Structural Phase Transition And Photoluminescence Properties Of Rare-earth Fluoride Under High Pressure

The rare earth (RE) fluorides and their RE ions doping materials are a family of important functional materials that have attracted much attention for their wide potential applications in optical telecommunication, lasers, diagnostics, and biological labels. The studies on the structural phase transition and physical properties of materials by means of high pressure experimental technology have been an important research subject in high pressure physical science, and it has important scientific significance and potential application value for obtaining new structures, synthesizing new materials, discovering new phenomena and revealing new rules. The studies on structural phase transition of rare earth fluorides, PL properties of doped materials and structural phase transition and PL properties of nano materials are very important for their application potential that have scientific significance. However, up to now, only the light rare‐earth trifluorides have been investigated under high pressure. The high‐pressure research for rare‐earth trifluorides with β‐YF3type structure has not been done. Thus in this thesis, we performed high pressure experimental researches on several typical samples of this kind of rare‐earth trifluoride, such as YF33, YF3:Eu+, YF3:Eu3+nanoparticals and GdF3by using diamond anvil cell (DAC) to make detailed studies on their structural phase transitions and PL properties under pressure.1. Structural phase transition of YF3under high pressure.The structural transformation of YF3was studied by in situ angular dispersive synchrotron X‐ray diffraction (ADXD) measurements under high‐pressure. The highest pressure is31.9GPa. The starting orthorhombic phase of YF3start to transforms into a new high‐pressure phase at7.5GPa and completely transforms into high‐pressure phase at15.4GPa. The high‐pressure phase is identified as hexagonal structure with space group P3c1, which can be stable up to the highest pressure and returned back to orthorhombic strcture after the pressure released. The bulk moduli of YF3of orthorhombic and hexagonal phases were estimated at146(6) GPa and267(45) GPa. These results reveal the structural phase transition routine of rare‐earth trifluorides with β‐YF3type structure, and enriched the understanding of high pressure behavior of rare‐earth trifluorides, which are very important for further high pressure research of rare‐earth trifluorides2. Structural phase transition and photoluminescence properties of YF33:Eu+under high pressure.We investigate high‐pressure induced phase transitions and photoluminescence properties of Eu‐doped YF3(YF3:Eu3+) by using the angular dispersive synchrotron X‐ray diffraction and in situ PL techniques at room temperature. The highest pressure is24.5GPa and25GPa, respectively. It is found that the starting orthorhombic phase transforms into a new high‐pressure phase which is identified as hexagonal structure at8.4GPa that is the same to YF3. The high‐pressure structure of YF+3:Eu3also returned to the orthorhombic phase after release of pressure. The bulk moduli of YF33:Eu+are similar with YF3. The doping ions almost have no effect on the high‐pressre structural phase transition behavior of YF3. The luminescence intensity increases with elevated pressure before phase transition due to the high‐pressure efect. These result enriched the understanding of the high‐pressure behaviors of rare‐earth doped PL materials, which are very important for obtaining new function material.3. Structural phase transition and photoluminescence properties of YF3+3:Eu nanoparticles under high pressure.High‐pressure behaviors of YF33:Eu+nanocrystals with an average grain size of40nm were investigated by in situ high‐pressure synchrotron radiation X‐ray diffraction and in situ PL measurements up to31.1GPa and25GPa at ambient temperature. The pressure‐induced structural phase transition starts at11.8GPa. The transition pressure is enhanced in nanosized YF33:Eu+as compared to submicrometer size sample, which is due to the surface energy differences. YF33:Eu+nanocrystals with a starting phase of orthorhombic structure transform into hexagonal structure and returned to the orthorhombic phase after release of pressure which is the same with the submicrometer size sample which is related to the atom arrangement difference between nanosized and bulk samples. Accompanied with the structural transformation, the Eu3+ion luminescence emerges obvious changes, which indicate the variation of the local symmetry of Eu3+ions. These results fill the gaps in the research of rare‐earth fluoride nanomaterials, which are very important for further high‐pressure research of this kind of material.4. Structural phase transition of GdF3under high pressure.The structural transformation of GdF3was studied by in situ angular dispersive synchrotron X‐ray diffraction (ADXD) measurements under high‐pressure. The highest pressure is up to29.5GPa. The structural phase transition routine of GdF3is the same with YF3, which transforms from orthorhombic structure to hexagonal structure and returned to the orthorhombic phase after release of pressure. The transition pressure of GdF3is3.2GPa which is much lower than that of YF3. The bulk modulus of high‐pressure phase in GdF3is also lower than YF3. We infer that the transition pressure and the bulk modulus are related to the radius of rare‐earth ions in rare‐earth fluorides. The bigger the radius, the lower the transition pressure and the bulk modulus. This result is very important for finding the regular of structural phase transition for rare‐earth fluorides system.