Synthesis Of CeF₃ Nanocrystal And CeF₃ NWs@CNT And The High Pressure Phase Transition Of CeF₃ Nanoplates

Lanthanide fluoride crystals have become the focus of recent investigations for their outstanding luminescent characteristics and potential applications in optical devices. As typical lanthanide fluorides crystal, Cerium fluoride (CeF₃) has been considered as one of the most promising scintillators of the next generation because of its high density, fast response and high-radiation resistance. Stimulated by both the promising applications and the interesting properties, great efforts have been devoted to the synthesis of CeF₃ nanostructures. For CeF₃ nanoplate previously, all the reports show that the obtained nanoplates were only round shaped with hexagonal (P63/mcm) structure synthesized via various methods. So it is still a challenge to prepare high crystallinity CeF₃ nanoplates with other regular shapes.High pressure technique provides us a very powerful tool to study the relations between the structure and physical properties and many new structures have been found under high pressure. High density is one of the most important parameters of conventional scintillator. With expectation to find higher density phase, several investigations have been focused on the high pressure phase transitions of bulk lanthanide fluorides crystals. However, there is no direct structural study on CeF₃ nanoplates, especially determinative X-ray diffraction data under high pressure. So it is important to study the phase transition and compressibility of CeF₃ nanoplates and the size effect on the phase transition process under higher pressure.The single crystalline CeF₃ nanoplates with regular hexagonal shape have been successfully synthesized by a ultrasound vibration assisted hydrothermal process for the first time. In the experiment , we chose to treat the mixing solution of CeCl3 and trisodium citrate with ultrasound irradiation(40 KHz). The mixing solution of CeCl3 , trisodium citrate and NaF was transferred into a Teflon bottle held in a stainless steel autoclave, sealed, and maintained at 180°C for 24 h. After centrifugation, washing with deionized water, and then dried in air we got final product. We set the molar ratio of the CeCl3, trisodium citrate and NaF at 1:2:6.25(0.4mmol, 0.4mmol, 2.5mmol). The phase structure of the as-prepared sample were investigated by XRD. All of the diffraction peaks can be identified to a pure hexagonal phase [space group : P63/mcm (193)] . From the ED patterns, the CeF₃ nanoplates is shown to be single crystal with high crystalline quality. SEM and TEM observations show that product is congeries of hexagonal CeF₃ nanoplates with an average diameter of 110 nm and thickness of 17 nm which is obviously different from the round nanoplates reported in study of Li et al. These results show that the ultrasound vibration assisted hydrothermal method is possible tool to synthesize nanomaterials with regular shapes and controllable size.The ADXD measurements were carried out at 4W2 High-Pressure Station of Beijing Synchrotron Radiation Facility (BSRF). High-pressure was generated by diamond anvil cell (DAC). CeF₃ powders were mounted in a 140-μm-diameter hole of the T301 stainless-steel gasket with thickness of 70μm. 4:1 methanol-ethanol mixture was chosen as the pressure-transmitting medium. The pressure was calibrated by the pressure dependent shift of the R1 ruby fluorescence line. The geometry correction for the radial integration of the two-dimensional data and the transformation into standard one-dimensional powder patterns were performed using Fit2d. The ADXD patterns of CeF₃ nanoplates showed that when the pressure increasing to 6.8 GPa two new peaks appeared which can be identified as (040), (222) peaks of orthorhombic (Cmma) structure. At 19.3 GPa another prominent change is that (021) ,(201), (220) and (002) peaks of orthorhombic structure appeared and the disappearance of (112) peak of hexagonal (P63/mcm) structure was observed. As the pressure reached 24GPa, all of diffraction peaks of the initial phase had disappeared which means that the CeF₃ nanoplates of hexagonal (P63/mcm) structure would completely transformed to the orthorhombic (Cmma) structure. During the transformation of lanthanide fluorides from hexagonal structure to orthorhombic structure, it have been guess that the structure of the high pressure phase is adistorted variant of the structure of a hypothetical cubic phase which is metastable under normal conditions and (040), (400), (222) peaks of orthorhombic structure is distorted from (220) peak of cubic phase. So we proposed a preferential phase transformation from (220) plane in the cubic structure to (040), (400), (222) plane of orthorhombic structure with compressing the (001) plane of initial hexagonal structure up to 6.8 GPa and the phase transition detailes accorded with conditions of phase transition pressure reduction with size decreasing in nanomaterials mentioned by Wang very well. In summary, the two parts of hexagonal-orthorhombic phase transition process should be attribute to size effect and anisotropy pressure-induced behaviors of 2D CeF₃ naoplates with high aspect ratio.The synthesis of CeF₃NWs@CNT was conducted in an arc discharge reactor in helium. A high-purity graphite tube filled with a mixture of polyetherimide (PEI) and CeF₃ powders in a weight ratio of 3:7 was used as the consuming anode while the cathode was a high-purity graphite rod. The arc discharge was conducted with a direct current of 70-90 A and voltage of 20-30 V. We found that when the He pressure was lower than 2.8x104Pa most of the product was carbon nanotubes and with He pressure increasing nanoparticles appeared in the product and there was no CeF₃NWs@CNT could be observed. As the He pressure up to 4.2x104Pa, CeF₃NWs@CNT came into observation and the content increase with the He pressure increasing. At 8.2x104Pa the content of CeF₃NWs@CNT obviously increased but the CNTs and amorphous carbon still exist in the product. In the experiment process, we found that the CNTs and amorphous carbon were mainly produce by graphite anode. So if we can find a polymer with good conductivity and high melting temperature as the anode, the outgrowth will decrease and the CeF₃NWs@CNT could be separated easily. The exploration of experimental conditions provides important detailed experiment data for synthesis of CeF₃NWs@CNT which is a great support for the primary exploration of the growth mechanism.