Grain Size And Defects Effect On High-Pressure Phase Transition Behavior Of MF₂（M=Ca,Sr） Nanocrystals

Numerous studies indicate that nanomaterials will always show novel and different phase transition behavior compared with bulk counterparts at high pressure.High-pressure phase transitions and structural stable states of nanomaterials upon compression have attracted abundant attention of scholars in many fields of material chemistry,physics and geology,etc.SrF₂,CaF₂and analogue compounds have been widely used in the fields of biomedical imaging,lasers,and scintillation sensing materials due to their excellent optical,electrical,and mechanical properties.So far,high-pressure studies on bulk SrF₂and CaF₂have been reported in detail,while there is still lack of comprehensive understanding on the pressure induced phase transition of nano-size SrF₂and CaF₂.There are many factors affecting the high-pressure structure and phase transformation path on nanomaterials,including grain size and lattice defects.Therefore,it is one of meaningful topics in the field of high pressure physics to conduct systematic studies on CaF₂and SrF₂nanomaterials with various sizes,forexploring the essential factors affecting the high-pressure structural phase transition and corresponding changes of physical properties.In this thesis,CaF₂and SrF₂nanocrystals with different sizes were successfully prepared using solvent heat method and LSS synthesis method.The effects of small-size as well as defects on the high-pressure structural phase transition and compressed lattice properties have been revealed by in-situ synchrotron radiation XRD measurements.This thesis mainly includes three aspects as the following:1.High-pressure structural phase transition and compression behavior of CaF₂nanocrystals with average grain size of 23 nm were studied by in-situ synchrotron radiation XRD measurements from ambient to 23.5 GPa.A pressure induced phase transition from cubic fluorite structure to orthorhombicα-Pb Cl₂-type structure occurred at 9.5 GPa.Upon decompression,cubic and orthorhombic phases co-exist at ambient pressure.After calculation,the bulk modulus（B₀）of the cubic and the orthorhombic phase of 23 nm CaF₂nanocrystalline is 103（2）and 78（2）GPa,respectively,of both which are larger than that of bulk CaF₂.This suggests that CaF₂nanocrystals have significantly higher incompressibility compared to bulk counterparts.Further analysis revealed that defects play a dominant role in structural stability of CaF₂nanocrystalline in this work.2.High-pressure structural phase transition and compression behavior of CaF₂nanocrystals with average grain size of 6 nm were studied by in-situ synchrotron radiation XRD measurements from ambient to 32.3 GPa.A pressure induced phase transition from cubic fluorite structure to orthorhombicα-Pb Cl₂-type structure occurred in 6 nm CaF₂nanocrystalline at 12.7 GPa.Compared with 23 nm and bulk counterparts,it is found that both grain size and defect affect the high-pressure structural stability and compression properties of different CaF₂samples.The defects mainly affect the compressibility of cubic structure before phase transition and the pressure of subsequent phase transition.After the phase transition,the size-effect becomes the main factor affecting the compressibility of high-pressure orthorhombic phase.3.High-pressure structural phase transition and compression behavior of SrF₂nanocrystals with average grain size of 29 nm were studied by in-situ synchrotron radiation XRD measurements from ambient to 41.7 GPa.Two pressure induced phase transition from cubic fluorite structure to orthorhombicα-Pb Cl₂-type structure and then to hexagonal Ni₂In-type sequently occurred in 29 nm SrF₂nanocrystalline at 8.3 GPa and 27.5 GPa The phase transition path at high pressure well coincides with the bulk SrF₂.However,the pressure of the 1^ststructural phase transition is higher than that of bulk counterpart due to the size-effect,and the pressure of the 2^ndstructural phase transition is similar mainly caused by the grain fragmentation during compression.Compared with bulk SrF₂,the bulk modulus（B₀）of the 29 nm SrF₂nanocrystalline shows the complex changes including firstly increasing,and then decreasing,finally increasing with the pressure.