Synthesis, structure, and properties of sodium or potassium-doped lanthanum orthomanganites from NaCl or KCl flux

Abstract

Lanthanum and oxygen-deficient La_1-x-yA_xMnO_3-δ(A=Na or K) ferromagnetic oxide phases have been synthesized from NaCl or KCl flux, starting from La₂O₃and MnCO₃at 900°C. Composition of the final product could be varied by varying initial composition La_yMnO₃(0.7<y<1). Final composition of these oxides was obtained from chemical analysis of elements present. As the lanthanum content increased, Na or K content decreased and the structure changed from rhombohedral to orthorhombic symmetry. Mn⁴⁺concentration was 27% in the sodium doped manganite, La_0.82Na_0.13MnO_2.93with Curie temperature 325 K, and Mn⁴⁺was 25% in the potassium doped La_0.82K_0.08MnO_2.89with T_c of 310 K. Structure of these two phases have been refined from neutron diffraction studies. Stoichiometric La_1-xA_xMnO₃(A=Na or K) have been obtained for the initial composition La_yMnO₃(y=0.85). Orthorhombic phases obtained were found to be Mn-deficient La_1-xNa_xMn_1-xMn_1-yO₃(x=0.02;y=0.05), which were ferromagnetic insulators, even though total Mn⁴⁺ contents were as high as 29%.