Structure And Characteristics Of Double Perovskite And Layered Perovskite Ceramics

Recently,multiferroic materials have attracted increasing scientific interests in the field of materials science and condensed matter physics due to their rich physics and the great potential applications.In the present thesis,the preparation,crystal structure and physical properties of LaRNiMnO6(R = rare earth)and Pr(SrxCa1-x)2Mn2O7 ceramics are systematically investigated,and the relationships between physical properties and structure are also discussed.The following primary conclusions have been achieved.All LaRNiMnO6 ceramics are determined as monoclinic symmetry in space group P2i/n with ordered arrangement in B-site ions(Ni2+ and Mn4+),which are similar with La2NiMnO6 ceramics.All samples show the ferromagnetic characteristic,which is ascribed to the Ni2+-O-Mn4+ superexchange interaction.Moreover,obvious magnetic hysteresis loops are detected below ferromagnetic Curie temperature TC.The Curie point TC decreases monotonously with decreasing<Ni-O-Mn>bond angle which in turn depends on the ionic radius of A site.The hopping of carriers between Ni2+ and Mn4+ cations,which equivalent to the reversal of polar regions,are responsible for the dielectric characteristic.Besides,a dielectric anomaly near the ferromagnetic transition temperature indicates the coupling of magnetic properties and dielectric properties,and a considerable negative magnetodielectric coefficient of-7.7%is observed at room temperature for the R=Sm ceramics.The negative magnetodielectric coefficient monotonously decreases with increasing the applied magnetic field,both intrinsic and extrinsic effects should be responsible for the magnetodielectric coupling effect.Pr(SrxCa1-x)2Mn2O7(x=0,0.1 and 0.2)ceramics with stable R-P layered structure are synthesized through a long time sintering processes.The space group can be fitted with either non-centrosymmetric Cmc21 or centrosymmetric Cmcm,where Cmc21 is a subgroup of Cmcm,more accurate determination requires further experimental analysis.DSC,temperature-programmed XRD and dielectric spectroscopy results show that there are two phase transition related to charge ordering in the present ceramics.The phase transition temperatures are close to room temperature and the process is diffused.The substitution of Sr2+ has a significant influence on phase transition behavior,the phase transition temperature T1 monotonously decreases with the increasing amount of x,while the phase transition temperature T2 decreases first and then increases with increasing x.All samples are weak ferromagnetic below antiferromagnetic Neel temperature TN(TN is detected in the temperature range of 130 K?170 K)and show the so-called CE-type antiferromagnetic configuration.The best ferromagnetic characteristics are achieved in the composition of x=0.25.Low temperature parts of M-T curves show the spin-glass(SG)behavior,which is an outcome of competition between ferromagnetic interaction and antiferromagnetic interaction,and the former is ascribed to the Mn3+-O-Mn4+ double exchange interaction.