Effect Of The A Site Cation On The Electrical Properties Of (La,D) 0.67 (Sr,Ca) 0.33 MnO 3 (D=Nd, Sm, Gd, Dy, Y)

The strong correlated manganites Re1-xAexMnO3(Re=trivalent rare earth elements,Ae=bivalent alkaline earth elements)is a kind of rare earth/alkaline earth doping manganese oxides with perovskite structure,in which the Re and Ae ions locate in A-site,the Mn ion locates in B-site,0<x<1.The representative material of perovskite structure is La0.67Ca0.33MnO3,which has been widely used as an important material to make bolometer,optical-electrical converter,high density magnetic storage head,magnetic sensitive sensor,due to its colossal magneto-resistance(CMR)effect and laser-induced voltage(LIV)effect.The reaearch of the thesis mainly focus on the A-site cation doping on the parent phase of the alkaline earth,and make the further study on the phase composition,crystal structure,microscopic morphology and electrical transport properties.On the basis of the optimized sintering process of La0.67Ca0.33MnO3(LCMO),the sol-gol method has been used to prepared a series of La0.67-xDxSr0.33MnO3(D=Nd,Sm,Gd,Dy,Y)and(La,D)0.67Sr0.33-xCaxMnO3(D=Nd,Sm,Gd,Dy)polycrystalline.The proportion of rare earth/alkaline earth was fixed,doping on the LCMO with different kind of rare earth and 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.XRD patterns confirm that all polycrystalline ceramics samples have single phase without any detectable secondary phases in the scope of instrument sensitivity at room temperature,besides La0.67-xYxSr0.33MnO3.It can be indexed with an orthorhombic structure for all polycrystalline ceramics samples with the space group Pnma.The sharp diffraction peaks reveal the good crystallization of the ceramics samples.It's worth noting that the enlarged view of the strongest diffraction peak shifts to higher degree with increasing elements doping of Nd,Sm,Gd,Dy,Y,which indicates that the decrease of the unit cell volume.The surface morphology of the polycrystalline ceramics samples was studied by SEM and the composition of the samples were analyzed by EDS,which suggests that the samples synthesized by sol-gel technique have barely pores and the grin size decrease with the decreasing of the effective ionic radius of the doping elements Nd,Sm,Gd,Dy,Y.The measurement of the resistivity-temperature curves shows that the metal-insulator transition at TP shifts to lower temperature,and the resistivity ? increases,at the same time,the metal-insulator transition broadening temperature gets narrow,which gives rise to the increasing in themperature coefficient of resistivity.In view of the experimental phenomenon above,the mechanism based on the Double Exchange theory was put forward to explain the phenomenon,as following.The A-site cation ionic radius in the system replaced by the small ionic radius decreases the A-site average cationic radius<rA>and the tolerance factor t,which increases the deformation of the MnO6 octahedron.It gives rise to the decrease of the eg electron exchanges between Mn3+-O2--Mn4+.As a result,the electrical transport properties of the system reduce,which behaves that the metal-insulator transition temperature TP shifts at low temperature,and the resistivity ? increase.