Combustion synthesis of alkaline-earth substituted lanthanum manganites; LaMnO3, La0.6Ca0.4MnO3 and La0.6Sr0.4MnO3

Abstract

Alkaline-earth substituted lanthanum manganites La_{1 − x}A_xMnO₃ (A = Ca, Sr; x = 0, 0.4) were synthesized in air by self-propagating high-temperature synthesis (SHS): a combustion process involving the reaction of lanthanum(III) oxide, calcium oxide (or carbonate), strontium peroxide, manganese metal powder and sodium perchlorate. A very sensitive thermal imaging method was used to study the combustion characteristics of the SHS-process. This method is based on the continuous measurement of the combustion process using an IR-camera and software developed by MIKRON Instrument Co., Inc. (M9100 Pyrovision Series – Imaging Pyrometer). X-Ray powder diffraction, scanning electron microscopy (SEM)/energy dispersive analysis by X-rays (EDAX), infrared spectroscopy and vibrating sample magnetometry were carried out on all samples. X-Ray diffraction data showed that single-phase orthorhombic LaMnO₃ and monoclinic La_0.6Ca_0.4MnO₃ and La_0.6Sr_0.4MnO₃ were formed. Magnetic properties were found to transform from antiferromagnetic for LaMnO₃ to weak ferromagnetic for La_0.6Ca_0.4MnO₃ to strong ferromagnetic for La_0.6Sr_0.4MnO₃. The experimental results show the SHS route is useful for the synthesis of multicomponent Mn-based oxides. The thermal imaging work enables accurate monitoring of the SHS combustion process and enabled velocity of propagation, maximum temperature, cooling rates, synthesis wave width and synthesis wave path to be determined with high precision.