- Remarkable changes in the optical properties of CeO2 nanocrystals induced by lanthanide ions doping
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Highly uniform and well-dispersed CeO2 and CeO 2:Eu3+ (Sm3+, Tb3+) nanocrystals were prepared by a nonhydrolytic solution route and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), UV/vis absorption, and photoluminescence (PL) spectra, respectively. The result of XRD indicates that the CeO2 nanocrystals are well crystallized with a cubic structure. The TEM images illustrate that the average size of CeO2 nanocrystals is about 3.5 nm in diameter. The absorption spectrum of CeO2:Eu3+ nanocrystals exhibits red-shifting with respect to that of the undoped CeO2 nanocrystals. Under the excitation of 440 nm (or 426 nm) light, the colloidal solution of the undoped CeO2 nanocrystals shows a very weak emission band with a maximum at 501 nm, which is remarkably enhanced by doping additional lanthanide ions (Eu3+, Tb3+, Sm3+) in the CeO2 nanocrystals. The emission band is not due to the characteristic emission of the lanthanide ions but might arise from the oxygen vacancy which is introduced in the fluorite lattice of the CeO2 nanocrystals to compensate the effective negative charge associated with the trivalent ions.
- Wang, Zhenling,Quan, Zewei,Lin, Jun
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- Preparation of REFeAsO1-xFx (R E=Sm and Gd) superconductors at a relatively low temperature
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A series of the SmFeAsO1-xFx and GdFeAsO 1-xFx (x=0.05, 0.1, 0.15, 0.2, 0.25) samples have been prepared using nano-scaled ReF3 as the fluorine resource at a relatively low temperature. The samples have been sintered at 1100 and 1120 °C for SmFeAsO1-xFx and GdFeAsO1-xF x, respectively. These temperatures are at least 5060° lower than other previous reports. All of the so-prepared samples possess a tetragonal ZrCuSiAs-type structure. Dramatically supression of the lattice parameters and increase in Tc proved that this low temperature process was more effective to introduce fluorine into REFeAsO. Superconducting transition appeared at 39.5 K for SmFeAsO1-xFx with x=0.05 and at 22 K for GdFeAsO1-xFx with x=0.1. The highest Tc was detected to be 54 K in SmFeAsO0.8F0.2 and 40.2 K in GdFeAsO0.75F0.25. The use of the nano-scaled ReF3 compounds has improved the efficiency of the present low temperature method in synthesizing the fluorine-doped iron-based superconductors.
- Cui,Chen,Cheng,Yang,Wang,Li,Zhao
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- Low temperature preparation and superconductivity of F-doped SmFeAsO
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A low temperature (1100 °C) process of preparing F-doped SmFeAsO samples has been developed using SmF3 with nanometer scale as the source of fluorine. A series of the SmFeAsO1 xFx (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3) samples have been prepared using the present method. Compared with previous reports, the present SmF3 is more effective to introduce F into SmFeAsO system in which a transition temperature of 39 K can be observed when x = 0.05. The superconductivity is definitely enhanced with the increasing F-doping level. All the samples presented to be layered structure and the crystal particle size is about three times larger with sintering time increasing from 36 h to 48 h. Except for the nanometer scale of SmF3, the flux effect of SmF3 is recognized to be another reason for the decrease of the sintering temperature. Further more, a relatively large amount of SmF3 was also employed in the raw materials to introduce excessive F and this has induced higher Tc (55 K) in SmFeAsO0.8F0.2+δ system.
- Chen,Cui,Cheng,Yang,Wang,Li,Zhang,Zhao
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