Preparation and magnetocaloric effect of self-doped La0.8-xNa0.2□xMnO3+δ (□ = vacancies) polycrystal

Single-phase perovskite-type manganese oxides La(0.8-x)Na(0.2)rectangle(x)MnO(3+delta) with x = 0.01-0.04 have been prepared by the modified sol-gel method and the influence of the vacancy concentration in the A site on their structures and magnetocaloric properties has systematically been investigated. The experimental results revealed that the samples calcinated at 1573 K remained single phase and were constituted by submicrometre homogeneous particles without deviation from the nominal stoichiometry upon heating. For all the compositions explored in this work, the average manganese oxidation state is practically constant, at 3.37 +/- 0.02. Magnetic measurement results show that an appropriate amount of vacancy concentration in the A site is favourable to both the Curie temperature and magnetic entropy change for polycrystalline La(0,8-x)Na(0.2)rectangle(x)MnO(3+delta) compounds. An approximately linear relation between Curie temperature and magnetic entropy change was observed. For La(0.77)Na(0.198)Mn(1.000)rectangle(0.029)O(2.94), the largest value of magnetic entropy change of 3.86 J kg(-1) K-1 under a field of 10 kOe and the Curie temperature of 364 K were obtained.