Stoichiometric LaMnO3 has an orthorhombic crystal structure with an antiferromagnetic (TN ~ 140 K) insulating nature (1). The substitution of aliovalent cations (Sr2+ or Na1+) induce holes in the partially filled 3d shell of Mn and oxidizes the Mn3+ (t2g3eg1) to Mn4+ (t2g3eg0) and transforms the LaMnO3 to ferromagnetic metal through the double exchange coupling of Mn4+-O-Mn3+ (2). Since the Na1+ (1.39 Å) has an ionic radii closer to La3+ (1.36 Å) this would induce a twice number of holes compared to Sr2+ substitution. Drastic variations in their physical properties have also been found in different synthesis methods (3,4). Therefore, in this work, we adopted a recently emerging microwave (MW) heating to synthesise La1-xNaxMnO3 (x = 0, 0.1, 0.3) from the oxide precursors. Conduction or dielectric loss heating by MW irradiation enables the fast and homogenous reaction in few minutes. We achieved 1000 oC in 10 min and dwelled the samples for 20 min. The operating MW power alone was varied from 1000 W to 1200 W and 1400 W for x = 0.3 to examine the effect of MW power on its physical properties. Temperature-dependent magnetization and resistivity of x = 0, 0.1, 0.3 with fixed MW power (1200 W) are shown in Fig. 1 (a)&(b). A broad Curie transition is observed for undoped LaMnO3 with TC of 200 K, unlike our previous report on solid-state prepared La1-xNaxMnO3 where we observed TN = 140 K for undoped LaMnO3 (5), and the transition becomes narrow with increased TC as x increases. Insulating LaMnO3 transforms to metallic below TC upon Na doping. TC is denoted in the figure by dotted vertical lines. As the MW power increases from 1000 W to 1200 W, the resistivity of x = 0.3 decreases drastically and the metal-insulator transition also becomes sharp at a temperature very close to TC (Fig. 1(c)). The obtained microwave absorption peaks at a specific field show only a little increase with increasing the frequency of the MW signal. These findings will be correlated and discussed with the structural and morphological features acquired by microwave irradiation.
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