La_0.67(Ca,A)_0.33MnO₃(A=Li,Na,K) Preparation, Structure And Electrical Properties

The unique properties such as colossal magnetoresistance（CMR）and metal-insulator transition of perovskite-based manganite as La_1-xA_xMnO₃（A is the divalent or monovalent metal ion）had been extensively studied.Various magnetic phases,Jahn-tellere effect and charge ordering are also some of interesting phenomena which is showed in these perovskite manganites.The research efforts in the CMR materials area have been focused on the optimization of doped compounds with the aim to enhancing and tuning their physical properties.The reaearch of thesis mainly focus on the A-site cation doping on the parent phase of the monovalent alkali metal ion,and two sets of samples were selected for doping Ag,and make further study on the phase composition,microscopic morphology,crystal structure,electrical transport properties and magnetic properties.On the basis of the optimized sintering process of La_0.67Ca_0.33MnO₃（LCMO）,the sol-gol method which use methyl alcohol and deionized water as mixed solvent has been used to prepared a series of La_0.67（Ca,A）_0.33MnO₃（A=Li,Na,K）and La_0.67（Ca,A）_0.33MnO₃:Ag_x（A=Na,K）polycrystalline.The proportion of rare earth/alkaline earth was fixed on 0.67/0.33,doping on the LCMO with different kind of alkaline earth as well as with different concentrations.The phase composition and crystal structure were tested by X-ray powder diffraction（XRD）,the surface morphology was investigated by scanning electron microscope（SEM）,the elements of the sample were analyzed by energy dispersive spectrometer（EDS）and the electrical property was measured by the standard four-probe method,the magnetic properties was measured by magnetic performance measurement system.XRD patterns confirm that Ag was ditected in La_0.67（Ca,A）_0.33MnO₃:Ag_x（A=Na,K）polycrystalline ceramics samples,and other polycrystalline ceramics samples have single phase without any detectable secondary phases at room temperature.It?s worth noting that the strongest diffraction peak shifts to lower degree with increasing elements doping of Li,Na,K,Ag,which indicates that the increase of the unit cell volume.SEM observed the surface morphology of the polycrystalline ceramics samples,the result suggests that the grain boundary becomes blurred and the quality of crystals reduced with increase of Li;the grain size decrease with increase of K,but in the samples which doping Na,the grain size was increase with the increase of Na.In addition,manganese depolymerization occurs at the grain boundary in the Na-doping samples;the pores appeared in the Ag-doping samples.EDS show that all elements are evenly distributed on the surface of the samples,except Na-doping samples.The measurement of the resistivity-temperature curves shows that resistivity decreases with the increasing Na,K,Ag content.The conduction mechanisms of resistivity were different in the different temperature ranges.Three temperature ranges were considered in there:metallic regime（T<T_P）,insulating regime（T>θ_D/2）and competing regime（T_P<T<θ_D/2）.The metal-insulator transition temperature（T_P）increase with increasing Na,K,Ag content.The temperature dependence of magnetization show that Curie temperature（T_C）decrease with the Na,Ag increasing.The change in electrical performance is due to dop element changing bond angle of Mn³⁺-O^2--Mn4⁺which effect the doble exchange.Furthermore,element doping change the electron activation energy which can affect the resistivity.