Hydrothermal synthesis of La 1−X Sr X MnO 3 dendrites Darko Makovec a,(n) , Tanja Goršak a , Klementina Zupan b , Darja Lisjak a a Department for Materials Synthesis, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia b Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškarčeva 5, SI-1000 Ljubljana, Slovenia article info Article history: Received 5 March 2013 Received in revised form 2 April 2013 Accepted 8 April 2013 Communicated by: T. Nishinaga Available online 17 April 2013 Keywords: A1. Crystal growth A1. Dendrites A1. Hydrothermal synthesis Magnetic perovskite abstract Single-crystalline dendrites of La 1−X Sr X MnO 3 (LSMO) perovskite were synthesized using a simple hydrothermal method without the use of surfactants. The Sr 2+ , La 3+ , and Mn 2+ ions were co- precipitated with aqueous NaOH under a flow of Ar. The aqueous suspension of the precipitates was hydrothermally treated in an autoclave filled with ambient air at temperatures ranging from 220 1C to 300 1C. The products were characterized using a combination of X-ray diffractometry (XRD) and transmission electron microscopy (TEM, HREM, EDXS). The dendrites formed either in a “tree-like” shape, with the trunk and the branches extending along the 〈111〉 directions of the quasi-cubic structure, or in the hexagonal shape of a “snowflake”. The mechanism of the dendrite nucleation was proposed, based on phase development. During the hydrothermal treatment at lower temperatures the hexagonal platelet crystals of Sr 1−X La X MnO 3 with the hexagonal perovskite structure form first. At higher temperatures the LSMO nucleates epitaxially at the edges of the hexagonal crystals and grows outward, forming the dendrite. To the best of our knowledge, this is the first report on the synthesis of crystalline dendrites of La 1−X Sr X MnO 3 perovskite. & 2013 Elsevier B.V. All rights reserved. 1. Introduction In recent years, new functional materials have been synthesized with architectural control of their micro/nanostructures. For applica- tions demanding a large surface area of material, the synthesis of hierarchically branched, fractal structures is of special interest. Dendrites are examples of fractal structures that form by crystal growth under specific, non-equilibrium conditions. Dendrites occur in the nature, for example, as snowflakes, and have been deliberately synthesized and tested as new materials. [1–17] The synthesis of dendritic microstructures of materials with a simple composition and structure, including simple metals (Ag, Cu, Co, and Ni) [1–5], alloys (CuNi, FeNi 3 ) [6,7], transition-metal chalcogenides (CdS, Ag 2 Se, PbTe) [8–10] and simple oxides (ZnO, CeO 2 , α-Fe 2 O 3 , WO 3 ) [11–14] is well documented, whereas the dendrites of mixed oxides with a more complex structure and composition have been successfully synthesized on fewer occasions. Starting from mixed oxides, spinel magnetite Fe 3 O 4 [15] and the perovskites BaTiO 3 [16] and SrTiO 3 [17] were synthesized in the form of dendrites. Because of their special, strongly correlated transport and mag- netic properties, mixed-valence manganites with a perovskite struc- ture such us La 1−X Sr X MnO 3 have received special attention in different areas of modern technology. By increasing X, the La 1−X Sr X MnO 3 pero- vskite changes from an insulator to a metallic conductor, while its magnetic properties change from antiferromagnetic to ferromagnetic. [18] It has been widely used for magnetic sensors in combination with colossal magnetoresistance (CMR), as cathode materials in solid-oxide fuel cells (SOFCs), for magnetocaloric refrigeration, as oxidation catal- ysts, and for mediator nanoparticles in the treatment of cancer using a self-regulating magnetic hyperthermia, etc. [19–24]. Many different chemical methods have been developed for the synthesis of La 1−X Sr X MnO 3 , including solid-state reaction, carbonate and oxalate co-precipitation, the sol–gel method, pyrolysis, the citrate method, the polymeric precursor route, the molten salt route, etc. [23,25–29] All these methods involve high temperatures or require high-temperature post-annealing (calcination) to obtain the final composition and the structure of the material. In contrast, the hydrothermal method [30–36] enables the synthesis of the La 1−X Sr X MnO 3 perovskite in situ, during a hydrothermal treatment of the corresponding hydroxides in water at moderate temperatures (usually below 300 1C.) and under the autogenous pressure. Usually, a mixture of Mn 2+ and Mn 7+ is employed in order to obtain the desired valence of the manganese in the product, usually in the form of cuboid, micron-sized particles. Nanoparticles of the La 1−X Sr X MnO 3 perovskite were also hydrothermally synthesized with the addition of surfactants. In this work, the hydrothermal method was applied to synthe- size the La 1−X Sr X MnO 3 perovskite in the form of single-crystalline dendrites. 2. Experimental procedure In a typical synthesis experiment a solution of the metal ions was prepared by dissolving 9.8 mmol of La(NO 3 ) 3  6H 2 O, 4.2 mmol of Sr (NO 3 ) 2 , and 10.0 mmol of Mn(NO 3 ) 2  4H 2 O in 100 mL of distilled water. The solution of metal ions was then admixed into 100 mL of Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.04.019 (n) Corresponding author. Tel.: +386 1 4773 579; fax: +386 1 2519 385. E-mail address: darko.makovec@ijs.si (D. Makovec). Journal of Crystal Growth 375 (2013) 78–83