Mechanosynthesis of lanthanum manganite

Materials Science and Engineering A 454–455 (2007) 69–74

Mechanosynthesis of lanthanum manganite A.M. Bolar´ın a,∗ , F. S´anchez a , A. Ponce b , E.E. Mart´ınez c a

Universidad Aut´onoma del Estado de Hidalgo-CIMyM, Carr. Pachuca-Tulancingo Km. 4.5, Pachuca, Hidalgo 42184, Mexico Departamento de Ingenier´ıa Mec´anica, Tecnol´ogico de Monterrey, C/Del puente, 222, Tlalpan 14380, M´exico, D.F., Mexico c Instituto de Metalurgia de la Universidad Aut´ onoma de San Luis Potos´ı, Av. Sierra Leona 550, Lomas 2a . Secci´on. San Luis Potos´ı, S.L.P. 78210, Mexico b

Received 5 July 2006; received in revised form 27 October 2006; accepted 13 December 2006

Abstract Lanthanum manganite LaMnO3 was prepared using a high energy ball milling SPEX 8000 D mixer/mill. Powders of Mn2 O3 and La2 O3 were mixed in stoichiometric proportion for obtaining this manganite. Mechanosynthesis processing (MCP) was carried out at room temperature in air, in hardened-steel vials with balls using a weight ratio of balls to powder of 10:1. X-ray diffraction (XRD) was used to evaluate phase transformation as function of milling time. Morphology and grain size of the LaMnO3 were characterized by means of scanning electron microscopy (SEM). Particle size was measured by means of a zeta size analyzer. Selected area electron diffraction (SAED) patterns in a transmission electron microscopy (TEM) were indexed to identify this phase in the obtained sample. Found compounds exhibited orthorhombic structure, and it was confirmed that lanthanum manganite was produced after 4.5 h of milling. Evolution of the phase transformation showed that increasing the milling time produced an exponential decreasing in particle size, up to 480 nm after 6 h of milling, while milling times longer than 20 h produce an important reduction in the powder size, although higher iron contamination from the vials and balls is also greater. © 2006 Elsevier B.V. All rights reserved. Keywords: Mechanosynthesis; Lanthanum manganite; Crystalline structure

1. Introduction Nanomaterials of manganites such as REMnO3 (RE = rare earth) have been recently the subject of interest of researchers due to its application possibilities. In first instance, the nanoscale crystal often exhibits magnetic properties that are different from those of their bulk countparts [1]. On the other hand, this type of materials have great potential for use in a wide range of applications, such as sensors, permanent magnets, catalysts and pigments [1–4]. The attention has been focused recently on their magnetic and electrical properties, because they display a variety of interesting properties, including the Jahn-Teller (JT) distortion, charge, orbital ordering, and the colossal magnetoresistance (CMR) effect [5–7] which are of great interest next to their possible use as electrode materials for solid-oxide fuel cell (SOFC) [7–9]. Synthesis of manganites with other rare earth elements instead lanthanum, REMnO3 have been reported; several synthe∗ Corresponding author. Tel.: +52 7 71 72000x6713; fax: +52 7 71 72000x6729. E-mail addresses: [email protected], [email protected] (A.M. Bolar´ın).

0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.062

sis methods such as solid–solid state reaction [10], sol–gel [11], co-precipitation [12], thin film deposition [13] and single-crystal growth [14] have been developed. An alternative method for mass production of these materials could be MCP, which is a process mechanically activated [15,16] that has been applied successfully in obtaining different kind of oxides [17–19]. Effectiveness of high energy ball milling technique to promote mechanosynthesis of nanostructured manganites by mechanical activation of chlorides and oxides compounds for synthesize Cax La1−x MnO3 has been demonstrated [3]. Zhang and Saito [4] synthesized LaMnO3 perovskites at room temperature by milling a mix of Mn2 O3 and La2 O3 powders using a planetary ball mill; synthesis was is completed after about 180 min, however, results related to crystalline structure; morphology and particle size were not reported in that research. These characteristics are very relevant in order to understand and establish relationships with their electric and magnetic properties. The purpose of this work was the obtaining of lanthanum manganite evaluating its crystalline structure, morphology and particle size. Contributing to define new parameters of the synthesis in order to obtain ternary doped manganites, such as

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M1−x REx MnO3 (where M is an alkaline), which are important materials in the electronic and magnetic development. In spite of the positive results obtained by Zhang and Saito [4], the goal of this work was to carry out the synthesis of this manganite (LaMnO3 ) without a posterior annealing treatment. In this way, we employed metal oxides as starting materials, manganese(III) oxide and lanthanum oxide powder mixture, in order to obtain the manganites by simple reaction in solid state. 2. Experimental part 2.1. Ball milling Mn2 O3 (Sigma–Aldrich, >99.9%) and La2 O3 (Sigma– Aldrich, >99.9%) powders were used as starting materials. They were mixed stoichiometricly according to the reaction: 0.5 Mn2 O3 + 0.5 La2 O3 → LaMnO3

(1)

The process was started with 5 g, this mixture placed in a vial together with hardened-steel balls of 12.7 mm diameter. The process was carry out at room temperature in air, using a SPEX 8000 D mixer/mill equipped with cylindrical hardenedsteel vials (60 cm3 ). The powder to ball weight ratio was 1:10. In order to prevent the excessive overheating of the vials, the experiments were carried out in cycles of 90 min of milling followed by 30 min of rest. The total time of milling was up to 20 h. 2.2. Phase identification X-ray diffraction (XRD) of powders was used to follow the phase transformation during the milling process. Samples of different milling times were characterized at room temperature using a Philips X’Pert diffractometer. Diffraction parameters were 2θ ranging of 20–140◦ with a stepsize of 0.05◦ (2θ). Cu ˚ radiation was used in all experiments. In order K␣ (λ = 1.5418 A) to compare the crystalline phase in the samples. Selected area electron diffraction (SAED) patterns acquired using a transmission electron microscope (TEM) Jeol 1000EX operating at 120 kV. 2.3. Particle size and morphology The morphology of the particles and distribution of chemical elements in the samples were observed by scanning electron microscope (SEM), using a Philips XL30, operating at 20 keV. This equipment has an energy dispersive spectrometer (EDS) EDAX-New XL30, for conducting chemical analysis. The particle size was measured with a zeta size analyzer Malvern Instruments 3000 HSA . Prior to the measurements, powders samples were dispersed in ethanol. 2.4. Contamination level The contamination level (weight percent of iron in the sample from milling media) was measured by inductively coupled

Fig. 1. X-ray powder diffraction patterns of oxides mixture milled at various times, 0–20 h. Phases: () La2 O3 , (䊉) Mn2 O3 and () LaMnO3 .

plasma spectrometer (ICP) using a Perkin-Elmer Optima 300 XL and qualitatively using EDS analysis. 3. Results and discussion 3.1. Phase identification 3.1.1. Evolution of the phase transformation Powder XRD patterns are shown in Fig. 1 for a sequence of different milling times, up to 20 h. This figure shows the starting mixture (tm = 0) made of La2 O3 [JCPDS: 74-1144] and Mn2 O3 [JCPDS: 41-1442] only. Mechanosynthesis performed for an hour exhibits peaks of the lanthanum manganite (LaMnO3 [JCPDS: 35-1353]) clearly as it can be noticed from peak located at 2θ, 32◦ . The amount of this compound increased with milling time. Simultaneously, peaks belonging to original oxides disappeared. The pattern of 4.5 h of milled sample shows no peaks of starting materials. The XRD spectrum shows that after 4.5 h of milling time, the reaction given by Eq. (1) is completed. XRD pattern of mixture milled during 20 h does not show differences in peak intensity, after this time, starting oxide patterns vanished. An important observation during the milling process is a remarkable peak profile broadening due probably to crystallite size reduction and lattice strain promoted by the milling process. 3.1.2. Structural characterization It is know that lanthanum manganite structure is a perovskite type when it is synthesized by chemical solution methods such as sol–gel and polymeric gel. However, the crystal structure of this compound may change as a function of a change in the tolerance factor. MCP of mixture of solids, is a nonequilibrium solid state processing method which leads to a variety of applications and allows development of novel crystal structures and microstructures with unique physical and chemical properties. Therefore, it was important to identify the materials obtained by this route. It can be assumed that XRD patterns show broad peaks and background noise as a consequence

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Fig. 2. Electron diffraction patterns of LaMnO3 sample milled during 7 h showing the d spacing.

of fine particle size, making difficult to discern other possible phases. TEM was used to determine the microstructure in the sample milled during 7 h. Selected area electron diffraction (SAED) patterns show crystals in several random orientations, the electron diffraction pattern is shown in Fig. 2. Each ring corresponds to a set of planes with spacing dh k l -spacing. The corresponding planes are shown in Fig. 2. In Table 1 a comparison among experimental and theoretical d-spacing results with the dh k l -spacing for the cubic and orthorhombic LaMnO3 crystal structures is summarized. The simulated pattern and the indexing of the experimental electron diffraction patterns have been simulated using the Java electron microscopy simulation (JEMS) software package as shown in Fig. 3. From the indexing of the SAED patterns, crystalline phase of LaMnO3 corresponds to orthorhombic lattice. Indeed experimental measurements of d-spacing do not correspond to any cubic diffraction in the simulated electron pattern. Other compounds such as La2 MnO4 and La2 MnO4.15 require low oxygen pressures and temperature above 1650 K [20], therefore they are not present in the obtained sample. Although most of the reported data for the lanthanum manganite show it orthorhombic lattice [21–24] there are also cubic [25] and tetragonal reports [26,27]. There is some controversy about the actual lattice of undoped LaMnO3 [28], although diagrams presented in some work [29] suggests cubic structure as possible, and it is even described purely as a perovskite [28] or orthorhombic perovskite [30].

Fig. 3. Simulated electron diffraction patterns and their corresponding polycrystalline rings of the reflection families for LaMnO3 orthorhombic crystalline structures.

Table 1 d-Spacing measurements of the rings in the diffraction patterns Cubic lattice Corresponding planes family

Orthorhombic lattice d-Spacing (nm)

Corresponding planes family

Theoretical

Experimental

{1 0 0}, {0 1 0} {1 1 0}

0.3960 0.2800

{2 0 0}, {0 2 0}

0.1980

0.39100 0.27836 0.22609 0.19598 0.16006

{0 2 0} {0 0 2} {0 2 2} {0 4 0} {0 4 2}

d-Spacing (nm) Theoretical

Experimental

0.3871 0.2801 0.2269 0.1935 0.1592

0.39100 0.27836 0.22609 0.19598 0.16006

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Fig. 4. Micrographs of LaMnO3 powders milled during different times: (a) 4.5 h, (b) 7 h, (c) 9 h and (d) 20 h.

It is supposed in MCP that mechanical energy will activate thermally the atoms in the material in such a way that they take new positions as thermodynamical feasibility allows. High pressure could be locally achieved and even change the electronic properties of the product [30]. Just as a side comment, this feature could be usefully even for modifying the properties of a material that was already synthesized. The mechanism were the raw materials are kept vibrating until the atoms find their location has not actually been seen, however there are several successfully demonstrations [17–19]. Similarities of the lattices among the compounds as well as coherent interfaces are a factor that most be considered. Lattice of the obtained LaMnO3 was affected by iron presence, which could substitute any of the cations due to its double valence. Therefore, a conclusion regarding the kind of structure that is being produced assigned to MCP only is not easy to address, especially because low iron contamination was achieved at short milling times, which were not enough to produce LaMnO3 . However, the synthesis was carried out completely meaning that this procedure is suitable for conducting the synthesis. Another aspect is that kinetic energy that was being applied to the powder became less effective as the tests between 6 and 20 h confirm with changes on the size of the agglomerates only.

consisted of at least two types of particles: small, roughly spherical particles about 200 nm in diameter and other large, also rounded about 2 ␮m in diameter. The powder becomes rounder as milling time increases, close to roughly spherical particles down to 1000 nm in diameter. Particle size distribution of sample milled during 20 h seems narrow with agglomeration particle of about 400 nm as shown in Fig. 5. Particle size of powders was determined quantitatively using a potential zeta analyzer. The results are shown in Fig. 6, confirming what it is presented in Figs. 4 and 5, as the milling time

3.2. Particle size and morphology Fig. 4 shows micrographs of powder samples milled at different times. The sample powder from 4.5 h of milling time

Fig. 5. Micrographs of agglomerates of LaMnO3 powders milled during 20 h.

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Fig. 6. Effect of milling time on particle size. Fig. 7. Effect of milling time on Fe contamination level.

increases, particle size decreases exponentially up to 480 nm for 6 h of milling time. Agglomerates of fine grain with irregular shapes can be observed after 9 h of milling time. The agglomeration of powders increases with the milling time due to the refinement of the particle size. 3.3. Chemical analysis The MCP might generate iron impurities from the vials and balls of the system, this is inevitable during milling process [15], and it affects magnetic and electric properties of the materials. Therefore, iron contamination was measured in this work.

In Fig. 7 is presented the relation between the amount of iron (wt.%) against milling time. It can be observed that iron contamination increases continuously from 0.5 wt.% for 1 h to 1.2 wt.% for 7 h of milling time. A prolonged milling time (>20 h) produces a higher contamination, around 3.4 wt.% of the powder mixture, due to the friction between oxides and metallic parts of the miller, during the milling process. In order to confirm whether the iron impurities in milled powders existed as an atomic form substituted for manganese in the particles or as isolated metal form, EDS elemental analysis were carried out. The results are presented in Fig. 8, which shows the distribution of iron atoms in a cross-sectioned particle milled during 20 h. It can be seen that iron was well distributed in a par-

Fig. 8. SEM micrograph of cross-sectioned powder milled for 20 h and EDS dot mapping of La, Mn and Fe atoms.

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ticle. Hence, it could be speculated that the iron has substituted Mn sites in LaMnO3 . 4. Conclusions Lanthanum manganites, LaMnO3 , were synthesized using a mechanosynthesis process, this process involving high energy ball milling of La2 O3 and Mn2 O3 , using a weight to ball–powder mass ratio of 1:10, after 4.5 h of milling. The lattice of material powder obtained was orthorhombic. A prolonged milling time (>20 h) induces a high contamination of the powder mixture, due to metallic diffusion from vial and balls. The iron atoms are substituting manganese sites in orthorhombic perovskite of LaMnO3 which certainly affects its electric and magnetic properties. The lanthanum manganite powder consists in aggregates, consisting of nanometric size particles with nanocrystalline structure. If the aggregates formation is prevented, this method would be a favorable way to prepare nanosize lanthanum manganites. Acknowledgements The authors would like the financial support by the Promep program, of Educational Secretary of Mexico and also would like to thank C. Flores for his technical support in the TEM analysis. References [1] M. Muroi, R. Street, P.G. McCormick, J. Appl. Phys. 87 (1999) 3424. [2] J.H. Voorhoeve, D.W. Johnson Jr., J.P. Remeika, P.K. Gallagher, Science 195 (1977) 827. [3] M. Muroi, R. Street, P.G. McCormick, J. Solid State Chem. 152 (2000) 503–510. [4] Q. Zhang, F. Saito, J. Alloys Compd. 297 (2000) 99–103. [5] A. Urushibara, Y. Morimoto, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103.

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