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NANOCRYSTALLINE PEROVSKITE-LIKE OXIDES FORMATION IN Ln2O3 - Fe2O3 - H2O (Ln = La, Gd) SYSTEMS
E. A. Tugova, O. N. Karpov
Ioffe Physical Technical Institute of RAS, Saint Petersburg, Russia
PACS 61.46.-w; 82.80.Ej
Nanocrystalline LnFeO3 (Ln = La, Gd) ferrites have been prepared by the co-precipitation method followed by heat treatment in air. The formation mechanisms for LaFeO3 and GdFeO3 in Ln2O3 - Fe2O3 - H2O (Ln = La, Gd) systems under the mentioned conditions are investigated. The phase interaction scheme, reflecting ways which lead to the target, synthesis product yield, as well as the common tendency of LaFeO3 and GdFeO3 formation mechanisms, are constructed. The mean sizes of coherent scattering regions of LaFeO3 and GdFeO3 were determined to be 30±3 and 40±4 nm, respectively.
Keywords: LnFeO3, ferrites, nanostructures, precipitation technique, phase formation, X-Ray diffraction. Received: 5 November 2014 Revised: 27 November 2014
1. Introduction
Perovskite-like oxides, produced by different synthetic methods and having a variety of structural peculiarities, possess a complex array of electric, magnetic, catalytic, thermal, and other properties being of interest, in terms of practical applications [1-20]. Conventionally, the most common technique for obtaining complex oxides, including perovskite-like phases, is the method of solid-state synthesis [21-23]. Usually, for solid-state synthesis, simple oxides, carbonates, nitrates, oxalates, citrates or other salts of proper metals can be used as reagents. To ensure stoichiometry of the compounds synthesized, definite composition of gaseous medium can be prescribed, i.e. air, nitrogen, oxygen, hydrogen, etc. Despite a great number of papers devoted to the solid-state synthesis of perovskite-like oxides and to research into the peculiarities of their structure and properties [12, 17-21], limited data on the formation processes for perovskite-like oxides are available. There is also insufficient data on state equilibria for Ln2O3 - Fe2O3 (Ln = La, Gd) systems, which include perovskite-like oxides. Additionally, since the composition of the substances determines their basic properties, the lack of the mentioned data may affect the preparation of materials based on perovskite-like oxides with function-oriented characteristics.
Recently, the preparation of perovskite-like oxides using "soft chemistry" techniques has become of greater interest [24-25]. This is determined by the practical application of nanomaterials with unique structure and properties prepared using "soft chemistry" techniques [26]. However, mechanisms for the formation of the considered compounds remain mostly unstudied, which is associated, inter alia, with a lack of reliable generalized data on stability regions, processes of decomposition of intermediate substances, typically, p-, d- and f -elements hydroxides resulting from the reactions of precipitation, hydrolysis, etc., i.e. those proceeding, primarily, in aqueous media. Shafer and Roy attempted to determine stability of
rare-earth oxides and hydroxides in studying the state equilibria of Ln2O3 - H2O (Ln= La, Nd, Sm) systems [27]. Lanthanum oxide was found to differ from neodymium and samarium, primarily, in its greater tendency towards hydration. The authors were not able to determine whether LaOOH monohydrate or La2O3 forms via the decomposition of lanthanum hydroxide, or its crystal structure. The authors of [28] studied the dehydration kinetics for Gd(OH) 3 obtained via hydrothermal conditions. The chemical transformations within the Fe2O3 - H2O system within media of different pH and with different P-T-conditions were investigated in [29, 30]. Papers [2, 5, 31] also discussed the formation of LaFeO3 during the dehydration of components of the La2O3 - Fe2O3 - H2O system. It has been reported that for the preparation of GdFeO3 nanoparticles using polyol technique, the authors obtained an until-then unknown compound corresponding to the formula Gd3FeO6, extending the number of compounds formed in the Gd2O3-Fe2O3-H2O system [32].
Thus, in literature, there are a number of works concerning the study of nanocrys-talline perovskite-like oxides using "soft chemistry" techniques. However, to describe the mechanisms of perovskite-like oxides formation using "soft chemistry" techniques requires application of a comprehensive approach, taking into account information about phase equilibria in the systems where perovskite-like oxides could form, and analysis of the impact chemical background could have on the formation of the synthetic targets.
Therefore, the data investigation aims at studying of the formation processes of nanoscale perovskite-like oxides in Ln2O3 - Fe2O3 - H2O (Ln = La, Gd) systems differing in synthetic prehistory.
2. Experimental
2.1. Synthesis procedure
LnFeO3 (Ln = La, Gd) samples were prepared the by precipitation method from aqueous solutions of stoichiometric amounts of 1M Ln(NO3)3-6H2O and Fe(NO3)3-9H2O. Aqueous 10 wt.% NH4OH was used as the precipitating medium. To the NH4OH medium, the aqueous solutions of lanthanum, gadolinium and iron nitrates were added in a drop-wise manner by adjusting the pH value to 8-9. The co-precipitated mixtures were filtered immediately after preparation to remove traces of NO- ions. Samples were then dried at room temperature in air. These initial mixtures were then pressed and calcined in air at 500-900°C for 3 h.
2.2. Characterization of prepared nanocrystalline perovskite-related oxides
The purity and crystal structure of LnFeO3 (Ln = La, Gd) samples were characterized by powder X-ray diffraction (XRD) using Shimadzu XRD-7000 with monochromatic CuKa radiation (A= 154.178 pm). a-Al2O3 was used as an internal standard. Crystallite sizes of the obtained powders were calculated by the X-ray line broadening technique based on Scherer's formula.
The elemental composition and the composition of separate phases were analyzed by means of scanning electron microscopy (SEM) using Quanta 200, coupled with EDAX microprobe analyzer. The error in determining the elements content by this method varies with the atomic number and equals to 0.3 mass% on average.
The IR spectroscopic investigations of samples were carried out on an FSM-1202 Fourier-transform spectrometer from 400-4000 cm-1. Samples were prepared as KBr pellets.
3. Results and discussion
X-ray phase analysis of precipitated mixtures testifies to their X-ray amorphous condition. Elemental analysis showed that the stoichiometric ratio of Ln:Fe is 1.00:1.06, which is close to that of the synthetic targets, LnFeO3 (Ln = La, Gd). Furthermore, the obtained samples were exposed to thermal treatment from 500 to 900° in air (pO2 =0.21 atm.).
Fig. 1a shows the data obtained from X-ray phase analysis for samples corresponding to the stoichiometry of LaFeO3 after thermal treatment from 500 to 900°C in air. According to the X-ray phase analysis, when the initial mixture was exposed to thermal treatment at 500°C for 3 hours, La2O2CO3 is crystallized as a monoclinic structure. The IR spectrometric analysis of this sample showed presence of bandwidths at 3,650-3,300; 2,936; 2,862; 1,774; 1,639; 1,500; 1,450-1,400; 1,388; 1,057-1,076; 856-900; 700-725 cm-1 (see table), which can be attributed to valence and deformation vibrations of crystallization water, Fe-O groups typically found in Fe(OH)3, Fe2O3, La2O2CO3 (Fig. 1b) spectra. Raising the initial mixture processing temperature to 600°C will lead to a decrease in the bandwidth intensity, associated with the removal of crystallization water (3,500-3,200; 1,630 cm-1), and with initiation of decarbonation of La2O2CO3, forming La2O3. According to the data presented (see Fig. 1, table) the reaction mixture thermally treated at 600 and 900°C contained a wide bandwidth at frequency 670-530 cm-1, which can be referred to as a band corresponding to La-O-Fe vibrations in perovskite structure, by analogy with [33, 34]. The data obtained agree with the results of X-ray phase analysis for samples exposed to thermal processing at 600°C when the formation of LaFeO3 is recorded. The size of the regions of coherent scattering for LaFeO3, prepared by thermal treatment of the co-precipitated mixture at 600°C, was 30±3 nm.
• LaFeO; ♦La;0;C0;
20 2-1 2s 32 36 -to 44 4s 52 56 60 w 6s 750 »» 12»2««MS« Jlf»3«037EB
29(f) wavenumber (cm" )
Fig. 1. ) X-Ray diffraction patterns and b) IR transmission spectra of initial mixture corresponding with stoichiometry of LaFeO3 heat treated in the air
Thus, the activation temperature for the formation of LaFeO3 is connected with the decarbonation of La2O2CO3, proceeding from 500 - 600°C, with La2O3 thus formed, initiating the generation of nanoscale perovskite-like lanthanum ferrite from 600°C.
Fig. 2 shows the X-ray phase analysis for samples corresponding to a stoichiometry of GdFeO3, prepared after thermal treatment of the initial mixture from 500 to 900°C in air. According to the presented X-ray phase analysis data (Fig. 2), the samples thermally treated from 500 to 600°C contain Gd2O3 and Gd2O2CO3 phases. The appearance of Gd2O2CO3 in the reaction mixture may be caused by utilizing synthetic precursors that were previously
Table. IR spectroscopic data of the initial mixture on stoichiometry closed to LaFeO3, heat treated at 500, 600, 900°C temperatures versus reference data [33-34]
Experimental data Reference data [33-34]
wavenumber (cm-1) wavenumber (cm-1)
500°C 600°C 900°C H2O La2O2CO3 La(OH)3 Fe(OH)3 a-Fe2O3
515-670 530-670 530-670 650 660-680-730 655 500-620
858 860-875 860-870-880
1075 1080 1050-1060-1080 1080
1382 1375 1388 1380 1353 1388
1473 1468-1500 1470-1500 1500
1638 1645 1650 16001630 1634 1656
2862 2859 2859 2865
2928 2934 2934 2930 2951
33503600 33503600 34003600 32003550 3420, 3600
dried in air (and thus exposed to CO2). According to [35], Gd2O2CO3 exists in layered structures consisting of alternating (Ln2O2+)n and carbonate groups (CO3)nn- layers. Raising the thermal treatment temperature will result in decarbonation, leading to the formation of cubic crystallized active-reactive Gd2O3. According to [35], Gd2O2CO3 decomposes as
650° C
follows: Gd2O2CO3 ->Gd2O3+ tCO2. According to the presented data of X-ray phase
analysis (Fig. 2), starting from 700°C, the GdFeO3 phase is formed. The mean size of coherent scattering regions, calculated from reflex with 111 index for the GdFeO3 sample obtained after thermal treatment of the initial mixture at 700°C, was 40±4 nm.
Thus, the activation temperature for the formation of GdFeO3, in the case when one of the reagents is Gd2O2CO3, shall be determined by its decarbonation temperature at 600-700°C, leading to the formation of active-reactive Gd2O3. In such a case, the formation of nanoscale perovskite-like gadolinium ferrite will be recorded as starting from 700°C.
Thus, in case when initial compositions were obtained by the co-precipitation method, the processes for LnFeO3 nanoparticle formation can be represented as a scheme depicted in Fig. 3, exemplified by the formation of GdFeO3 nanocrystals. The diagram is given with experimental and literature data [27-30, 33-35] taken into consideration. The formation of FeO(OH) and Gd(OH)3 hydroxides, mixed on the molecular level at the initial stages of synthesis, apparently, may lead to the formation of GdFeO3 nanoparticles, with simultaneous dehydration of both components. The second mechanism is connected with both dehydration and decarbonation processes, the latter proceeding at a higher temperature. In connection with this, localized in space Fe2O3 and Gd2O3 will interact at the next stage. This will lead to a rise in the synthesis temperature for GdFeO3 and to an increase in the size of the formed crystals. The obtained results correlate with the previous conclusions [22]. Note that if GdFeO3 nanocrystals are prepared via hydrothermal methods, the initial mixture obtained according to the technique described in this paper, we also record a difference in the nature of intermediate products, despite carrying out the synthesis in an aqueous medium [1].
Fig. 2. X-Ray diffraction patterns of initial mixtures corresponding with sto-ichiometry of GdFeO3 after sintering within the temperature range 500 to 900°C in the air
Fig. 3. LnFeO3 nanocrystals formation scheme exemplified by the forming of GdFeO3 under the heat treatment of co-precipitated initial mixture in air Remarks: I - the mechanism of co-precipitated hydroxides simultaneous dehydration with GdFeO3 formation; II - the scheme of step-by-step transformations of co-precipitated components with spatial-isolated Fe2O3 and Gd2O3 nanoparticles formation followed by solid state interaction for GdFeO3 production
4. Conclusion
Therefore, the mechanism for perovskite-related lanthanum and gadolinium ferrites formation depends heavily on chemical processes resulting in the spatial separation of components of a target product. In particular, these processes may determine the temperature at which perovskite-related ferrite begins to form, as well as the size of the formed nanocrystals.
Acknowledgments
The authors would like to thank Prof. V.V. Gusarov very much for useful discussions and valuable advice.
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