【作者】 王婧姝；

【导师】 崔啟良；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2012， 博士

【摘要】 碱土金属氟化物在高压下的结构行为十分丰富，是研究物质在极端条件下结构和物性的重要研究对象。利用高压技术来研究纳米材料的高压相变行为和压缩行为，是崭新的而且具有挑战的课题之一。本论文利用水热合成法制备不同尺寸的形貌均一的单分散碱土金属氟化物纳米晶。采用原位金刚石对顶砧高压实验技术对不同尺寸的碱土金属氟化物纳米晶的原位高压结构进行了研究，揭示纳米材料小尺寸效应对物质结构相变行为的影响规律。1.利用水热合成法制备单分散碱土金属氟化物纳米晶，通过控制反应温度、反应物浓度等条件制备不同粒径、形貌均一的纳米晶。2.利用原位高压同步辐射X-ray衍射技术，研究了粒径为8 nm和11 nmCaF₂纳米颗粒的高压相变行为。发现两种纳米晶相变压力点的差异是由于材料的尺寸不同引起的，两种纳米晶粒高压相变行为体现出了纳米小尺寸效应，表现出明显的尺寸相关性，尺寸越小其相变压力点越高。3.利用原位高压同步辐射X-ray衍射技术和高压Raman技术，对SrF₂体材料的高压相变行为和压缩行为进行了研究。分析了包括传压介质，实验温度等因素对SrF₂高压相变行为的影响。4.采用同步辐射X-ray衍射技术研究SrF₂纳米颗粒和SrF₂纳米板的高压相变行为。对晶格膨胀、样品暴露面和缺陷作用等影响这两种纳米晶高压行为差异的原因进行了探讨。5.采用同步辐射X-ray衍射技术研究BaF₂纳米板的高压相变行为。探讨影响BaF₂纳米材料相变行为主要因素。更多还原

【Abstract】 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.更多还原