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

Using the co-precipitation–calcination technique, lanthanum aluminate (LaAlO3) 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 LaAlO3 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 LaAlO3 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 LaAlO3 (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 LaAlO3 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.