements which show paramagnetic behavior downto LNT should be addressed. It is evident that as Tc is approached from above, thenumber of small polarons increases, and reaches a maximum at Tc, resulting in theobserved anomalies at this temperature [17]. The observed progressive increase inmagnetization in the temperature range down to about 75 K could thus be associatedwith the progressive increase in the number of polarons, and their alignment in theapplied field direction. It could be inferred from the magnetic data that the polaronsstart to interact below about 270 K, and coalesce at temperatures below about 75 K,marking a true polaronic to ferromagnetic transition. This interpretation is consistentwith the reported results on La0.7Ca0.3MnO3 [24], and with our Mössbauer resultswhich indicates the presence of Fe ions in a magnetically disordered state.

4 Conclusions

The present study shows that Mössbauer spectra for the systems Pr0.5Sr0.5Mn1−x

FexO3 (with x = 0.05–0.30) are all quadrupole split with no magnetically splitcomponents. This indicates that the magnetic moments of Fe exhibit paramagneticrelaxations in all samples at temperatures down to LNT. Since all studied samples arefully oxidized, all Fe ions are in octahedral sites coordinated with 6O2− ions. Analysisof the spectra leads to the following conclusions:

1. Room temperature Mössbauer spectra consist of two quadrupole components,corresponding to Fe3+ ions in sites of different lattice distortions.

2. The value of the quadrupole splitting for each of these two components increasesslightly and monotonically with increasing Fe concentration due to the changingof Mn4+/Mn3+ ratio, and the consequent changes in the number of polarons.

3. Reducing the temperature down to LNT results in a third quadrupole componentwith small quadrupole splitting (between 0.04 and 0.16 mm/s). This is an evidenceof charge delocalization and the consequent disappearance of lattice distortionproduced by the polaronic effect at room temperature.

4. Charge ordering transition is responsible for the appreciable increase in quadru-pole splitting and intensity for the high quadrupole split component uponlowering the temperature.

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