phys. stat. sol. (c) 3, No. 9, 3266–3271 (2006) / DOI 10.1002/pssc.200567105

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Praseodymium deficiency effects on the physical properties of

Pr1.2–xxSr1.8Mn2O7 bilayer manganese oxides

M. Triki1, S. Zouari

1, A. Cheikhrouhou

*, 1, 2 and P. Strobel

3

1 Laboratoire de Physique des Matériaux, Faculté des Sciences de Sfax, B. P. 802, 3018 Sfax, Tunisie 2 Laboratoire de Magnétisme Louis Néel, B.P. 166, 38042 Grenoble Cedex 9, France 3 Laboratoire de Cristallographie, CNRS, B.P. 166, 38042 Grenoble Cedex 9, France

Received 5 September 2005, revised 8 January 2006, accepted 26 April 2006

Published online 1 August 2006

PACS 61.10.Nz, 61.66.Dk, 61.72.Ww, 72.80.Jc, 75.30.Kz, 75.60.Ej

The praseodymium deficiency effects on the structural, magnetic and electrical properties of the lacunar

bilayers manganese oxides Pr1.2–xxSr1.8Mn2O7 (0 ≤ x ≤ 0.15) were studied. Rietveld refinements of the

X-ray diffraction patterns show that all our samples are single phase and crystallize in the tetragonal struc-

ture with I4/mmm space group. All our samples exhibit a paramagnetic-ferromagnetic transition on cool-

ing. The praseodymium deficiency leads to a decrease of the Curie temperature. Resistivity measurements

as a function of temperature show that only the parent sample Pr1.2Sr1.8Mn2O7 exhibits a semiconducting-

metallic transition with decreasing temperature.

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction At present there is great technological interest into the materials which exhibit mag-netoresistance effects, because of their use in magnetic information storage or as magnetic field sensors [1–5]. The bilayered manganese oxides with formula A3B2O7, also known as Ruddlesden-Popper phases, exhibit magnetoresistance effect and are characterized by pronounced physical anisotropy [6–16]. The structure of these compounds is described as an intergrowth of 2 layer perovskite blocks and an inter-spersing single rock salt layer which decouples the blocks magnetically and electrically [17–19]. As a consequence, the double exchange mechanism is expected to be much weaker in the layered compounds [20, 21]. In this work we studied the effect of praseodymium deficiency on the structural, magnetic and electrical properties of Pr1.2–xxSr1.8Mn2O7 (0 ≤ x ≤ 0.15), in which Mn4+ content increases with x. In these sam-ples the average ionic size of the elements occupying the A site may give lattice effects and also the interaction between Mn4+ and Mn3+ moments will be influenced. 2 Experimental Polycristalline samples Pr1.2–xxSr1.8Mn2O7 (0 ≤ x ≤ 0.15) were elaborated using the standard ceramic process by mixing Pr6O11, SrCO3 and MnO2 up to 99.9% purity in the desired propor-tions according to the following reaction:

(0.2-x/6)Pr6O11 + 1.8SrCO3 + 2MnO2 → Pr1.2–xxSr1.8Mn2O7 + δCO2 +δ’O2 The starting materials were intimately mixed, ground and calcined at 1000 °C for 48 hours in air. The resulting powders were ground again, then pressed into pellets and sintered at 1400 °C for 72 hours with intermediate grindings.

* Corresponding author: e-mail: abdcheikhrouhou@yahoo.fr, Phone: +216 74 676607, Fax: +216 74 676607

phys. stat. sol. (c) 3, No. 9 (2006) 3267

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The pellets were rapidly quenched at room temperature in air in order to freeze the structure at the an-nealed temperature. The samples were characterized by X-ray diffraction at room temperature using Mo radiation and the XRD profiles were refined by Fullprof program. Magnetization measurements were carried out using an extracting sample magnetometer, and resistivity was measured by the standard four probe method. 3 Results and discussion

3.1 X-ray diffraction analysis Powder X-ray diffraction measurements at room temperature and Rietveld structure refinements using the Fullprof program [22] indicate that all our synthesized samples Pr1.2–xxSr1.8Mn2O7 (0 ≤ x ≤ 0.15) crystallize in the tetragonal structure with the I4/mmm space group. A typical X-ray diffraction pattern, including the observed and calculated profiles and the difference profile of Pr1.150.05Sr1.8Mn2O7 is given in Fig. 1. All reflections fit the tetragonal Ruddlesden-Popper-type structure (with the exception of a weak reflection at 18 degrees 2θ).

Fig. 1 Observed, calculated and difference X-ray powder diffraction patterns at room temperature for

Pr1.150.05Sr1.8Mn2O7. Reflection positions are marked for the ideal phase.

Table 1 lists the data obtained from the refinements. The A3B2O7 contains two sites within which the A cations (Pr and Sr) can be distributed: the 2b site at 0 0 ½ with 12-fold coordination (perovskite site, abbreviated “P”), and the 4e site at 0 0 z with 9-fold coordination (rocksalt site, abbreviated “R”). The occupations on these sites were refined, and the fraction of praseodymium deficiency in both P and R sites was deduced. The results in terms of vacancy fraction are listed in Table 2. The structural analysis shows that there is no significant difference in the Pr/Sr occupation ratio (2/3 within experimental errors) on the P and R sites in the stoichiometric sample. With increasing x in Pr1.2–xxSr1.8Mn2O7, the Sr occupancy remains remarkably constant on both sites. On the contrary, the occupancy of praseodymium decreases, and this change is much more pronounced on the 4e site than on the 2b one, resulting in a larger fraction of vacancies in the former. In other words, vacancies are formed

3268 M. Triki et al.: Pr deficiency effects on the physical properties of Pr1.2–x

xSr

1.8Mn

2O

7

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

preferentially in the rocksalt site (9-fold coordinated) rather than in the perovskite one (12-fold coordi-nated). This is in agreement with the tendency for A cations to fill A sites with highest coordination [24]. Table 2 also includes the total fraction of vacancies Σv per Pr1.2–xxSr1.8Mn2O7 formula unit deduced from the structural refinements; it is given by the formula Σv = 2v(R)+v(P), where v stands for the va-cancy fraction on a given site. The excellent agreement with the nominal vacancy concentration is worth pointing out.

Table 1 Crystallographic data for Pr1.2–xxSr1.8Mn2O7 (0 ≤ x ≤ 0.15).

x atom Site x y z Biso(Å2) Occupancy †

0 Pr/Sr (P) 2b 0 0 0.5 0.5158(9) 0.408(15)/0.604(15) Pr/Sr (R) 4e 0 0 0.3175(4) 0.3723(3) 0.395(16)/0.597(16) Mn 4e 0 0 0.0974(5) 0.1365(8) O(1) 2a 0 0 0 0.8476(9) O(2) 4e 0 0 0.1982(4) 0.8476(9) O(3) 8g 0 0.5 0.0970(0) 0.8476(9) 0.05 Pr/Sr (P) 2b 0 0 0.5 0.7551(2) 0.389(14)/0.603(14) Pr/Sr (R) 4e 0 0 0.3169(3) 0.7348(7) 0.379(13)/0.598(13) Mn 4e 0 0 0.0975(3) 0.1869(8) O(1) 2a 0 0 0 2.0421(8) O(2) 4e 0 0 0.1962(7) 2.0421(8) O(3) 8g 0 0.5 0.0976(9) 2.0421(8) 0.1 Pr/Sr (P) 2b 0 0 0.5 1.1595(7) 0.386(14)/0.611(14) Pr/Sr (R) 4e 0 0 0.3169(7) 0.6154(3) 0.357(8)/0.594(8) Mn 4e 0 0 0.0975(0) 0.8817(3) O(1) 2a 0 0 0 1.7770(1) O(2) 4e 0 0 0.1989(0) 1.7770(1) O(3) 8g 0 0.5 0.0987(6) 1.7770(1) 0.15 Pr/Sr (P) 2b 0 0 0.5 0.8636(6) 0.365(8)/0.610(8) Pr/Sr (R) 4e 0 0 0.3175(7) 0.5682(3) 0.342(6)/0.594(6) Mn 4e 0 0 0.0972(9) 0.8274(9) O(1) 2a 0 0 0 1.9313(3) O(2) 4e 0 0 0.1965(7) 1.9313(3) O(3) 8g 0 0.5 0.0967(7) 1.9313(3) † occupancy fixed for Mn and O atoms.

Table 2 Distribution of Pr and vacancies in the P (2b) and R (4e) sites.

x Occupancy on P (2b) site

Pr vacancies

Occupancy on R (4e) site

Pr vacancies

Total vacancies

per formula

0 0.408(15) 0 0.395(16) 0 0

0.05 0.389(14) 0.01(1) 0.379(13) 0.023(13) 0.056 0.1 0.386(13) 0.01(1) 0.357(8) 0.049(8) 0.109 0.15 0.365(8) 0.025(8) 0.342(6) 0.064(6) 0.153 Figures 2 and 3 display the evolution of the lattice parameters and the cell volume, respectively, as func-tion of Pr defect concentration x. The lattice parameters a, b and c and the cell volume decrease with increasing x up to 0.05. This result can be explained by the increase of Mn4+ content with smaller ionic radius [r(Mn3+) = 0.785Å, r(Mn4+) = 0.68Å] [23]. The increase of these parameters for x > 0.05 can be explained by the Coulombian repulsion between cations.

phys. stat. sol. (c) 3, No. 9 (2006) 3269

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3,844

3,846

3,848

3,85

3,852

20

20,05

20,1

20,15

20,2

0 0,05 0,1 0,15

a=b(Å

) c(Å)

x

a = b

c

295

296

297

298

0 0,05 0,1 0,15

V(Å

3)

x 3.2 Magnetic measurements The temperature dependence of magnetization for Pr1.2–xxSr1.8Mn2O7 at a magnetic applied field of 0.05 T is shown in Fig. 4. All samples exhibit a transition from paramag-netic to ferromagnetic state with decreasing temperature. As shown in Fig. 5, the deficit of praseodymium leads to a decrease in Curie temperature Tc from 265 K for x = 0 to 230 K for x = 0.15. It leads also to a weakness of the ferromagnetism at low temperatures.

0

0,1

0,2

0,3

0,4

0,5

0 50 100 150 200 250 300 350

x = 0x = 0.05

x = 0.1x = 0.15

M (µB/f

orm

ule

)

T(K)

Pr1.2-x

Sr1.8

Mn2O

7

220

230

240

250

260

270

0 0,05 0,1 0,15

Tc

(K)

x

3.3 Electrical measurements In Fig. 6, we plot the temperature dependence of the resistivity (in zero applied magnetic field). The stoichiometric sample exhibits two transitions with decreasing temperature, the first at 66 K from semiconducting to metallic state and the second at 35 K to another semiconducting state. This can be explained by the spin canted state observed at low temperature in this sample [24] which makes difficult the hopping of charges between neighbours sites. In contrast, the non-stoichiometric compounds exhibit a semiconducting behaviour in the whole temperature range.

Fig. 2 Evolution of the lattice parameters with x

for Pr1.2–xxSr1.8Mn2O7. Fig. 3 Evolution of the unit cell volume with

x for Pr1.2–xxSr1.8Mn2O7 .

Fig. 4 Temperature dependence of magnetisation for

Pr1.2–xxSr1.8Mn2O7 at 0.05 T.

Fig. 5 Curie temperature as a function of x for

Pr1.2–xxSr1.8Mn2O7 samples.

3270 M. Triki et al.: Pr deficiency effects on the physical properties of Pr1.2–x

xSr

1.8Mn

2O

7

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

0

1 104

2 104

3 104

0 100 200 300

x = 0

x = 0.05

0.1

x = 0.15

ρ (

Ω.cm)

T(K)

0

1000

2000

3000

4000

0 100 200 300

x = 0

ρ (Ω

.cm

)

T(K)

It is interesting to note the evolution of the resistivity values with composition. At a given temperature

below 100 K, the resistivity value increases sharply with x up to 0.05 and than decreases for x > 0.05.

Interestingly, such an evolution is parallel to the evolution of the crystallographic parameters (see Figs. 2

and 3).

4 Conclusion The effect of introducing praseodymium vacancies on the structural, magnetic and

electrical properties of Pr1.2–xxSr1.8Mn2O7 was investigated. All synthesized samples crystallize in the

tetragonal structure with the I4/mmm space group. The presence of Pr vacancies was confirmed by Riet-

veld refinements, showing that these vacancies concentrate on the rocksalt-type A sites of the structure.

Magnetic investigations showed that all samples exhibit a paramagnetic-ferromagnetic transition with

decreasing temperature. The electrical characterizations showed that the parent compound exhibits two

transitions. However these transitions are suppressed in lacunar phases, which are semiconductors in the

whole temperature range. These results also showed a strong correlation between the structural and elec-

trical properties.

Acknowledgements This work has been supported by the Tunisian Ministry of Scientific Research, Technology

and Development of Competences.

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