Low-temperature synthesis of lanthanum monoaluminate powders using the co-precipitation–calcination technique

Abstract

Abstract Using the co-precipitation–calcination technique, lanthanum aluminate (LaAlO 3 ) powders were successfully synthesized from ammonia and metal chloride aqueous solutions as starting materials. The thermal decomposition and crystallization behavior of the gel precursors were characterized by means of DTA/TGA analyses. The phase evolution of the powders obtained after gel calcination in the temperature range of 600–900 °C was studied using X-ray diffraction analysis. The crystallization temperature of LaAlO 3 depends on the concentration of the starting solutions and on the presence or absence of ammonia chloride. The gel was precipitated from the high-concentration starting solution and then washed—this procedure guaranteed its homogeneity and led to the direct formation of phase-pure LaAlO 3 at temperatures as low as 835 °C. On the other hand, failure to remove co-precipitated ammonia chloride from either the high-concentration or low-concentration starting solution prior to calcination results in an increase in the crystallization temperature of LaAlO 3 (up to 903 °C), and to the deterioration in gel homogeneity—aluminum and lanthanum oxides are formed aside from lanthanum aluminate. TEM investigations show that the best LaAlO 3 powder, obtained after calcination at 900 °C for 2 h in air, consists of isometric particles no larger than 15 nm in diameter. The obtained powder may be used to produce single-phase bulk samples with excellent microstructure.