RESUMO
By reaction of PbC2O4 and TiO2 in the eutectic NaCl-KCl salts, both sphere- and rod-like PbTiO3 (PTO) powders were synthesized via molten salt synthesis (MSS) and template MSS methods, respectively. X-ray diffraction patterns reveal that all the PTO powders crystallize in a tetragonal phase structure. Increasing the molar ratio of PbC2O4:TiO2:NaCl:KCl from 1:1:10:10 to 1:1:60:60 in the MSS process has little effect on the sphere-like morphology of the PTO powders synthesized at 950 °C for 5 h. Large-scale polycrystalline rod-like PTO powders with diameters of 480 nm-1.50 µm and lengths up to 10 µm were synthesized at 800 °C for 5 h by template MSS method, where the rod-like anatase TiO2 precursors were used as templates and the molar ratio of PbC2O4:TiO2:NaCl:KCl was equal to 1:1:60:60. X-ray energy dispersive spectroscopy spectra reveal that all the PTO powders are composed of Pb, Ti, and O elements, and the measured Pb:Ti atomic ratios are close to 1:1. In the template MSS process, the molten salt content plays an important role in forming the rod-like PTO powders. Under low molten salt content, the rod-like PTO powders cannot be synthesized even if the rod-like TiO2 templates are used. In addition, prolonging the reaction time suppressed the formation of rod-like PTO powders but promoted the formation of sphere-like PTO nanoparticles. The dielectric properties the sphere- and rod-like PTO powders were comparatively investigated. At room temperature, the dielectric constant and dielectric loss of the spherical PTO powders synthesized by MSS method with the molar ratio of PbC2O4:TiO2:NaCl:KCl equal to 1:1:30:30 were ~ 340 and 0.06 (measured at 106 Hz), respectively. The corresponding values for the rod-like PTO powders synthesized by template MSS method with the molar ratio of PbC2O4:TiO2:NaCl:KCl equal to 1:1:60:60 were 140 and 0.08, respectively. The present results demonstrate the sphere-like PTO powders have better dielectric properties, which have promising applications in the fields of multilayer capacitors and resonators.
RESUMO
We report on microstructural, magnetic, and optical properties of Pr-doped perovskite manganite (La1 - xPrx)0.67Ca0.33MnO3 (LPCMO, x = 0.0-0.5) nanoparticles synthesized via sol-gel process. Structural characterizations (X-ray and electron diffraction patterns, (high resolution) TEM images) provide information regarding the phase formation and the single-crystalline nature of the LPCMO systems. X-ray and electron diffraction patterns reveal that all the LPCMO samples crystallize in perovskite crystallography with an orthorhombic structure (Pnma space group), where the MnO6 octahedron is elongated along the b axis due to the Jahn-Teller effect. That is confirmed by Raman spectra. Crystallite sizes and grain sizes were calculated from XRD and TEM respectively, and the lattice fringes resolved in the high-resolution TEM images of individual LPCMO nanoparticle confirmed its single-crystalline nature. FTIR spectra identify the characteristic Mn-O bond stretching vibration mode near 600 cm- 1, which shifts towards high wavenumbers with increasing post-annealing temperature or Pr-doping concentration, resulting in further distortion of the MnO6 octahedron. XPS revealed dual oxidation states of Mn3+ and Mn4+ in the LPCMO nanoparticles. UV-vis absorption spectra confirm the semiconducting nature of the LPCMO nanoparticles with optical bandgaps of 2.55-2.71 eV. Magnetic measurements as a function of temperature and magnetic field at field cooling and zero-field cooling modes, provided a Curie temperature around 230 K, saturation magnetization of about 81 emu/g, and coercive field of 390 Oe at 10 K. Such magnetic properties and the semiconducting nature of the LPCMO nanoparticles will make them as suitable candidate for magnetic semiconductor spintronics.