A “Green Chemistry” Approach to the Synthesis of Rare‐Earth Aluminates: Perovskite‐Type LaAlO₃ Nanoparticles in Molten Nitrates

Abstract

Perovskite-type LaAlO 3 nanoparticles have been prepared by a facile, rapid, and environmentally friendly molten salts method using alkali metal nitrates as low-temperature fluxes. Starting from hydrated lanthanum and aluminum nitrates and alkali metal hydroxides, the proposed methodology consists briefly of two steps: a mechanically induced metathesis reaction followed by short firing at temperatures above nitrates melting points. The purpose of the first is twofold: on the one hand to generate in situ the alkali metal nitrate flux and on the other hand, to obtain a La and Al-containing precursor material suitable for the synthesis of bulk LaAlO 3 nanoparticles in molten nitrates. Different alkali metal nitrates and eutectic mixtures were used to analyze the influence of melt basicity in the reaction outcome. Single phase LaAlO 3 was obtained directly, without any purification step when using three molten media: LiNO 3 , NaNO 3 , and their mixture; using KNO 3 as flux either alone or as part of eutectic compositions, prevents complete conversion, and the title material is obtained mixed with additional crystalline phases such as lanthanum hydroxinitrates and carbonates. As-prepared LaAlO 3 powders are composed of loosely agglomerated nanoparticles with very fine crystallite size (32-45 nm). The present method reduces considerably previously reported synthesis time/temperatures for this material. D. Lupas-contributing editor Manuscript No. 30188.J ournal might well be useful to prepare other perovskite and garnettype complex oxides based on Ln 2 O 3 -Al 2 O 3 combinations (Ln: lanthanides and yttrium) for optical, electronic, and structural applications.

A two step molten method for low temperature synthesis of La0.9Bi0.1AlO3 relaxor nanocrystalline

“…It is obvious that a mechanically induced metathesis reaction took place in the milling stage and NaNO 3 is the only crystalline product identified in the XRD pattern and La, Al and Bi species are present as amorphous phases. This phenomena is similar to the observation in the molten synthesis of LaAlO 3 by Mendoza-Mendoza et al [11]. It is deduced that in mechanically induced metathesis reaction, the exchange of ionic species between reactants generates in situ sodium nitrate (NaNO 3 ) and at the same time produces La-, Al-and Bi-containing precursor materials.…”

“…It is deduced that in mechanically induced metathesis reaction, the exchange of ionic species between reactants generates in situ sodium nitrate (NaNO 3 ) and at the same time produces La-, Al-and Bi-containing precursor materials. The obtained NaNO 3 can not only play as non-aqueous melt salt role, but also behave as Lux-Flood base providing oxide ions [11]. Moreover, though the detailed structure characterization of the La-, Al-and Bi containing precursor materials is difficult within the resolution of the XRD technique, it can be expected that the poorly crystalline precursor materials possess high reactivity, facilitating the formation of LBAO in molten nitrates.…”

Optical properties of Eu and Er doped LaAlO3 nanopowders prepared by low-temperature method

“…However, the methods applied for synthesis of aluminum-containing perovskites are in general time-consuming and challenging since they demand the optimization of many different experimental parameters such as pH, nature of starting materials, temperature and mixing procedures. Very recently, we have reported synthesis of LaAlO 3 , La 0.9 Dy 0.1 AlO 3 , La 0.9 Er 0.1 AlO 3 and La 0.8 Dy 0.1 Er 0.1 AlO 3 nanopowders by a novel ''green chemistry'' approach using alkali metal nitrates as low-temperature fluxes [25,26]. This method allowed obtaining pure and doped perovskite-type LaAlO 3 powders with very fine crystallite size (32-60 nm) at remarkably low temperatures, i.e.…”

Molten salts synthesis and electrical properties of Sr- and/or Mg-doped perovskite-type LaAlO3 powders

“…Alkali metal nitrates are preferred reaction media because of their low cost, low melting points (e.g., NaNO 3 = 303°C), high stability over a wide liquid temperature window (*550°C), and high water solubility which facilitates recovery of the target phase by a simple washing step [21]. As recently shown in different studies published by some of the present author group, both a metathesis reaction and molten salts synthesis might be combined to allow the preparation of pure and Er-and/or Dy-doped lanthanum aluminate nanoparticles at temperatures much lower than those previously reported in the literature using Li, Na, or K nitrates as fluxes [22,23]. In this article, we will give additional details on the reaction mechanism in sodium nitrate and show that with small changes, the method might be also further extended for preparing acceptor-doped, and thus oxygen-ion-conducting, LaAlO 3 materials.…”

“…Pristine and doped LaAlO 3 powders were prepared by a twosteps method consisting of a mechanically induced metathesis reaction followed by short firing at temperatures above sodium nitrate's melting point as previously described [22,23]. Analytical grade metal nitrates (Al(NO 3 …”