Structure and Magnetic Properties of Lanthanum Strontium Ferrites Nanopowders Synthesized by Thermal Decomposition of Mixed Metal Acetyl Acetonates

Structure and Magnetic Properties of Lanthanum Strontium Ferrites Nanopowders Synthesized by Thermal Decomposition of Mixed Metal Acetyl Acetonates

E. A. Nforna

Lanochee Department of Chemistry,

University of Dschang, Cameroon,

P. A. Joy

Mmu, Pmcd,

CSIR-National Chemical Laboratory, Pune 411008,

India

1. N. Ghogomu

   Lanochee Department of Chemistry,

   University of Dschang, Cameroon,

   1. N. Lambi

      Lmc Department of Chemistry,

      ENS Yaoundé, University of Yaoundé I, Cameroon

      Abstract Single phased, polycrystalline mixed metal oxides, La1-xSrxFeO3- (x = 0.0, 0.2, 0.3, 0.5) have been prepared via the thermal decomposition of mixed metal acetylacetonate precursors. The precursors were synthesized at 55 °C by coprecipitation and decomposed at 450 oC to give multi-phased lanthanum strontium ferrite nanoparticles which were further calcined at 1000 oC to obtain the single-phased perovskite ferrites. The decomposition temperature of the precursors was determined by thermogravimetric analysis (TGA). The perovskite ferrites were then characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). SEM and XRD analyses showed that the particles of the ferrites were spherical and nano-sized with structures ranging from orthorhombic to rhombohedral as the Sr content increased. For x = 0.0, 0.2, and 0.3 (orthorhombic), the average FeO bond length and the unit cell volume decreased with increasing Sr content.

      Magnetic properties were investigated at room temperature (27 °C) using a vibrating sample magnetometer and the hysteresis loops showed that all the compositions exhibit anti- ferromagnetic ordering with a weak ferromagnetic interaction related to their structures. Increasing coercive fields were also observed with increasing Sr content.

      Keywords La1-xSrxFeO3-; acetylacetonate; co-precipitation; X-ray diffraction; ferromagnetic; anti-ferromagnetic.

      1. INTRODUCTION

         A variety of materials of extreme importance to modern technology have ABO3 structures. The most important being the perovskite-type materials which present structural compositions with ferroelectric properties and with high dielectric constants. Their immense interest or wide

         applications are ascribed to their tunable shapes. Substitution in the A or B-site of the ABO3 perovskite structure modifies the properties of the perovskite material for specific areas of applications [1]. Lanthanum strontium ferrites, La1-xSrxFeO3, have recently been the focus of major research due to their applications in solid oxide fuel cells as cathode materials [2, 3, 4], in gas sensors [5], in catalysis [6] and in magnetic data storage, logic, sensors [7, 8, 9]. These applications have been attributed to the unique structural features of the rare earth ferrite perovskites [9, 10].

         Rare earth ferrites with general formula, ReFeO3 (where Re represents a rare earth element), have a tilted orthorhombic perovskite structure with unit cell dimensions a, b and c such that c is usually twice the length of either a or b. The cation sites are highly substitutable, thus modifying the structure and physical properties of the perovskite [10]. This may lead to new properties such as ferro-electric/magnetic, conductivity, piezoelectric, pyroelectric or magneto-optic properties for specific applications [1, 11, 12]. LaFeO3 is a perovskite material where the unit cell consists of six O2- ions around Fe3+ ions in an octahedral coordination. The La3+ ions enter the interstitial sites of the FeO6 octahedron [13, 14]. Deviating from the ideal cubic perovskite, LaFeO3 shows an orthorhombic setting with magnetic spins pointing along the a-axis and an anti-ferromagnetic component parallel to the a- axis. There is a weak ferromagnetic component caused by a slight canting (0.5o) of the antiparallel spins towards the c-axis [13, 14]. The magnetic properties of LaFeO3 is due to Fe3+ ions and there is no magnetic interaction between Fe3+ and La3+ since La3+ is non-magnetic. Substituting Sr2+ for La3+ in (La1-xSrxFeO3-) shows extensive modification of the structure and interesting magnetic, electronic and catalytic properties

         [15]. The solid solution La1-xSrxFeO3- shows an orthorhombic structure from x=0.0 to x=0.2; mixed orthorhombic – rhombohedral at x=0.3; rhombohedral from x=0.4 to 0.5 [13].

         Synthetic techniques strongly influence the structure and particle size of perovskites. Lanthanum strontium ferrite has been prepared by a variety of synthesis techniques such as solid state reaction method [16, 17], solgel [7], precipitation

         [18], combustion [11], polymeric method [14], etc. However, there are some disadvantages associated with these synthetic methods such as high temperatures [16, 17], long reaction time or slow diffusion [16, 19], formation of secondary or impurity phases [1] etc.

         In this study, a circumvention or mitigation of the problems enumerated above is proposed wherein nanosized lanthanum strontium ferrites [La1-xSrxFeO3-; x = 0, 0.2, 0.3, 0.5] have been prepared under atmospheric conditions by thermal decomposition or pyrolysis of the mixed metal acetylacetonates at a relatively low temperature of 450 oC. Secondary phases that appeared at the decomposition temperature were eliminated after calcination at 1000 oC for 6 hours. The advantages of this technique include: the use of metal acetylacetonates as single source precursors which are easy to prepare in solution at low temperatures, their stablility in air, their decomposition at relatively low temperatures to form the metal oxides and the fact that mixing of the metal ions in solution at the molecular level is homogenous thus ensuring the desired stoichiometry in the final product [20, 21]. The synthesized powders are then characterized by different spectroscopic methods.

      2. EXPERIMENTAL

         The following reagents of analytical grade were used: C5H8O2 (99.5%, BDH); La(NO3)3.6H2O (99% ), Sr(NO3)2 (99%), Fe(NO3)3.9H2O (99.5%), Co(NO3)2.6H2O (99%), and

         NaOH (98%) (Sigma-Aldrich),

         The different perovskite nanopowders were prepared according to the formula La1-xSrxFeO3-: {x=0.0 (LFO), 0.2 (LSF82), 0.3 (LSF73), 0.5 (LSF55)}, and the non

         stiochiometric La0.7Sr0.3Fe1.05O3- (LSF7305 – 5 % excess Fe) and La0.7Sr0.3Fe1.1O3- (LSF7310 – 10 % excess Fe) from the corresponding mixed metal acetylacetonate precursors by thermal decomposition [20, 22, 23].

         The mixed metal acetylacetonate precursors were prepared by co-precipitation [24, 25]. La0.7Sr0.3Fe(C5H7O2)6 (x=0.3, LSF73acac) was prepared as follows: Sodium acetylacetonate, a precipitating agent, was first prepared by adding dropwise stoichiometric amount of 3 ml (2.916 g) acetylacetone (acacH) to a solution of NaOH (2.4 M) maintained at 50 oC while stirring on a magnetic stirrer for 20 minutes.

         1.47 g of La(NO3)3.6H2O , 0.309 g of Sr(NO3)2 and 1.959

         g of Fe(NO3)3.9H2O were dissolved in 10 ml double distilled water and slowly added to the hot solution of sodium acetylacetonate while stirring for 30 minutes. Red precipitate of La0.7Sr0.3Fe(C5H7O2)6 was formed. The precipitate was allowed to cool, filtered by suction, washed three to four times with distilled water to remove by-products and unreacted reactants and dried for 5 hours at 80oC in a dessicator. The yield was 92.7%.

         The synthesis of LaFe(C5H7O2)6 (LFOacac), La1- xSrxFe(C5H7O2)6 {x=0.2 (LSF82acac), 0.5 (LSF55acac)} and the non stoichiometric La0.7Sr0.3Fe1.05(C5H7O2)6 (LSF7305acac) and La0.7Sr0.3Fe1.1(C5H7O2)6 (LSF7310acac)

         was each carried out by the same procedue.

         The mixed metal oxides were formed by thermal decomposition of the mixed metal acetylacetonate complexes by putting into a preheated muffle furnace at 450 oC in air for three hours. The powders formed were ground and calcined at 1000 oC for 6 hours at a heating and cooling rate of 5 oC/min.

         Thermogravimetric studies of one of the acetylacetonate precursors LSF73acac was carried out using a TG analyser (Model: SDT Q600 V20.9 Build 20 DSC-TGA Standard) in air, at a heating rate of 10 °C/min in an alumina crucible. The temperature range was between room temperature and 900 °C. For each composition calcined at 1000 oC, phase purity and crystal structure were determined by powder X-ray diffraction ( PANalytical XPert PRO 1712) in the 2 range of 10-80o, using Cu-K radiation. The structural refinements were carried out with the Rietveld method of profile analysis using GSAS. Crystallite size was determined based on Scherers formula from the line broadening of the peak at around 32°. The morphology of the powders calcined at 1000 oC was examined using a JEOL JSM-840A scanning electron microscope fitted with an energy dispersive X-ray spectrometer (EDS). The magnetization measurements were performed using a vibrating sample magnetometer, VSM (model EG&G PAR 4500) at room temperature (300 K) with applied field from 0 to 15 KOe.

      3. RESULTS AND DISCUSSION

         1. TGA and DTA

            It is vital to study the thermal decomposition chemistry of the acetylacetonate precursors as it indicates the total weight loss on heating, the formation of intermediate phases, and the temperature at which all the organic fraction is removed and final product formed.

            The TGA and DTA curves of one of the precursors, LSF73acac is presented in fig. 1. The DTA curve indicates four important temperature points namely: 74.02, 172.9, 454.5 and 617.9 °C respectively. The first weight loss of 3.8 % occurred in the temperature range 70.4 -140.4 oC and is probably due to the partial vaporization of the compound. Acetylacetonates sublime at low temperatures [24]. The second weight loss of 65.3 % occurred in the temperature range 148.3 230.9 oC and is attributed to the decomposition of the acetyacetonate group as six molecules of acetone (theoretical 57.6%) and CO2 (theoretical 5.7%). The third weight loss of 7.9 % occurred in the temperature range 352.1 – 500 oC and is attributed to the decomposition of secondary phases like La2(CO3)2 to release CO2 (5.7%). %). Lanthanum oxycarbonate was obtained at 550 oC as intermediate by [26] during the thermal decomposition of lanthanum metalorganics.The fourth weight loss of 2.6 % occurred between 613 648.7 oC and is attributable to the removal of H2O (theoretical 2.3%) from a secondary phase such as La2O2(OH)2. XRD patterns were recorded for the resulting powders formed at different decomposition temperatures (between 400 – 1000 C) and indicated the presence of lanthanum oxy-hydroxy carbonates at temperatures less than 800 C.

            With about 3% of the compound lost by sublimation, the percentage residue was equal to 20.2% (theoretical 21.1%) and corresponds to La0.7Sr0.3FeO3. Fe2O3 was obtained as residue by [23] from the decomposition of iron(III) acetylacetonate.On their own part, [27] observed the evolution of acetone and CO2 from the decomposition of iron(III) acetylacetonate.

            Fig. 1. TGA and DTA curves for LSF73acac

         2. Synthesis of nanopowders

            Lanthanum ferrite (LaFeO3) and lanthanum strontium ferrite (La1-xSrxFeO3-) were synthesized from the acetylacetonate precursors by thermal decomposition. Various synthetic parameters such as decomposition temperature of the precursors, calcination temperature and time for the formation of single phase were investigated. Process optimization was

            carried out on the composition La0.7Sr0.3FeO3-. Decomposition temperature of each metal acetylacetonate is less than 450 oC [24, 27]. Based on the optimization conditions, the TGA/DTA curves and the fact that the decomposition temperature of each of the metal acetylacetonates is less than 450 oC [24, 27], thermal decomposition of the metal acetylacetonate precursors was carried out at 450 oC to ensure rapid decomposition of all the components in the mixture at the same time to form the desired perovskite phase. The small amounts of secondary phases formed at 450 oC merged to a single phase after calcination at 1000 oC for 6 hours. All characterizations were carried out on the La1-xSrxFeO3- polycrystalline powders obtained at 1000 oC.

         3. Structure and particle size (XRD)

            The XRD patterns of all the compositions of La1-xSrxFeO3-

            obtained at 1000 oC are presented in fig. 2.

            The grain sizes were calculated at 2 = 32 (most intense peak) according to the Scherrer formula:

            D = K/ cos (1)

            Where K is a constant, its value 0.9 for spherical particles, is the x-ray wavelength, is the Braggs angle and is the full width at half maxima at reflecting intensity peaks. All the compositions showed particle sizes less than 100 nm (table 1).

            Fig. 2. XRD pattern of LaFeO3 simulated, LFO, LSF82, LSF73, LSF7305, LSF7310 and LSF55

            TABLE I. PARTICLE SIZES OF LA1-XSRXFEO3- AND THE NON STIOCHIOMETRIC COMPOSITION

            Sample	LFO	LSF82	LSF73	LSF7305	LSF7310	LSF55
            	X=0 LaFeO3	X=0.2 (La0.8Sr0.2FeO3-)	X=0.3 (La0.7Sr0.3FeO3-)	X=0.3 (La0.7Sr0.3Fe1.05O3-)	X=0.3 (La0.7Sr0.3Fe1.1O3-	X=0.5 (La0.5Sr0.5FeO3-
            Mean grain size (nm)	61	48	56	45	51	40

            The XRD patterns of the solid solution, La1-xSrxFeO3- all show the formation of a single phase perovskite. From the diffraction pattern, the unsubstituted lanthanum ferrite, LFO (x=0.0) shows an orthorhombic structure, with space group Pbnm (62) according to JCPDS file No 74-2203, in agreement with results previously reported [1, 11]. LSF82 (x=0.2), the stoichiometric composition LSF73 (x=0.3), and the non stoichiometric compositions LSF7305 and LSF7310 (x=0.3) show single phase orthorhombic structures as indexed in JCPDS No 35-1480 [1, 11, 18]. The composition La0.5Sr0.5FeO3- (x=0.5, LSF55) shows a rhombohedral structure as indexed in JCPDS No 82-1962, with space group R-3c (167). Increasing Sr2+ substitution for La3+ changes the structure from orthorhombic to rhombohedral. Lattice parameters and some selected bond lengths and angles from Rietveld refinement of the orthorhombic perovskite phases (La1-xSrxFeO3- x=0.0, 0.2, 0.3), with space group Pbnm, is summarized in table 2. The observed and calculated XRD patterns of the typical refined XRD patterns of LaFeO3 is given in fig. 3. There is good agreement between the observed and calculated patterns. From table 2, increasing Sr2+ substitution results in decrease of the unit cell volume from

            24230 – 241.511 Ã… for the orthorhombic structures. There is abrupt increase in unit cell volume for the rhombohedral structure, x=0.5 according to the literature (JCPDS No 82- 1962).

            Although the ionic radius of Sr2+ (118 pm) is larger than that of La3+ (116 pm) [28] the replacement of La3+ by Sr2+ results in a contraction of unit cell volume which is explained hereafter. Sr2+ substitution for La3+ results in the formation of Fe4+ ions to maintain electrical neutrality. The Fe4+ ions are smaller than Fe3+. The smaller Fe4+ ions strengthen the FeO bond in the FeO6 octahedra according to Paulings rule [29] leading to increased covalency and shorter bond length. These effects are stronger than the effect of the substitution of the large Sr2+ ions. Hence, there is contraction of unit cell volume for low Sr substitutions (x=0.1- 0.3), similar to other results reported [4, 7, 18, 30]. As the Sr content increases formation of oxygen vacancies dominates the formation of Fe4+. The unit cell volume increases due to the substitution of the larger Sr2+ ions as observed for x=0.5 [30]. Even increasing the amount of Fe by up to 10% in excess still forms monophase perovskite.

            Fig. 3. Observed (cross) and calculated (continuous line) intensities in the XRD pattern of LFO. The differences between observed and calculated intensities are plotted at the botto

            TABLE II. RIETVELD REFINED STRUCTURAL PARAMETERS FOR LA1-XSRXFEO3- X=0.0, 0.2, 0.3 LITERATURE VALUES (JCPDS) IN PARENTHESES

            Compounds Literature – JCPDS No	LaFeO3 [74-2203]	La0.8Sr0.2FeO3- [35-1480]	La0.7Sr0.3FeO3-	La0.5Sr0.5FeO3- [82-1962]
            Crystal system Space group		Orthorhombic Pbnm (62)		Rhombohedral
            Lattice				parameters
            a (Ã…)	55522(7)	5.5506(10)	5.55157(19)
            	[5.553]	[5.532]		[5.511]
            b (Ã…)	55597(4)	5.5516(7)	5.54860(19)
            	[5.563]	[5.553]		[5.511]
            c (Ã…)	78494(6)	7.8444(14)	7.84037(26)
            	[7.867]	[7.835]		[13.416]
            Cell volume (Ã…3)	24230(4)	241.73(7)	241.511(14)
            	[243.02]	[240.68]		[352.87]
            Bond lengths (Ã…)
            FeO1	1.99319(14)	1.97001(34)	1.94957(5)
            FeO2	1.95106(13)	1.96772(22)	2.02487(6)
            FeO2	2.08045(18)	2.03273(22)	2.02553(5)
            Bond angles (°)
            FeO1Fe	159.811(3)	169.097(3)	150.938(1)
            FeO2Fe	154.042(1)	157.741(4)	161.694(0)

            (b)

         4. Microstructure and morphology (SEM)

            The SEM micrographs of LFO, LSF82, LSF73, LSF7305, LSF7310 and LSF55 calcined at 1000 oC are shown in fig. 4. The micrographs show spherical particles with grain sizes less than 100 nm in agreement with XRD results. The particles have a great tendency of agglomeration in agreement with results previously reported [11, 31]. These SEM images also reveal that the powders show homogeneity in form and compositions.

            (a)

            (c)

            (d)

            Fig. 4. SEM micrographs of (a) LFO, (b) LSF82, (c) LSF73, (d) LSF7305, (e)LSF7310 at 2 µm and (f) LSF55 at 1 µm

            Elemental analysis (EDS)

            Table 3 summarizes the elemental weight percentages in each composition from EDS. The experimental metal ratios are better compared with theoretical values by calculating (La+Sr):Fe ratios rather than the single metal ratios since EDS is a surface technique which focuses on small regions of the sample. The experimental metal ratios are presented thus: LFO La:Fe is 1:1; LSF82 (La,Sr):Fe is 0.9:1; LSF73 (La,Sr):Fe is 1.1:1; LSF7305 (La,Sr):Fe is 1:1.05; LSF7310

            (La,Sr):Fe

            (e)

            (f)

            is 1.1:1.1; LSF55 (La,Sr):Fe is 1:1. The metal-metal ratios, within acceptable error limits, are therefore close to the expected stoichiometric values (theoretical La+Sr:Fe 1:1). No other elements were detected.

            TABLE III. ELEMENTAL ANALYSIS OF LFO, LSF82, LSF73, LSF7305, LSF7310 AND LSF55 FROM EDS

            Compound	LFO	LSF82	LSF73	LSF7305	LSF7310	LSF55
            Element	Weight % (theoretical)	Weight % (theoretical)	Weight % (theoretical)	Weight % (theoretical)	Weight % (theoretical)	Weight % (theoretical)
            La	52.29 (57.2)	49.20 (47.8)	49.45 (42.7)	49.02 (42.3)	45.97 (41.8)	38.67 (32)
            Sr		4.46 (7.5)	9.69 (11.5)	8.07 (11.4)	10.68 (11.3)	13.07 (20.2)
            Fe	21.79 (23)	22.95 (24)	21.78 (24.5)	24.88 (25.5)	22.86 (26.4)	23.41 (25.7)
            O	25.92 (19.8)	23.39 (20.6)	19.07 (21.1)	18.03 (20.9)	20.49 (20.6)	24.84 (22.1)
            Total	100	100	100	100	100	100

         5. Magnetic measurements

         Magnetic measurements were performed on the La1-

         xSrxFeO3- nanopowders obtained after calcination at 1000 oC

         with a maximum applied magnetic field of 15 kOe. Fig. 5 presents the hysteresis loops measured at 27 oC and table 4 shows the coercive force and magnetization at maximum applied field. We observe spontaneous magnetization and the appearance of hysteresis loops at room temperature (fig.5). The maximum field applied, 15 kOe does not saturate the magnetization. We also note a closed loop at the maximum applied field. The shape of the hysteresis loops indicates weak ferromagnetism and the non-saturation of the magnetization is characteristic of anti ferromagnetic ordering of the spins in the nanoparticles. Other researchers have reported similar results [7, 14, 32]. The magnetization increases with Sr content. Majunath B. And Manivannan, 2010 [33] observed increased magnetization with increased K substitution for La in lanthanum ferrite. Increasing Sr substitution (up to x=0.3) increases the coercive force while there is a little decrease when x=0.5. These observations can be understood from structural variation. LaFeO3 is a G-type spin ordered anti ferromagnetic orthoferrite [34]. The magnetic moments of two neighbouring Fe3+ ions are coupled through the O2- ions between the Fe3+OFe3+ known as super-exchange interactions in anti ferromagnetic spin order along the a-axis [7, 14, 35]. The spin canting resulting from the tilted FeO6 octahedra gives rise to a weak ferromagnetism along the c- axis. With increasing Sr substitution, the FeO bond reduces

         and the introduction of large Sr2+ cations decreases the tilts of the octahedra which strengthen the antiferromagnetic super- exchange ordering. Formation of Fe4+ ions resulting from the substitution of Sr2+ for La3+ leads to Fe4+OFe3+ interaction called double exchange interaction. This interaction strengthens the ferromagnetic component [7] and the presence of oxygen vacancies can also disturb the super-exchange interactions, increasing the ferromagnetism. From structural considerations, for low Sr substitutions, the double exchange interaction is sigificant and when x=0.5, oxygen vacancies are more important. It was similarly observed as reported by

         [7] that for nanoparticles, magnetic moments due to uncompensated surface spins contribute to the increase of magnetization.

         TABLE IV. COERCIVE FORCE OF LA1-XSRXFEO3- AND THE NON STOICHIOMETRIC COMPOSITIONS.

         Compound	LFO	LSF8 2	LSF7 3	LSF730 5	LSF731 0	LSF5 5
         Coercive force (Oe)	356. 4	621.6	1255	1209.7	998.5	920.1
         Magnetizatio n at maximum applied field (emu/g)	0.16	1.06	1.5	1.53	2.4	5.66

         A

         B

         Fig. 5. Hysteresis loops of (A) LFO, LSF82, LSF73, LSF7305, LSF7310 and LSF55 and LFO in the inset at room temperature (B) enlarged loops to show variation of coercive fields.

      4. CONCLUSION

The solid solution with formula La1-xSrxFeO3- (x=0, 0.2, 0.3, 0.5) were synthesized by metal organic decomposition of the mixed metal acetylacetonate complexes. XRD patterns showed monophasic orthorhombic to rhombohedral structures with increasing Sr content and contraction of the lattice for the orthorhombic compositions. There is spontaneous magnetization of all compositions when an external magnetic field is applied. All samples exhibit a hysteresis loop characterized by coexisting anti ferromagnetic and weak ferromagnetic interactions. Sr substitution increases the coercive force. These compounds have potential magnetic applications in data storage, logic, magnetic storage etc.

ACKNOWLEDGEMENTS

This work was partly financed by the Government of India through FICCI under the C V Raman International Fellowship for African Researchers program to E. A. Nforna. Special thanks equally go to the University of Dschang (Cameroon) for research grants to meritorious postgraduate students which helped a great deal towards the completion of this research project.

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