Studies On Phase Transitions And Photoluminescence Of Several Rare Earth Salts And Antimony Trioxide In Extreme Conditions

Temperature and pressure are two important physical parameters that decide the material state,so play an important role in changing the material structure,causing phase change.The pressure can shorten the distance between the atoms,adjust the crystal structure and the electron orbit,and the temperature can help the surface activity to overcome the barrier between the different structures.The physical and chemical properties of the substance depend on its structure,size and morphology.Therefore,extreme conditions including low temperature,high temperature and high pressure can change the existence state of the material and obtain the novel phenomenon hidden at ambient temperature and pressure.In this paper,the luminescent materials with different sizes and morphologies were prepared,and the changes of the structure,morphology and size of the materials were studied.The physical and chemical properties of the samples were investigated under extreme conditions,which provided clues for the exploration of excellent new materials.The full text is six chapters,as follows:In chapter 1,some progresses in high pressure technology and experimental methods,high pressure field and luminescent materials are briefly introduced.In chapter 2,we presented the detailed synthesize process of hexagonal NaYF₄:Yb,Ln microcrystals.The upconversion and downconversion photoluminescence intensity of NaYF₄:Yb,Ho microcrystals at high temperature were studied.Two different types of luminous intensity have been increasing during heating up to 500°C and cooling.After falling to room temperature,the upconversion and the downconversion luminous intensity were hundreds and several times greater as compared to without heating.In combination with XRD,IR spectra,absorption spectra and electron spin resonance spectroscopy,we found that the heat treatment removes the residual organic groups from the surface of the sample and enhance the upconversion luminescence and the resulting C=C and other unsaturated bonds Luminous enhancement.At the same time,we studied the upconversion spectra of rare earth doped NaYF₄ at different temperatures and pressures.Er's thermal coupling levels 2^H_11/2 and 4^S_3/2 ratio decrease with increasing temperature,and the law can be used to fit the temperature between 100-700 K.The ratio of the luminous intensity of the thermally coupled energy level also changes monotonically at high pressure and can also be used to measure the change in pressure.In chapter 3,we report the detailed synthesize process of YVO₄:Yb,Er phosphors i.e by hydrothermal and then annealing method and the conditions were optimized.The phosphors were able to realize both the downconversion and the upconversion.The YVO₄:Yb,Er phosphors can be applied to solar cells,while achieving UV and infrared to visible light energy conversion.Followed by the synthesis of a series of Tm₃+,Er₃+,Ho₃+ and Yb₃+ doped YVO₄ upconversion phosphor,which were emitted blue,green and yellow lights.We tried to use the composite up-conversion phosphor assembled with blue,green and red on the conversion LED.By adjusting the mixing ratio of Ho₃+ and Tr₃+,the upconversion white light is realized,which is close to the standard white light using CIE coordinates.From the variable temperature upconversion spectrum,when YV0₄:Yb,Tm,Ho is used as a white LED,the temperature effect of the fluorescence should be taken into account.In chapter 4,pure hexagonal phase EuF₃ nanocrystals were synthesized by aqueous reaction.The high pressure fluorescence spectra show that the transition point of the orthorhombic phase EuF₃ to hexagonal phase is about 1.5 GPa higher than that of the bulk.Unlike the steady-state presence under hydrostatic pressure,hexagonal EuF₃ undergoes cyclic transitions from hexagonal to orthorhombic and then back to the hexagonal under non-hydrostatic pressure.The pressure from the hexagonal to the orthorhombic phase transition is very small?0.07 GPa?,and under 3?10 GPa it changes from the orthorhombic phase to the hexagonal phase.Similarly,hexagonal EuF₃ undergoes cyclic transitions from hexagonal to orthorhombic and then back to the hexadecimal at high temperatures.In chapter 5,the cubic Sb₂O₃ microcrystals and orthorhombic Sb₂O₃ nanobelts were synthesized by hydrothermal and solvothermal methods.The structural stability of cubic Sb₂O₃ microcrystals at room temperature was studied by in situ high pressure Raman spectroscopy method.At pressures above 25 GPa,Sb₂O₃ translates into a high density amorphous phase,which can be facilitated by laser irradiation.A few minutes of irradiation can reduce the pressure required for complete phase change from more than 35 GPa to 27 GPa.After completely depressurizing,the amorphous phase can be partially reverted to a cubic phase,and laser irradiation can also accelerate the process.The Raman spectra and synchrotron radiation XRD are used to study the phase transition under high pressure.Both high pressure Raman and high pressure XRD results show that the orthorhombic Sb₂O₃ nanoribbons are transformed into a high pressure phase with unknown structure at 13 GPa.In chapter 6,summarizes the results in previous chapters.