| Perovskite type strontium manganate(Sr Mn O3) have attracted increasing attention due to their abundant physical properties such as spin, charge orderings and magnetism. However, there are two main shortcomings for its applications: high-presuure synsysis conditions and lack of ferromagnetic structure at or near room temepature. Thus, most studies of Sr Mn O3 are only focused on theory. Aimed at above unsolved problems, the synthesis of single-phase Sr Mn O3 ceramic and powders were studied. The phase structure by doping with Ba, Nd elements and quenching process were characterized.The magnetic and dielectric properties of Sr Mn O3 before and after the structure phase transition were slso systematically investigated in this paper.The phase-pure Sr Mn O3 powders and ceramics were successfully prepared by a simple and faciled sol-gel methods with glacial acetic acid as a solvent and ethylene glycol as a complexing agent. It was found that the problems of the high synthesis temperature, long time and more impurities in Sr Mn O3 can be solved. At first, the TEM observation revealed that the phase-pure Sr Mn O3 powders were prepared by calcining at 1350~1400oC under an oxygen atmosphere for 2h. Secondly, in the case of oxygen atmosphere, phase-pure Sr Mn O3 ceramics were then fabricated at 1400 oC for 4h. The relative density of resulting smaples is as high as 96.4%. Thirdly, the analysis of XPS showed that partial Mn4+ transmfored to Mn3+ because of oxygen vacancy in Sr Mn O3 ceramics, which decreased the insulativity in samples.It results of XRD and TEM observation showed that the phase structure of Sr Mn O3 ceramics were obviously changed by doping with Ba, Nd elements. On the one hand, the single-phase Sr1-x Bax Mn O3(0≤x≤0.5) powders were obtained by calcining at 1300 o C in the air for 2h. The Sr1-x Bax Mn O3 ceramics were obtained by sintering at 1380 oC in the oxygen atmosphere for 2h. The crystal structure gradually manifested lattice distortion and pseudo-hexagonal in the Ba-substituted compounds with increasing Ba content. On the other hand, the single-phase Sr1-x Ndx Mn O3(0≤x≤0.5) powders were synthsized by calcining at 1300 oC in the air for 2h, average particle sizes of powders are about 10 μm. The Sr1-xNdx Mn O3 ceramics were obtained by sintering at 1400 oC in the oxygen atmosphere for 2h, and the ceramic shrinkage rate was maximal for x=0.2. The result of TEM observation indicated that the original hexagonal crystal structure were shifted gradually towards orthogonal structure in the Nd-substituted compounds with increasing Nd content. The phase transition of Sr Mn O3 ceramics doped Nd3+ ions were bound up with the canting Mn3+O6 octahedra and Jahn-Teller lattice distortion interaction.The relative dielectric constant εr was 3700, dielectric loss tangen tanδ was 1.00 for Sr Mn O3 ceramics. The results of SQUID and the neutron powder diffraction indicated that the Néel transition temperature(TN) of paramagnetic to antiferromagnetic transition was determined to be around 60 K for Sr Mn O3 ceramics. At 2K, the M-H curves showed the spontaneous magnetization was largest, Ms = 0.91emu/g and coercive field Hc = 10800 Oe. And the same time, the magnetic structure of Sr Mn O3 ceramics was dominated antiferromagnetic state below TN.It was found that the magnetoelectric properties were distinctly changed after the phase transition for Sr Mn O3 ceramics by SQUID and the neutron powder diffraction. On the one hand, the doping substitution of Ba2+ ion obviously abated the dielectric properties of Sr Mn O3. The dielectric properties was supreme for x=0.2, relative dielectric constant εr was 1043.06 and dielectric loss tangen tanδ was 3.38. The descent of the dielectric properties of Ba2+ doped compounds is associated with increasing the concentration of oxygen vacancies. On the other hand, the doping substitution of Nd3+ ion obviously improved the magnetic behavior of Sr Mn O3 ceramics。 It was worth noting that that the Néel transition temperature(TN) is determined to be around 283 K for Sr0.8Nd0.2Mn O3 ceramics which was higher than 60 K for Sr Mn O3 ceramics. At 2K, the M-H curves showed the spontaneous magnetization was larger(Ms = 13.92emu/g) than Sr Mn O3 ceramics, and coercive field Hc = 1247 Oe. The magnetic structure of Sr0.8Nd0.2Mn O3 ceramics was dominated C-type antiferromagnetic structure below TN. Compared with single-phase Sr Mn O3 ceramics, the improved Néel temperature and magnetic properties were bound up with the Jahn-Teller lattice distortion interaction. |
PDF Full Text Request