Sol-gel derived Nd 1/3 La 1/3 Ca 1/3 MnO 3 : Phase evolution and preparation of films and nanopowders
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An all-alkoxide sol-gel route to the formation of Nd1/3La1/3Ca1/3MnO3 thin films and powders has been developed. The microstructural evolution on heat treatment of the gel to yield the perovskite oxide was monitored by means of thermogravimetric analysis-differential scanning calorimetry, powder x-ray diffraction (XRD), Fourier transform infrared spectroscopy, and transmission electron microscopy (TEM)-energy dispersive spectroscopy (EDS). It was found that the amorphous gel consists of hydrated oxo-carbonate without organic residues, and on heating it decomposes in several steps, finally forming single-phase perovskite at 680 °C. Further heating results in only slight changes in the cell parameters and crystal growth. Films were prepared by spin coating, followed by heat treatment in air to a temperature of 800 °C, and studied by scanning electron microscopy, TEM-EDS, and XRD. Films on Al2O3 were more porous, while films on Pt–TiO2–SiO2–Si were rather dense and consisted of areas with different crystal orientations.
I. INTRODUCTION
Perovskite ceramics exhibit a number of interesting chemical and physical properties, and a rich variety of crystallographic, electronic, and magnetic phases. The discovery of large magnetoresistance (MR) in holedoped manganite perovskites in the early 1990s created an enormous interest in this type of mixed-valence compound. Today, it is evident that the low temperatures and high magnetic fields required for this extraordinary MR effect make most applications based on it unrealistic. However, the La1-xAxMnO3 perovskites are still interesting from a fundamental aspect and for potential applications in, for example, uncooled infrared (IR) bolometers.1–4 The physical properties of materials are very sensitive to oxygen stoichiometry and the ratio of the constituent metals; therefore, the synthesis methods and conditions largely influence the properties. For the preparation of complex oxides, the solution-based routes have many advantages, such as precise control over the stoichiometry and low processing temperature. Some commonly used solution methods, such as the glycin–nitrate method and the Pechini technique, are limited to powder production, but the good mixture of the raw powder produced by dissolution, drying, and combustion makes it possible to shorten the annealing times and lower the temperature, compared to solid-state reactions. The sol-
gel methods combine the advantage of lower temperature and shorter processing time, with the possibility of making not only powders, but also films, fibers, and coatings. Here, we report the sol-gel synthesis of Nd0.33La0.33Ca0.33MnO3 (NLCMO) using only alkoxide precursors. Although there have been some reports of chemical solution routes to Ln–A–Mn oxides,5–10 no other research team has used a purely alkoxide route, although such systems normally yield the best homogeneity of the constituent elements.11 Although there is a vast amount of literature on the sol-gel preparation of perovskite titanate films, when it comes to manganites, films are
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