Fabrication And Characterization Of Rare Earth Doped Perovskite Manganite Nanocrystals

Perovskite manganites R1-xAxMnO3(R = La,Pr and other rare earth elements,A = Ca,Sr,Ba,and other alkaline-earth elements)demonstrate complex interplays of spin,charge,orbital,and lattice degrees of freedom,which results in novel physical phenomena such as the colossal magnetoresistance(CMR)effect,giant magnetic entropy effect,metal-insulator(M-I)transition,electronic phase separation,charge/orbital ordering,and complex structural phases in their phase diagrams.As a prototypical system of perovskite manganites,La1-xCaxMnO3(LCMO)system exhibits much rich structural phases such as canted antiferromagnetic(CAF),charge ordered,ferromagnetic(FM)metallic,paramagnetic(PM)insulating phases and others in the LCMO system,which are dependent upon the Ca-doping concentration.Studies of the CMR effect in LCMO manganites have led to many other new phenomena and properties such as charge ordering and orbital ordering,which makes the physical characteristics of perovskite manganites interesting and challenging.Nowadays,advances in the integration and miniaturization of electronic devices have resulted in their feature sizes continued to be decreased,and now the feature sizes of electronic devices based on the LCMO perovskite manganites are down-scaled into nanometric scale.At nanoscale the surface-to-volume ratio in LCMO nanocrystals becomes increasingly important.Various finite size effects in low-dimensional LCMO manganite nanostructures will lead to more interesting novel properties of this system.In this thesis,(La1-xPrx)0.67Ca0.33MnO3 nanocrystals were synthesized by sol-gel method and the effects of annealing temperature and doped concentration of Pr(x)on the microstructures of the(La1-xPrx)0.67Ca0.33MnO3 nanocrystals were also investigated.First,(La0.6Pr0,4)0.67Ca0.33MnO3 nanoparticles were synthesized by sol-gel method and annealed at 700?,800?,900?,and 1000?,respectively.X-Ray diffraction(XRD)patterns reveal that all the samples are well crystallized without impurity phase,and their crystal structures are of orthorhombic system.Their XRD patterns match well with the La0.6Ca0.4MnO3(JCPDS card no.70-2665).The lattice parameter a and the unit cell volumes of the samples have a slight increase with increasing the annealing temperature.Scanning electron microscopy(SEM)images demonstrate that the samples annealed at lower temperatures exhibit much particle agglomeration,and some of them have sheet morphology,whereas the samples annealed at higher temperatures exhibit particle morphology as well as better dispersity.The chemical compositions of the samples obtained from the X-ray energy dispersive spectra(EDX)spectra are close to the nominal values.Transmission electron microscopy(TEM)and high-resolution transmission electron microscopy(HRTEM)images reveal that the average particle size was in the range of 10-100 nm,which was increased with increasing the annealing temperature.The clear lattice fringes resolved in the HRTEM images taken from a single nanoparticle reveal the well crystallinity of the nanoparticle.The interplanar spacing of the lattice fringes is in agreement with the XRD data.Selected area electron diffraction(SAED)patterns taken from many nanoparticles exhibit the characteristics of polycrystalline diffraction rings,and from which the lattice parameters are determined,matching well with the XRD data.The magnetic properties of the nanocrystals were tested by superconducting quantum interference device(SQUID).Their M-T curves were measured under a magnetic field of 0.01 T in the field-cooling(FC)and zero-field cooling(ZFC)modes.The results show that all the samples undergo a PM-FM transition upon cooling,and the Curie temperature Tc was determined to be 171 and 183 K for the samples annealed at 800? and 1000?,respectively.The M-H hysteresis loops of the samples measured at different temperatures(e.g.2K,10K,300K)demonstrate that all the samples exhibit ferromagnetic behavior at temperatures below Tc(e.g.2K and 10K),and paramagnetic behavior at temperatures above Tc(e.g.300K).At 10K the saturation magnetization and coercive field of the(La0.6Pr0.4)0.67Ca0.33MnO3 sample post-annealed at 800? were 60 emu/g and 621 Oe,respectively.Second,Pr-doped(La1-xPrx)0.67Ca0.33MnO3 nanocrystals with x = 0,0.1,0.2,0.3,0.4,0.5 were synthesized by sol-gel method,and post-annealed at 800? for 5 hrs.Similarly,their XRD patterns match well with La0.6Ca0.4MnO3(JCPDS card no.70-2665),which reveals that all the samples are well crystallized without impurity phase,and their crystal structure is of orthorhombic system.The lattice parameter a and the unit cell volumes of the samples have a slight decrease with increasing the Pr-doped concentration.That is mainly ascribed to that the radius of La3+(103.2 pm)is slightly larger than that of Pr3+(99 pm).SEM images reveal that all the samples exhibit particle morphology,and the X-ray energy dispersive spectra(EDX)data demonstrate that the chemical compositions of the samples are close to the nominal values.TEM and HRTEM images reveal that the average particle sizes are in the range of 20-100 nm.The lattice fringes resolved in the HRTEM image of a single nanoparticle confirm the well crystallinity of the particle.The interplanar spacing of the lattice fringes is in agreement with the XRD data.The angle between the two sets of crystal faces matches well with the theoretical value.The SAED patterns taken from lots of the Pr-doped(La1-xPrx)0.67Ca0.33MnO3 nanoparticles also exhibit the feature of polycrystalline diffraction rings,which are composed of the discrete diffraction spots.The lattice parameters determined from the SAED patterns are in agreement with the XRD data.