Preparation of La0.67Ca0.33MnO3 Nanophase Powders and Films

Apr 4, 2002 - The perovskite could also be obtained by annealing at 560 °C, but in this case there remained in the product a small amount of amorphou...
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Chem. Mater. 2002, 14, 1981-1988

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Preparation of La0.67Ca0.33MnO3 Nanophase Powders and Films from Alkoxide Precursors Annika Pohl and Gunnar Westin* Department of Materials Chemistry, The Ångstro¨ m Laboratory, Uppsala University, SE-751 21, Uppsala, Sweden

Kjell Jansson Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91, Stockholm, Sweden Received May 17, 2001. Revised Manuscript Received February 5, 2002

A sol-gel route has been developed which, as far as we know, is the first purely alkoxidebased route to nanophase powders and thin films of perovskite La0.67Ca0.33MnO3. The microstructural evolution, on heat treatment, of free gel films obtained by air hydrolysis of the methoxy-ethoxide precursors, to nanophase oxides of the target oxide was investigated by means of thermogravimetric analysis, differential thermal analysis, thermomass spectroscopy, FT-IR spectroscopy, powder X-ray diffraction, SEM-EDS, and TEM-EDS. The xerogel consisted of a hydrated oxo-carbonate, almost without remaining solvent or alkoxo groups, and the metal elements appeared homogeneously distributed when studied with a ca. 20 nm wide EDS probe. Heating without annealing decomposed the carbonate groups and yielded the pure perovskite La0.67Ca0.33MnO3 at 690 and 800 °C, with heating rates of 5 and 20 °C min-1, respectively. The perovskite could also be obtained by annealing at 560 °C, but in this case there remained in the product a small amount of amorphous material containing La and Mn but almost no Ca. The orthorhombic La0.67Ca0.33MnO3 prepared by heating to 800 °C at rates of 20 °C min-1 or less had rather constant cell dimensions of ca. a ) 5.45, b ) 7.65, and c ) 5.47 Å, while heating at 20 °C min-1 to 800 °C with 1 h annealing, heating at 2 °C min-1 to 800 °C without annealing, or heating at 20 °C min-1 to 1000 °C gave a cell with ca. a ) 5.45, b ) 7.71, and c ) 5.47 Å, fitting the literature data quite well. The former cell is believed to have a higher Mn valency than the latter. Films were prepared by addition of a 0.6 M alkoxide solution to Si/SiO2/TiO2/Pt or polycrystalline R-Al2O3 substrates spinning at 3500 rpm, followed by heat treatment in air at 800 °C. Whereas crack-free, homogeneous 100-150 nm thick La0.67Ca0.33MnO3 films formed at heating rates of 20 °C min-1 or less, large bubbles were observed in films heated at 50 °C min-1 or more. Thicker films were obtained by repeating this procedure, and thinner films were obtained with more dilute alkoxide solutions.

1. Introduction Recent discoveries of colossal magnetic resistance (CMR) effects in mixed-valence perovskite manganates of the La1-xAxMnO3 type, where A is a divalent cation and 0.2 < x < 0.5, have made these materials leading candidates for the next generation of magnetic sensors and reading heads. The electric and magnetic properties of these materials are related to their structure, and intensive studies are going on to find the mechanism of CMR in these ceramics. In parallel with these studies, the synthesis parameters for optimization of the CMR properties are being investigated.1-6 (1) Jin, S.; Tiefel, T. H.; McCormack, M.; O’Brian, H. M.; Chen, L. H.; Ramesh, R.; Schuring, D. Appl. Phys. Lett. 1995, 67, 557. (2) Bae, S.-Y.; Wang, S. X. Appl. Phys. Lett. 1996, 69, 121. (3) Zhang, W.; Boyd, W.; Elliot, M.; Herrenden-Harkerand, W. Appl. Phys. Lett. 1996, 69, 3929. (4) Balbashov, A. M.; Karabashev, S. G.; Mukovskiy, Y. M.; Zverkov, S. A. J. Cryst. Growth 1996, 167, 365. (5) Va´zquez-Va´zquez, C.; Blanco, M. C.; Lo´pez-Quintela, M. A.; Sa´nchez, R. D.; Rivas, J.; Oseroff, S. B. J. Mater. Chem. 1998, 8, 991.

Compared to various vapor deposition techniques, the sol-gel route to complex oxides as films and nanosized powders is cost-effective and versatile. Few studies on sol-gel preparation of La-Ca-Mn oxide films have been conducted,2,5,7 and as far as we know, none of them involves a purely alkoxide route, although such precursors normally yield the best homogeneity of the constituent elements. The dearth of such studies might be due to the lack of a suitable Mn alkoxide source, because most alkoxides of low-valent transition metals, such as the methoxides, ethoxides, and isopropoxides, are insoluble polymers and are difficult to prepare in a pure form. In the work described here, the first all alkoxide based sol-gel route using the novel precursor [Mn19O12(MOE)14(MOEH)10]MOEH8 and the La and Ca methoxy(6) Mahendiran, R.; Mahesh, R.; Raychaudhuri, A. K.; Rao, C. N. R. Phys. Rev. B 1996, 53, 12160. (7) Faaland, S.; Knudsen, K. D.; Einarsrud, M.-A.; Ro¨mark, L.; Ho¨jer, R.; Grande, T. J. Solid State Chem. 1998, 140, 320. (8) Pohl, A.; Westin, G.; Kritikos, M. Chem.sEur. J., in press.

10.1021/cm010487q CCC: $22.00 © 2002 American Chemical Society Published on Web 04/04/2002

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ethoxides as precursors was developed. This route yielded La0.67Ca0.33MnO3 as polycrystalline films and nanophase powders at comparatively very low temperatures. Polycrystalline, 100-150 nm thick films of La0.67Ca0.33MnO3 on Si/SiO2/TiO2/Pt and Al2O3 substrates were prepared by deposition of the precursor solution by spin-coating, with subsequent heating to 800 °C, followed by different annealings. The conversion of the gel to the perovskite phase was studied in some detail, using nanoparticle samples obtained by heating thin pieces of gel in a thermogravimetric apparatus to different temperatures and quenching them to room temperature. These samples were characterized by powder X-ray diffraction, FT-IR spectroscopy, and TEMEDS. Further information on the reactions converting gel to oxide was obtained by differential thermal analysis and thermomass spectroscopy. The films were investigated by SEM and, after being removed from the substrate, by TEM-EDS. 2. Experimental Section 2.1. Equipment and Chemicals. a. Equipment Used. The IR spectra were obtained with a Fourier transform spectrometer (FT-IR; Bruker IFS-55) using samples as KBr tablets and paraffin mulls between KBr plates. It was observed that the spectra of the gel and the 220 °C sample were affected by pressing with KBr, and presented less well-defined peaks, compared to the paraffin mull spectra especially for the water bending mode around 1600 cm-1. Therefore, the paraffin mull spectra are presented for the gel and the 220 °C sample in Figure 4. The decomposition process on heating was studied with a thermogravimetric apparatus (TGA; Perkin-Elmer TGA7), differential thermal analysis apparatus (DTA; Setaram Labsys 1600), and a mass spectrometer (TMS; Balzers Quadrupol spectrometer QMG 420) connected to a home-built tube furnace with a 20% O2:80% He flow.9 Only the signals due to CO2 (molecular weight ) 12 (C), 28 (CO), and 44 (CO2)) were observed, since the water released was adsorbed in the capillary leading to the spectrometer. The crystalline materials were identified, and the unit cells determined, with GuinierHa¨gg focusing powder cameras using Cu KR1 radiation and Si as internal standard (PXRD). A transmission electron microscope equipped with an energy-dispersive X-ray spectrometer (TEM-EDS; JEOL 2000FX-Link 10000AN) was used for studies of the habit, metal ion homogeneity, and crystallinity of the samplessthe latter by the use of electron diffraction patterns (ED). The probe diameter in the EDS analyses was normally ca. 20-30 nm. Scanning electron microscopes equipped with energy-dispersive X-ray spectrometers (SEMEDS: JEOL 820-Link 10000AN and JEOL 880-Link GEM ISIS 3.00) were used for studies of the habit of the films on substrates. b. Chemicals Used. The MnCl2 (Merck, anhydrous), La metal chips (Aldrich, 99.9%), and Ca shot (Johnsson Mattey, 99.9%) were used as purchased. The p.a. quality methoxyethanol (MOEH) and toluene were distilled over CaH2 under inert atmosphere. The preparations involving the syntheses and mixing of precursors were made in a glovebox, using dry, oxygen-free argon. 2.2. Preparation of the Precursors. The lanthanum precursor was prepared by dissolving 5.67 g (40.8 mmol) of La chips in 35 mL of MOEH, with ca. 0.5 mg of HgCl2 as catalyst, at 50 °C for 30 h. The resulting pale yellow solution contained suspended green-black fine particles, which were removed by centrifugation. The La concentration was determined gravimetrically as La2O3 formed by annealing at 1050 °C for 12 h. The La methoxy-ethoxide (La-MOE) concentration was adjusted to 0.75 M by addition of MOEH. (9) Andersson, M.; Jansson, K.; Nygren, M. Catal. Lett. 1996, 39, 253.

Pohl et al. The calcium precursor was prepared by dissolving 2.00 g (49.9 mmol) of Ca shot in 40 mL of methoxyethanol at room temperature for 24 h. After centrifugation, the faint yellow solution was separated from a small amount of solid residue. The Ca concentration was determined gravimetrically as CaO formed by annealing at 1050 °C for 12 h. The Ca methoxyethoxide concentration was adjusted to 0.75 M by addition of MOEH. The [Mn19O12(MOE)14(MOEH)10]MOEH precursor was prepared according to the literature,8 by reacting potassium methoxy-ethoxide in methoxyethanol with MnCl2 in a 2:1 ratio, at room temperature for 24 h. After the KCl formed in the reaction had been removed, the solution part was evaporated to obtain the raw Mn oxoalkoxide. After recrystallization from toluene-MOEH, the alkoxide was free of residual K and Cl, according to SEM-EDS analysis. The Mn content of the Mn oxoalkoxide was checked gravimetrically as Mn3O4 after hydrolysis and annealing at 1050 °C for 12 h. 2.3. Preparation of the Nanophase La0.67Ca0.33MnO3. Free-standing Ca0.33La0.67Mn gel films were produced for the studies on the phase development during heat treatment, by depositing a 0.6 M (total metal concentration) toluene:MOEH (1:1) La0.67Ca0.33Mn solution onto aluminum foil, from which the dry gel films could easily be removed. The freed gels were heat treated in air with the TG apparatus, to investigate the decomposition of various groups and formation of phases, mainly at a heating rate of 20 °C min-1, but also in less detail at 2, 5, 50, and 100 °C min-1. The heating rate 20 °C min-1 was also used for the DTA and thermomass spectrometer studies, the latter performed in an 80:20 (He:O2 vol:vol) mixture instead of air. After quenching in the TG apparatus from different temperatures, the samples were analyzed with PXRD, IR spectroscopy, and TEM-EDS. To investigate how the heating rate and time influenced the final oxide, IR spectroscopy and PXRD studies were made on samples heated to 800 °C with rates from 2 to 100 °C min-1. Samples were also annealed for 2 and 24 h at 560 °C, 1 h at 700 °C, and 1 and 4 h at 800 °C, after heating at a rate 20 °C min-1 to the annealing temperature. 2.4. Preparation of Films on Substrates. Gel films were deposited on Si/SiO2/TiO2/Pt, and polycrystalline R-Al2O3, by adding a 0.6 M toluene:MOEH (1:1) solution to the spinning (3500 rpm for 20 s) substrates. Thicker films were prepared by a multicoating procedure with intermittent heat treatments. To study the effects of changed heating rates, gel films on Si/SiO2/TiO2/Pt were heated to 800 °C in a tube furnace in air, at rates from 2 to 100 °C min-1. Gel films were also heated at 20 °C min-1 to 800 °C and were annealed for 1 and 4 h; they were studied by SEM and in some cases by TEM-EDS, after scraping off from the substrate parts of the film to be studied.

3. Results and Discussion 3.1. Precursors. The La and Ca methoxy-ethoxide precursors were prepared in a way closely adhering to the literature.10,11 The solid-state structure of the Ca alkoxide has been described as consisting of Ca9(MOE)18 molecules by Goel et al.,11 but the solution structure in the present toluene-MOEH solvent might involve smaller, solvated oligomers. No structure has been reported for an La methoxy-ethoxide or oxo-methoxyethoxide. Unfortunately, so far our attempts to prepare single crystals suitable for X-ray studies of the compound has been unsuccessful. As a Mn source, the novel compound [Mn19O12(MOE)14(MOEH)10]MOEH was prepared by metathesis of MnCl2 and KMOE, and spectroscopic UV-vis studies indicated that the oxo-oxygens (10) Andersson, L.; Westin, G.; Jansson, K. AIChE J. 1997, 43, 2874. (11) Goel, S. C.; Matchett, M. A.; Chiang, M. Y.; Buhro, W. E. J. Am. Chem. Soc. 1991, 113, 1844.

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Figure 1. TEM image of the La0.67Ca0.33Mn gel and at top right the ED pattern of the area.

Figure 3. IR spectra of the La0.67Ca0.33Mn gel (A) heated to 220 (B), 300 (C), 400 (D), 465 (E), 620 (F), 690 (G), 800 (H), and 1000 °C (I). Spectrum A and B were obtained as paraffin mulls, while the other spectra were obtained as KBr tablets. The graph has been removed where the paraffin peaks were too strong to be corrected for.

Figure 2. PXRD patterns of the La0.67Ca0.33Mn gel (A) heated to 220 (B), 300 (C), 400 (D), 465 (E), 620 (F), 690 (G), 800 (H), and 1000 °C (I).

are formed by auto-decomposition immediately after the reaction of MnCl2 and KMOE.8 It is stable for long times, both in the solid state and in solution, provided that no traces of oxygen or water are present, and no changes were observed on heating to temperatures less than ca. 90 °C. The solid-state structure of [Mn19O12(MOE)14(MOEH)10]MOEH consists of flat molecular entities with an Mn-O coordination similar to that of brucite; this is unique among oxo-alkoxides, which are normally globular to keep the polar oxide parts in the interior and the nonpolar organic parts directed toward the organic solvent.12 FT-IR and UV-vis spectroscopic studies indicated that at least some changes occurred on dissolution of the compound, but we believe the main features of the structure remain also after dissolution in toluene-MOEH.8 3.2. Studies of the Phase Development in Free Gel Films on Heat Treatment. a. The Gel. Deposition of the precursor solution on aluminum foil resulted in an immediate dark brown coloring of the solution before it formed a sticky gel and eventually a xerogel. (12) Westin, G. J. Sol-Gel Sci. Technol. 1998, 12, 203.

Figure 4. TG graph (A) and DTA graph (B) of the La0.67Ca0.33Mn gel heated in air at a rate of 20 °C min-1. Corresponding thermo-MS graph (C) obtained in He:O2 (80:20 vol: vol).

This indicates that oxidation of Mn2+ took place even faster than the hydrolysis when the solution was subjected to air. TEM-EDS and ED analyses showed that the resulting gel was rather dense, homogeneous in its metal element composition, and completely amorphous. The gel was composed of layers, seemingly a few nanometers thick as shown in Figure 1. The PXRD pattern (Figure 2) showed no peaks, and the ED pattern contained very diffuse bands, indicating a wide range of M-M distances. The IR spectrum of the gel, shown in Figure 3, exhibits a band assigned to M-O stretching below 700 cm-1, and peaks assigned as due to carbonate stretching (1480 and 1390 cm-1) and vibration (1070 and 850 cm-1) modes. There are also at least two different H-O-H bending modes, one at 1645 cm-1 and the other at ca. 1580 cm-1, and an O-H stretch band at 3700-2700 cm-1. No toluene and only very little MOE(H), giving rise to some weak peaks between 1200

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and 1000 cm-1 due to C-C and C-O bending, remained in the gel. The carbonate groups most probably stem from absorption of CO2 present in the air, by the hydroxides formed by hydrolysis of the basic Ca2+ and La3+ alkoxides. A possible structure could consist of layers of MnOx capped by layers of La,Ca(CO3)y(H2O)z, separated by carbonate ions and water molecules in similarity to the hydrotalcites.13 A layered structure like this has also been observed in the alkoxide-based solgel synthesis of YMnO3.14 The two different H-O-H bending modes seen in the IR spectrum indicate that the H2O molecules are situated in two different and relatively well-defined positions, such as one with weak bonds to the surfaces of the particles and one with tighter bonds between the layers. By assuming the end point at 1000 °C of the TG curve (Figure 4) to be La0.67Ca0.33MnO3, the formula weight of the unheated gel was calculated to 345 g mol-1, and with the IR data of the gel and the observations of the phase evolution on heating, described in the following; the formula La0.67Ca0.33MnO2(CO3)1.33(H2O)3.8 seems plausible. However, long-time aged gels sometimes showed TG graphs different from those aged for less than a few weeks. The differences, believed to stem from structural and compositional changes, were more pronounced at temperatures below 400-500 °C, but the final oxide forming temperature was the same for both types of gels. Although the TG graphs and IR spectra of the fresh gels were reproducible, with only minor changes after storage at room temperature, the gels were quite sensitive to heating by the electron beam in the TEM and easily developed a structure with nanopores and blisters that was very similar to the material obtained on quenching from 220 °C. b. Heat Treatment of the Free-Standing Gel. To study the phase development on heating, powdered samples of the La0.67Ca0.33Mn gel were heated at a rate of 20 °C min-1 in air, using a TG apparatus, to the temperatures 220, 300, 400, 465, 620, 690, 800, and 1000 °C. The same heating rate was also used for studies with DTA and thermomass spectroscopy (Figure 4). The products were characterized, after quenching in the TG-apparatus in air, by IR spectroscopy (Figure 3), PXRD (Figure 2), and TEM-EDS. c. 220 °C. The material obtained by heating to 220 °C contained, as can be seen in Figure 5, 5-15 nm sized pores. It was stable in the electron beam, homogeneous in TEM-EDS, and amorphous by PXRD and ED. In the DTA curve, a strongly endothermic peak was observed in this temperature region, and the IR peak corresponding to the more weakly bonded water (1645 cm-1) had been reduced in intensity compared to that of the more strongly bonded water. This indicates that loosely bound water was lost without much reconstruction of bonds in the solid, and by back calculation from the TG curve, the formula weight of 295 g mol-1 was obtained, giving a probable approximate composition La0.67Ca0.33MnO2(CO3)1.33(H2O), which means that approximately 2.8 H2O had been lost from room temperature. (13) Costantino, U.; Marmottini, F.; Nocchetti, M.; Vivani, R. Eur. J. Inorg. Chem. 1998, 1439. (14) Westin, G.; Jansson, K. Advances in Science and Technology, 9th CIMTEC-World Ceramics Congress Ceramics: Part B 1999, 14, 159.

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Figure 5. TEM image of the La0.67Ca0.33Mn gel heated to 220 °C.

Figure 6. Bright-field image (left), dark-field image (right), and ED pattern of the La0.67Ca0.33Mn gel heated to 300 °C.

d. 300 °C. The TEM studies showed that the material had been densified to some extent but was still rather similar to the 220 °C sample (Figure 6). The EDS analyses showed that the material was homogeneous in its elemental composition. The PXRD patterns contained a few very weak and diffuse peaks, corroborated by the TEM-ED study, which showed both diffuse diffraction rings from the amorphous material and some weaker spots from crystalline material with d values that fitted the X-ray pattern. Dark-field imaging revealed ca. 1-7 nm sized crystallites as a minority phase. The identity of the crystalline material could not be determined, although data for a large variety of Mn oxides, La and Ca carbonates, oxo-carbonates, and hydroxo-oxo-carbonates were used for comparison. At 300 °C, the lowered intensity of the O-H stretch and bend peaks in the IR spectrum showed that only very small amounts of water remained, and the material could therefore be regarded as an oxo-carbonate. The DTA curve showed an exothermic peak in the temperature region 220-300 °C, despite the weight loss observed in the TG curve, which indicates that many new bonds are formed, more than compensating for the energy of the endothermic loss of gaseous water. At this temperature, the formula weight corresponded to 276 g mol-1, and the composition La0.67Ca0.33MnO2(CO3)1.33 seems probable, meaning that ca. 1 H2O was lost in this step. e. 400 °C. As seen in Figure 7, the material had begun to form a more particulate structure. The ED studies showed that the material was still mainly amorphous, but with a minor component of ca. 1-7 nm sized crystalline particles observable in the dark-field images. The PXRD pattern showed a few, very weak peaks with

Preparation of La0.67Ca0.33MnO3

Figure 7. TEM image and diffraction pattern of the La0.67Ca0.33Mn gel heated to 400 °C.

Figure 8. TEM micrograph of the La0.67Ca0.33Mn gel heated to 465 °C. Bright-field image (left) and dark-field image (right) of crystallites giving rise to the ED pattern.

d values corresponding closely to the strongest ED spots, but the identity of the crystalline phase could not be determined. According to the IR spectrum, the evaporation of water was complete at 400 °C. Also, at this temperature the peaks of C-C and C-O vibrations between 1200 and 1000 cm-1 were completely gone, showing that the evaporation of residual MOE(H) groups was complete. The first release of CO2 was observed in the thermo-MS graph from ca 300 °C, and the weight loss in the range 300-400 °C stems mainly from decomposition of carbonate ions into oxide ions and carbon dioxide. f. 465 °C. The TEM studies showed that the material was somewhat more densified and consisted of both crystalline and amorphous agglomerated particles in sizes of ca. 5-10 nm, but in some cases up to 50 nm (Figure 8). The amount of crystalline material observed by dark-field imaging was larger than in the 400 °C sample, but the very weak PXRD peaks observed at d values of 3.02, 2.57, 2.22, 1.57, and 1.11 Å could not be assigned to any known compound. Some further crystallization was also observed to occur when the material was heated by the electron beam in the TEM, showing that the material crystallized easily. Since only carbonate groups remained as decomposable groups, the weight loss from 400 to 690 °C is ascribed to release of CO2, which was also observed in the thermo-MS graph. g. 620 °C. As seen in Figure 9, the structure had become more particulate with normally 10-20 nm sized and strongly agglomerated particles, but there was also a large amount of amorphous material mixed with the crystalline particles. Both the ED and the PXRD pattern could be indexed with a cubic perovskite cell of (PXRD)

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Figure 9. TEM micrograph of the La0.67Ca0.33Mn gel heated to 620 °C. The specimen is composed of both amorphous and crystalline material, resulting in the ED pattern seen at top right.

a ) 3.854(5) Å. Since only the strongest reflexes were observed, and those expected for an orthorhombic distortion are very weak, it was not possible to ascertain a distortion of the perovskite at this temperature. An overlapped peak emerged around 600 cm-1 in the infrared M-O stretch band is the first sign of the perovskite formation observed by TEM and PXRD. Crystallization caused by the electron beam occurred even easier than for the 465 °C sample. The stepwise decomposition into oxide and CO2, seen between 300 and 690 °C in the TG curve, was accompanied by a reduction of the intensity of the carbonate stretching (1390-1480 cm-1) and bending (850 and 1070 cm-1) peaks in the IR spectrum. The calculated formula at 620 °C was 228 g mol-1, which corresponds to a composition of ca. La0.67Ca0.33MnO3.1(CO3)0.2. h. 690 °C. The material consisted almost entirely of agglomerated ca. 10-25 nm sized crystallites and a small amount of amorphous material. The TEM-ED and PXRD patterns were consistent, and the PXRD pattern was indexed with a cubic perovskite cell being equal, within experimental error, to that of the 620 °C sample (Table 1). The change of the amorphous material into crystalline perovskite, seen by PXRD and ED between 620 and 690 °C, was also visualized in the IR spectrum by the formation of a more distinct Mn-O stretching peak close to 600 cm-1, which is associated with the La0.67Ca0.33MnO3 perovskite, but the presence of small carbonate stretching peaks showed that some carbonate still remained. i. 800 °C. At this temperature, the entire sample was well crystallized as ca. 20-60 nm sized particles such as those seen in Figure 10, and no carbonate or other residues were indicated by the IR spectrum. Very weak peaks indicating orthorhombic distortion were observed in the TEM-ED and PXRD patterns, and an orthorhombic cell of a ) 5.458(2), b ) 7.640(7), and c ) 5.469(3) Å was obtained, fitting the La0.67Ca0.33MnO3 composition well for the a and c axes, but with a shorter b axis, compared to values extrapolated from the La0.70Ca0.30MnO3 and La0.60Ca0.40MnO3 cells reported by Faaland et al.7 (see Table 1). j. 1000 °C. The slow weight loss observed in the TG curve from 690 to 1000 °C corresponded to ca. 5 g mol-1 and might stem both from decomposition of residual

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Table 1. PXRD Cell Dimensions and FT-IR M-O Peak Maximum PXRD cell no. of reflexes quenched samples (20 °C min-1) 620 °C 690 °C 800 °C [a] 1000 °C [b] different heating rates, quenched at 800 °C 2 °C min-1 [b] 50 °C min-1 [a] 100 °C min-1 annealing 800 °C, 1 h [b] 800 °C, 4 h [b] 700 °C, 1 h [b] 560 °C, 2 h 560 °C, 24 h av [a]: fast heating rates up to 800 °C; [b]: slow heating rate, prolonged annealing or higher temperature [a] [b] literature data for La1-xCaxMnO37 x ) 0.4 x ) 0.3 x ) 0.33a a

cubic a (Å)

4 5 7 7

3.85(5) 3.8517(6) 3.85(12) 3.855(1)

9 9 7 9 (cubic 7) 8 8 8 8

orthorhombic a (Å)

orthorhombic b (Å)

orthorhombic c (Å)

orthorhombic vol (Å3)

FT-IR spectra M-O band max cm-1 532 (overlapped) 576 (overlapped) 584 594

5.458(2) 5.448(1)

7.640(7) 7.722(9)

5.469(3) 5.454(2)

228.05 229.45

3.852(3) 3.855(2) 3.865(9)

5.449(2) 5.451(2)

7.701(3) 7.665(7)

5.488(5) 5.462(2)

230.33 228.21

598 588 585

3.8569(8) 3.8553(7) 3.8534(5) 3.855(2) 3.854(1)

5.4525(1) 5.449(6) 5.448(1) 5.450(1) 5.447(2)

7.7076(1) 7.698(3) 7.702(1) 7.66(2) 7.66(2)

5.4703(1) 5.458(2) 5.463(3) 5.464(3) 5.459(3)

229.90 228.97 229.23 228.06 227.83

593 594 590 567 (overlapped) 609

3.853 3.854

5.454 5.449

7.652 7.706

5.466 5.467

228.13 229.57

586 594

5.4473(1) 5.4615(1) 5.4568

7.6940(1) 7.7306(1) 7.7107

5.4621(1) 5.4967(1) 5.4758

228.92 232.07 230.40

Extrapolated value from Faaland et al.7

Figure 10. TEM image of the La0.67Ca0.33Mn gel heated to 800 °C.

carbonate groups and from loss of oxygen. The latter should be indicated by changes in the IR Mn-O stretching frequency and PXRD patterns. It was found that, from 800 to 1000 °C, the changes in the a and c axes were very small while the b axis length was increased, giving the cell a ) 5.448(1), b ) 7.722(9), and c ) 5.454(2) Å, fitting the literature data7 quite well. Hence, the weight loss between 690 and 1000 °C is probably due to both decomposition of a small amount of residual carbonate groups and changes in the oxygen content, with the former being finished before 800 °C. In parallel with the changes in the X-ray cell dimensions, the Mn-O stretch peak maximum changed from 584 cm-1 at 800 °C to 594 cm-1 at 1000 °C. The b axis length has been shown to be affected by the oxygen content for the Ca0.30La0.70MnO3 phase, and higher oxygen contents were associated with a shorter b axis.15 This can be understood as the smaller and more (15) Herrero, E.; Alonso, J.; Martı´nez, J. L.; Vallet-Regı´, M.; Gonza´lez-Calbet, J. M. Chem. Mater. 2000, 12, 1060.

symmetric Mn4+ ions, yielding a shorter b axis than the larger and Jahn-Teller distorted Mn3+ ions. Thus, it seems that the crystalline La0.67Ca0.33MnO3 phase obtained from the gel has a higher oxidation state and loses oxygen on heating, but more studies are needed to draw conclusions about the details of this perovskite. 3.3. Effects of Heating Time and Rate. a. Heating Rate. By studying the effect on the phase development of heating rates from 2 to 100 °C min-1, it was found that the weight-loss steps were about the same regardless of the heating rate but occurred with better definition and at lower temperature with the slower heating rates. On heating the gel at 5 °C min-1, the IR spectra showed that the carbonate groups were almost gone at 640 °C and had completely disappeared at 690 °C, whereas the gel heated at 100 °C min-1 contained a small amount of carbonate even at 800 °C. This indicates that kinetic factors are involved in the carbonate decomposition. The orthorhombic perovskite cell changed mainly in the b-axis length with the heating rate: from roughly 7.65 Å for the higher rates to 7.70 Å for the slowest rate, 2 °C min-1. The Mn-O stretch maximum was shifted from 585 to 598 cm-1, with the higher wavenumber corresponding to the longer b axis, as was found in the quenching study described above. b. Isothermal Heat Treatment. The weight loss at 800 °C, according to the TG measurements, was 0.4% after 30 min and 0.9% after 6 h, corresponding to ca. 0.05 and 0.1 oxygen, respectively. It was found that the a- and c-axis dimensions changed very little on heating at 20 °C min-1 and annealing at 800 °C, whereas the b-axis length increased from 7.64 Å after 0 min to 7.71 Å after 1 h and 7.70 Å after 4 h, indicating that an equilibrium metal/oxygen ratio had been obtained after less than 1 h. By annealing for 1 h at 700 °C, a temperature just above the final rapid carbonate decomposition, the

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Figure 11. SEM images of films on Si/SiO2/TiO2/Pt substrates heated to 800 °C at 100 (a), 20 (b), and 2 °C min-1 (c) and annealed for 30 min.

residual small amount of carbonate found after quenching from 690 °C was removed, and the perovskite-cell b axis was 7.702 Å, similar to the results of the annealings at 800 °C. Annealing at 560 °C, which is below the final rapid carbonate decomposition step, was also investigated. After 2 h, the material consisted of strongly agglomerated ca. 10-30 nm sized particles, and TEM-EDS showed no inhomogeneities in the metal content. About half of the material was amorphous and the other half crystalline, consisting of more angular particles that yielded the ED pattern of an orthorhombic a perovskite cell. The PXRD pattern showed that the orthorhombic perovskite had the short b axis (7.66 Å) found in the high-oxidation-state perovskite. The IR spectrum showed that some carbonate groups remained, which indicates that at least part of the amorphous material observed was in the form of oxo-carbonates. The overlapped M-O band had a maximum at 567 cm-1. After 24 h at 560 °C, 50-200 nm sized perovskite crystals with composition close to La0.67Ca0.33MnO3 constituted ca. 97-99% of the material. The remaining part consisted of 100300 nm sized areas of porous networks of ca. 5-10 nm thick loops of an amorphous material containing La and Mn, but almost no Ca, which indicates a tendency toward phase separation when the material crystallizes slowly at low temperature. This might be due to sequential decomposition of the carbonates, releasing Ca and La ions at different times, thus inducing a deficiency of one component during crystallization. The b-axis length of the orthorhombic cell remained unchanged at 7.66 Å, but the Mn-O stretch maximum was 609 cm-1, which does not fit the perovskites with homogeneous composition that were found at temperatures from 690 °C and up. The IR spectrum also showed that there was a very small amount of carbonate groups left. The homogeneous orthorhombic perovskite La0.67Ca0.33MnO3, with cell dimensions of ca. a ) 5.45, b ) 7.71, and c ) 5.46 Å could thus only be obtained at temperatures higher than the final rapid carbonate decomposition step ending at ca. 680 °C with a heating rate of 20 °C min-1, although it seems possible to prepare somewhat inhomogeneous perovskites by longer annealings at lower temperatures. 3.4. Studies of Film Preparation. Thin gel films were obtained by spin-coating a 0.6 M solution on substrates of Si/SiO2/TiO2/Pt and, in the case of the 20 °C min-1 heating rate, also on polycrystalline R-Al2O3. Heating to 800 °C in air converted them into oxide films. To study the influence of heating rate on the oxide film quality, four different heating rates were used: 2, 20,

50, and 100 °C min-1. The SEM surface studies showed the film heated at 100 °C min-1 to be covered with homogeneously distributed bubbles, 1-2 µm in diameter, as seen in Figure 11. The film heated at 50 °C min-1 also contained some bubbles and pores, but far fewer than the 100 °C min-1 sample, and the 20 and 2 °C min-1 films were quite homogeneous. Thus, heating rates at or below 20 °C min-1 are required to obtain high-quality films. The TEM-EDS studies of the films showed them all to have homogeneous element composition. The crystal sizes were about 50-150 nm for the two higher heating rates and 80-250 nm for the lower rates, showing that the grain size can be influenced by the heating rate. The film thickness was 100-150 nm, for a single deposition, but thicker films could be obtained by adding several consecutive layers with intermittent heating. The film heated at 50 °C min-1 and scraped off from the substrate showed a monolayer of 50-150 nm crystallites with no amorphous parts, and the ED pattern obtained was consistent with the orthorhombic Ca0.33La0.67MnO3 phase, in accordance with the PXRD pattern discussed above. Thinner films could be prepared by using a more dilute solution, and with a 0.2 M solution, the obtained film thickness was 30-50 nm. 4. Conclusions An all-alkoxide route to films and nanophase powders of the Ca0.33La0.67MnO3 perovskite, using the novel Mn precursor [Mn19O12(MOE)14(MOEH)10]MOEH, has been has been developed. The gel obtained by air hydrolysis of the alkoxide precursor consisted of a hydrated oxo-carbonate with only very small amounts of organic groups remaining. Heating in air first removed the water and then decomposed the carbonate groups in several steps, yielding the pure perovskite at 800 °C with a 20 °C min-1 or at 690 °C with a 5 °C min-1 heating rate. Although inhomogeneous perovskites could be obtained by annealing at 560 °C, the pure and compositionally homogeneous orthorhombic La0.67Ca0.33MnO3 perovskite modification could be obtained only by short-time annealing at temperatures from 700 °C and up or by using a low heating rate (2 °C min-1) up to 800 °C. Two types of orthorhombic modifications were observed, one obtained on quenching from temperatures up to 800 °C after heating at 20 °C min-1 (ca. a ) 5.454, b ) 7.652, and c ) 5.466 Å), and one obtained after heating to higher temperatures or annealing for 1 h at temperatures from 700 °C and up (ca. a ) 5.449, b ) 7.706, and c ) 5.467 Å). The cells differ mainly in the b-axis length, with the latter fitting the

1988

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literature data quite well and the former having a shorter b axis, which is probably due to an increased amount of Mn4+ ions. The crystallite sizes were in the range 20-60 nm, after heating at 20 °C min-1 to 800 °C. Crack-free and homogeneous La0.67Ca0.33MnO3 perovskite films, 100-150 nm thick, were obtained by spincoating of Si/SiO2/TiO2/Pt or polycrystalline R-Al2O3 substrates with an 0.6 M alkoxide solution, followed by heating to 800 °C, at heating rates of or below 20 °C min-1. With heating rates above 20 °C min-1 the resulting films contained a large number of bubbles.

Pohl et al.

Thus, we have shown that the novel Mn oxo-alkoxide precursor can be used in a purely alkoxide route, at comparatively very low temperatures, 700 °C, in the preparation of elementally homogeneous orthorhombic La0.67Ca0.33MnO3 perovskite in the forms of nanophase powders and thin films. The route is also suitable for various types of doping, and such studies are in progress. Acknowledgment. The Swedish Natural Science Research Council (NFR) is thanked for financing this study. CM010487Q