Synthesis And Characterization Of Mg-doped The Rare Earth Perovskite Manganites

Since the recent decades, the reseach development of rare earth mangnites has always been an interesting subject because this family has diverse structures and rich optic, electric and magnetic properties. Generally, this system can fall into two distinct groups according to the radius of rare earth cation: RMnO₃ and RMn2O5 . between these two; RMnO₃ is a very important compound which has important applications in the field of magnetism, electronics, optics, catalysis. If we introduce alkaline earth such as Ca, Sr, Ba in the parent compound LaMnO₃ at the A-site. It can cause lattice distortions, and presenced Mn³⁺ and Mn⁴⁺ mixed valance state on B-site. Physical properties show remarkable changes with the substitution of different ions of different elements. As for Be²⁺ and Mg²⁺,Ra²⁺ion, because size of Be²⁺ ion is too small, Ra² is radioactive element, There is no previous evidences or reports to show that these can be doped into system of LaMnO₃. In 2001 J H Zhao claim for the substitution of Mg for La, he showed the effect of the substitution of divalent cations of small ionic radius (rA) at the A-sites in the manganese perovskites. He proposed the phase diagram and the date highlighted the suppression of a metal-insulator transport in these systems with small average A-site radius, but his result arosed controversy, someone proposed different ideas, they said that Rietveld analysis clearly shows Mg does not enter into the La-site .Rare earth manganites are traditionally synthesized by solid state reaction. These mixed Stoichiometric ratio raw materials are sintered at high temperature for a few hours or for a few days. Repeated cycles of milling and calcinations are carried out to achieve the completion of solid state reactions. High temperature solid state reaction has the advantage of being simple and easy to operate, and reagents are easy obtained.In this thesis, we focused on preparation, characterization and investigation of the magnetic properties for the perovskite manganites LaMn_1-XMg_XO₃(X=0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8,). Structural properties were studied by means of x-ray diffraction .A double perovskite is observed for x≥0.4 whereas no long Mn/Mg ordering is detected for x≤0.3. The local structure around Mn evidences the absence of Jahn-Teller distortion since x=0.1. The samples with low Mg content have a ferromagnetic ground state behaviour while samples with x≤0.3 undergo a spin-glass transition at low temperatures. The frequency dependence of the freezing temperature follows a Vogel-Fulcher law and it is similar to canonical spin-glasses. The high temperature dc susceptibility of all samples obeys the Curie-Weiss law but the effective paramagnetic moments do not agree with a spin-only contribution for x≥0.4. The samples with low Mg content have a ferromagnetic ground state while samples with x.>0.3 undergo a spinglass transition at low temperatures. In addition we have attempted to prepare R-P phase perovskite manganites La3-XMgxMn2O7, but through Rietveld analysis show that this are not R-P phase.Cu₂O can be sonochemically prepared in the solid-liquid interface of CuCl and water. The products are characterized by powder X-ray diffraction and the scanning electron microscopy (SEM). The results indicate that cuprous oxide morphologies are cubic, truncated cubic and octahedral shape respectively and vary from micron to nanometer in size. The formation and morphologies of Cu₂O are influenced by the high-intensity ultrasound, reaction temperature and addition of CuCl.