phys. stat. sol. (a) 201, No. 7, 1416–1420 (2004) / DOI 10.1002/pssa.200404443

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Deficiency effects on the physical properties of the lacunar La0.5Ca0.5 – xMnO3 manganese oxides

I. Walha1, W. Boujelben1, M. Koubaa1, 2, A. Cheikh-Rouhou*, 1, and A. M. Haghiri-Gosnet2 1 Laboratoire de Physique des Matériaux, Faculté des Sciences de Sfax, B.P. 802, 3018 Sfax, Tunisia 2 Institut d’Electronique Fondamentale, IEF/UMR 8622, Université Paris Sud, Bâtiment 220,

91405 Orsay Cedex, France

Received 26 January 2004, revised 21 March 2004, accepted 23 March 2004 Published online 11 May 2004

PACS 75.47.Gk, 75.60.Ej

We investigate the effects of calcium deficient on the structural and magnetotransport properties of non-stoichiometric La0.5Ca0.5 – xxMnO3 manganese oxides. Powder samples have been elaborated using the solid-state reaction technique. Our synthesized samples crystallize in the orthorhombic perovskite struc-ture with Pnma space group. Electrical measurements versus temperature in magnetic applied field up to 8 T on La0.5Ca0.5 – xxMnO3 compounds have been performed. Resistivity measurements of the stoichio-metric sample La0.5Ca0.5MnO3 show a semiconducting behaviour in the whole temperature range of 20–300 K. An increase of the resistivity at very low temperature can be attributed to the charge ordering (CO) effect. Calcium defect leads to an important decrease of the resistivity at low temperature and con-sequently a destruction of the CO effect observed at very low temperature in the parent compound. Elec-trical measurements show that 5% of calcium deficiency induces a semiconducting–metallic transition with decreasing temperature. The electrical transition temperature increases with increasing deficient con-tent. 5% of calcium deficient induces the same effect produced by an applied magnetic field of 8 T on the parent compound.

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction The perovskite-type manganites of the general formula Ln1–xAxMnO3 where Ln is a trivalent rare-earth and A is a divalent alkaline-earth or a monovalent alkali metal have been extensively investigated since the discovery of the colossal magnetoresistance effects in such materials [1–6]. In these materials, the magnetotransport could be explained on the basis of double exchange (DE) mecha-nism [7–9] between Mn3+–Mn4+ pairs with conductivity occurring by electron hopping between manga-nese ions along Mn–O–Mn bonds. However, the double exchange cannot alone explain the rich variety of phenomena found in these compounds. The system La1–xCaxMnO3 has a rich phase diagram [10], the half-doped manganite, with x = 0.5, is charge-spin-orbital ordered [11, 12]. The effect of divalent alka-line-earth element substitution in the stoichiometric perovskite manganites La1–xCaxMnO3 have been extensively studied, however, only few studies have been carried out on the deficiency effects in lacunar systems [13, 14]. In order to study the vacancy effects in La0.5Ca0.5MnO3, we investigate the structural and magneto-transport properties in the La0.5Ca0.5–xxMnO3 lacunar samples with 0 ≤ x ≤ 0.25. 2 Experimental Powder samples of La0.5Ca0.5–xxMnO3 (0 ≤ x ≤ 0.25) were prepared using the solid state reaction by mixing La2O3, MnO2 and CaCO3 up to 99.9% purity in the desired proportion according to the following reaction:

0.5La2O3 + 2(0.5 – x) CaCO3 + 2MnO2 2La0.5Ca0.5–xxMnO3 + δCO2.

* Corresponding author: e-mail: abdel.Cheikhrouhou@fss.rnu.tn

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© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Table 1 Chemical analysis results for lacunar La0.5Ca0.5–xxMnO3 samples.

x % Mn4+ theoretical % Mn4+ experimental relative error (%)

La0.5Ca0.5–xxMnO3

0.05 60 59.75 0.5 0.10 70 68.86 1.6 0.15 80 77.92 2.6 0.20 90 87.63 2.6 0.25 100 96.73 3.2

The starting materials were thoroughly mixed in an agate mortar and then heated in air at 1000 °C for 60 hours. A systematically annealing at high temperature is necessary to ensure a complete reaction. In fact the powders are pressed into pellets (of about 1 mm thickness) and sintered at 1400 °C in air for 60 hours with intermediate regrinding and repelling. Finally, the pellets were rapidly quenched to room temperature in air. Phase purity, homogeneity and cell dimensions were determined by X-ray diffraction at room tem-perature. Unit cell dimensions were obtained by least-squares calculations. Resistivity measurements as a function of temperature and magnetic applied field were carried out on dense ceramic pellets by the standard four-probe technique.

3 Results and discussion A vacancy in the A site implies a partial conversion of Mn3+ to Mn4+ leading to an increase in the Mn4+ content above 50%. According to the general formula, the Mn tetravalent and Mn trivalent contents are (0.5 + 2x) and (0.5 – 2x) in the lacunar perovskite oxides La0.5Ca0.5–xxMnO3, respectively. The calcium vacancy leads to a change in the average ionic radius ⟨rA⟩ of the A site. For electrostatic considerations, a vacancy must have an average radius ⟨rV⟩ ≠ 0. The Mn3+ and Mn4+ contents have been checked by chemical analysis. In Table 1 we list the chemical analysis results. The experimental results agree with the theoretical data. X-ray diffraction patterns at room temperature of La0.5Ca0.5–xxMnO3 show that all our samples are of single phase (Fig. 1). Our synthesized samples crystallize in the orthorhombic perovskite structure with Pnma space group. Calcium deficiencies do not modify the La0.5Ca0.5MnO3 structure. In Table 2, we list the crystallographic data for La0.5Ca0.5–xxMnO3 samples.

10 20 30 40 50

I(a.

u)

2θ(θ(θ(θ( degree)

x=0.00

x=0.05

x=0.15

x=0.20

x=0.25

x=0.10

200

121

220 04

0 042

321

400

161

402

440

600

28024

436

1 602

444

165

640

284

031

020

Fig. 1 X-ray powder diffraction patterns of La0.5Ca0.5–xxMnO3 samples.

1418 I. Walha et al.: Deficiency effects on the physical properties

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Table 2 Crystallographic data of lacunar La0.5Ca0.5–xxMnO3 samples.

x a (Å) b (Å) c (Å) V (Å3)

0.00 5.429(1) 7.693(4) 5.421(8) 226.45 0.05 5.430(2) 7.696(1) 5.422(1) 226.62 0.10 5.438(9) 7.707(5) 5.426(5) 227.48 0.15 5.450(0) 7.718(8) 5.434(3) 228.60 0.20 5.459(8) 7.739(7) 5.449(3) 230.27 0.25 5.470(6) 7.752(4) 5.456(9) 231.43

With increasing calcium deficiency content, the cell parameters a, b, c and the unit cell volume (V) increase. Such a result has been also observed in a previous work on Pr0.5Sr0.5–xxMnO3 samples [15], with increasing strontium defect amount. These samples crystallize in the orthorhombic structure with Imma space group. The increase of Mn4+ content due to the increase of the calcium-defect amount cannot explain the increase of the unit cell volume, and consequently other parameters as the evolution of the average ionic radius ⟨rA⟩ of the A site induced by the lacuna may explain such a behaviour. In order to study the magnetotransport properties of our synthesized samples, we have performed resistivity measurement versus temperature at different applied magnetic fields up to 8 T. In Fig. 2 we have plotted resistivity measurements of the stoichiometric sample La0.5Ca0.5MnO3 at different applied magnetic fields up to 8 T.

Using the sign of the temperature coefficient of resistivity d

dT

ρ

as a criterion d

dT

ρ

< 0 for a semi

conductor-like system and d

0 for a metallic-like systemdT

ρ >

, we found that the parent compound at

zero field shows a semiconducting behaviour in the whole temperature range of 20–300 K with a change in the slope at about 100 K. This behaviour can be explained by the charge ordering (CO) effect. Applied magnetic field leads to an important decrease of the resistivity values at low temperature and consequently a destruction of the CO effect observed at low temperature. The applied magnetic field induces also a semiconducting–metallic transition when the temperature decreases. With increasing applied magnetic field the electrical transition temperature Tρ increases and the resistivity peak becomes broader. In order to study the effect of calcium deficiency on the electrical properties of La0.5Ca0.5MnO3, we have performed electrical measurements at zero applied magnetic field of La0.5Ca0.5–xxMnO3 lacunar samples (x = 0.05 and 0.1) (Fig. 3). Calcium deficiency leads to an important decrease of the resistivity at low temperatures and induces a semiconducting–metallic transition when temperature decreases and consequently a destruction of the CO effect observed at very low temperature in the parent compound.

0 50 100 150 200 250 300100

101

102

103

104

105

106

107

0T1T2T4T8T

ρ(Ω

cm)

T(K)

Fig. 2 Resistivity evolution as a function of temperature at different applied magnetic fields for the stoichiometric sample La0.5Ca0.5MnO3.

phys. stat. sol. (a) 201, No. 7 (2004) / www.pss-a.com 1419

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

0 50 100 150 200 250 300100

101

102

103

104

105

106

107

H=0T

% (La0.5

Ca0.5

MnO3)

La0.5

Ca0.45

0.05

MnO3

La0.5

Ca0.40

0.10

MnO3

ρ(Ω

cm)

T(K) The electrical transition temperature increases with increasing calcium deficiency content. It is impor-tant to notify that 5% of calcium deficient induces the same effect produced by an applied magnetic field of 8 T in the parent compound. Figure 4 shows resistivity measurements versus temperature at applied magnetic fields of 0 and 8 T. The effect of the magnetic field of 8 T is to lower the resistivity values; the resistivity peak becomes broader and shifts to higher temperatures.

Defining the MR at a given temperature as MR = ρ

ρ

∆ =

(0) ( )

(0)

Hρ ρ

ρ

−

, where (0)ρ and ( )Hρ

are the resistivity values at zero and at applied magnetic field H, respectively, Fig. 5 shows the magne-toresistance evolution versus temperature at several applied magnetic fields for the sample La0.5Ca0.5MnO3. The MR remains constant at about 100% at low temperatures and than decreases with increasing tem-perature. The MR decrease depends strongly on the applied magnetic field. This decrease occurs rapidly for low field values and becomes more slowly with increasing magnetic field. Figure 6 shows the magnetoresistance evolution versus temperature at an applied magnetic field of 8 T for lacunar samples La0.5Ca0.5–xxMnO3 (x = 0.05 and 0.1). Contrary to the parent compound, the MR for calcium-deficient samples exhibits a maximum with decreasing temperature. The temperature corresponding to the maximum of the MR increases while the maximum of the MR decreases when the calcium deficiency content increases.

0 50 100 150 200 250 300

10

100

ρ(Ω

cm)

T(K)

0T8T(b)

0 50 100 150 200 250 300

10

100

1000

ρ(Ω

cm)

T(K)

0T8T

(a)

Fig. 4 Resistivity evolution as a function of temperature at H = 0 T and 8 T for both lacunar samples La0.5Ca0.450.05MnO3 (a) and La0.5Ca0.40.1MnO3 (b).

Fig. 3 Resistivity evolution as a function of temperature for La0.5Ca0.5–xxMnO3 samples (0 ≤ x ≤ 0.1) at H = 0 T.

1420 I. Walha et al.: Deficiency effects on the physical properties

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

50 10 0 1 50 200 250 300

0

20

40

60

80

100

MR

%

T (K)

1 T2 T4 T8 T

0 50 100 150 200 250 300

0

10

20

30

40

50

60

70

80

90

100

110

MR

%

T(K)

La0.5

Ca0.5

MnO3

La0.5

Ca0.45

0.05

MnO3

La0.5

Ca0 .4

0.1

MnO3

At 300 K, calcium deficient leads to an increase of the MR value. The MR is found to be 35% for x = 0.1, 20% for x = 0.05 and about 10% for x = 0. A previous study on lacunar Pr0.5Sr0.5–xxMnO3 samples [16] shows that the effect of strontium va-cancy on the electrical properties is in agreement with that observed in our lacunar La0.5Ca0.5–xx MnO3 samples. 4 Conclusion In this work we have investigated the calcium deficiency effect on the structural and magnetotransport properties of La0.5Ca0.5–xxMnO3 samples. Our studies show that the structural and electrical properties depend strongly on the vacancy content. Synthesized samples crystallize in the orthorhombic structure with Pnma space group. Calcium-deficient samples exhibit a semiconducting–metallic transition when the temperature decreases. The magneto-transport investigations of our samples show that 5% of calcium deficient induces the same effect produced by an applied magnetic field of 8 T on the parent compound La0.5Ca0.5MnO3.

Acknowledgements This work has been supported by the Tunisian Secretary of State for Scientific Research and Technology.

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Fig. 5 Magnetoresistance evolution as a function of temperature for the sample La0.5Ca0.5MnO3 at several mag-netic applied fields.

Fig. 6 Temperature dependence of the magnetore-sistance at H = 8 T for lacunar La0.5Ca0.5–xxMnO3

samples (0 ≤ x ≤ 0.1).