Study On The Phase Transition Of The Alkaline Earth Fluorides Nanocrystals Under High Pressure

It is well known naomaterials exhibit various novel physical and chemicalproperties compared with their bulk counterparts due to their unique properties, suchas size effect, confinement effect and so on. High pressure can effectively change thedistances between molecules and atoms. The changes of structure will do strongeffects on the properties of materials. Thus, high-pressure technology is an importantand effective route for us to understand the structures, properties and even theirrelationship. In recent years, with the rapid development of nanotechnology,nanotechnology combines with high-pressure technology, and it will reveal manyattractive and novel phenomena, structures and mechanics.As the important kind of fluoride, alkaline earth metal MF₂(M= Ca, Sr, Ba) havegreat applications in microelectric, optoelectric, biological and mineralogy due to itsanionic conductivity, high resistivity, high ionicity and low-energy phonons etc.The structural stability and phase transformation of nanocrystals (NCs) haverecently triggered significant interests in fundamental scientific research and practicalapplications in chemistry, physics, materials science and geophysics. At ambientconditions, bulk MF₂crystallizes in the cubic fluorite structure with a space group ofFm3m. The sequence of the pressure-induced phase transition would follow thestructural progression from cubic (Fm3m) fluorite structure to orthorhombic (Pnma)α-PbCl2-type structure to hexagonal (P63/mmc) Ni2In-type structure, which in turn would be characterized by a progression in the cation coordination number from 8 to9 to 11. However, to our knowledge, there has been no report of phase transformationstudy on nanoscale system of alkaline earth metal. In this work, we studied theinfluences on the pressure behavior and compressing characteristics of the differentsizes of CaF₂/ SrF₂/ BaF₂nanocrystals.The alkaline earth fluorides (MF₂) nanocrystals with different size weresuccessfully synthesized by a facile hydrothermal process. The MF₂nanocrystals witha controllable size were successfully synthesized by modulating the temperature andconcentration of the monomers. Moreover, high pressure studies on MF₂nanocrystalswith different size were carried out through in-situ X-ray diffraction and Ramantechniques. We found the size effect do a strong impact on the high-pressurebehaviors of MF₂.The high-pressure behaviors of CaF₂nanocrystals with sizes of 8 nm and 11 nmhave been investigated by angle-dispersive synchrotron x-ray powder diffractionmeasurement up to 46.5 GPa and 28.5 GPa at room temperature. A pressure-inducedfluorite structure (Fm3m) to orthorhombic PbCl2-type structure (Pnma) transitionstarts at 14.0 GPa and 12 GPa, respectively. The differences of transition pressure inCaF₂nanocrystals are strongly depend on their grain size. The high-pressure behaviorof CaF₂nanocrystals show a noticeable size-dependence, the onset of transitionpressure showed a significant increase with decreasing particle size. This studyshowed that a size effect in CaF₂nanocrystals can lead high-pressure metastablematerials at ambient conditions. The bulk modulus of cubic phase in CaF₂nanocrystals strongly depend on their grain size, and the increased bulk modulus ofcubic phase indicates higher incompressibility than bulk CaF₂.The high-pressure behavior of SrF₂has been investigated by angle-dispersivesynchrotron x-ray powder diffraction measurement up to 50.3 GPa at roomtemperature. Under pressures up to 50.3 GPa, SrF₂transforms from the cubic fluoritestructure to an orthorhombic cotunnite-type structure at about 6.8 GPa and then to ahexagonal Ni2In-type structure at 29.5 GPa. After decompression to ambientcondition, a few peaks attributed to orthorhombic phase have been retained, suggesting that the cubic and orthorhombic phase co-exist at ambient pressure. Theresidual stress in sample is not completely released upon decompression, which mightbe the reason of irreversibility at the ambient pressure. We hypothesized that thesample can recover the lower phase completely after set aside a long time. Afterrepeat in situ high-pressure X-ray diffraction and Raman spectroscopy, the phasetransformation is reversibility after complete pressure release.The isothermal bulkmodulus of the cubic, orthorhombic and hexagonal phases increased gradually,indicating higher incompressibility of SrF₂under high pressure.We synthesized SrF₂nanoparticles and nanoplates with an average size of around11 nm and 21 nm in a fluorite-type structure. The high-pressure behaviors ofnanocrystalline SrF₂samples have been investigated by angle-dispersive synchrotronx-ray powder diffraction measurement up to 46 GPa at room temperature. Two phasetransitions from fluorite-type toα-PbCl2-type and Ni2In-type phases of the SrF₂nanoparticles occur at 10 and 34.3 GPa, which is much higher than that in bulk SrF₂(6.8 GPa and 29.5 GPa). Upon decompression, the pureα-PbCl2-type metastablephase is retained when the pressure is released. In contrast, the two phasetransformations of nanoplates occur at 6.3 GPa and 27.7 GPa, and the high-pressurebehavior of SrF₂nanoplates is similar to that observed in bulk material. The SrF₂nanoparticles exhibit amazing pressure responses which are different than those forbulk materials. Decreasing particle size reduces the hosted ratios of defect anddislocation in SrF₂nanoparticles. In SrF₂nanoplates with sizes of 21 nm, the hosteddefect (or dislocation) acts to behave similar to that in bulk. The defect site couldserve as a weak point and induce a stress concentration, so a new phase prefers tonucleate at such a defect site. Therefore, high pressure phase nucleated in SrF₂nanoplates certainly has a reduced nucleation pressure relative to that in SrF₂nanoparticles.The high-pressure behavior of BaF₂nanoplates has been investigated byangle-dispersive synchrotron x-ray powder diffraction measurement up to 21.2 GPa atroom temperature. BaF₂nanoplates transforms from the cubic fluorite structure to anorthorhombic cotunnite-type structure at about 5.8 GPa and then to a hexagonal Ni2In-type structure at 14.4 GPa, which is much higher than that in bulk BaF₂. Upondecompression, the pureα-PbCl2-type metastable phase is retained when the pressureis released. This study showed that a size effect effect in BaF₂nanoplates can lead theenhancement of transition pressure and high-pressure metastable materials at ambientconditions.