The Impact Of High Pressure On The Crystal Structure Of Crystalline Sphene And Radiation-damaged Sphene

Titanite(CaTiSiO5)is proposed as a host phase for the immobilization of radioactive nuclides,especially high level of nuclear wastes,due to its good anti-radiation properties.High pressure research on titanite is quite limited so far,and focused on investigations of static pressure-induced structural phase transitions.It is of scientific significance to study the behavior of titanite under high pressure shocking,because of its lack of researches.Furthermore,many current researches on the high-pressure response of substances is mostly limited to crystalline materials.There is a lack of systematic research on the structure changes of radiation damaged materials that are subjected to high pressures,especially under high shock-wave pressures.Hence,the possible structure changes of radiation damaged titanite under high pressure impact is taken into consideration for depth understanding of the physical and chemical behavior of the radioactive nuclides host phase under extreme conditions.Radiation damaged titanite generally contain high levels of water or hydrogen relative to their crystalline state.The effect of high pressure shock on the diffusion behavior of hydrogen in radiation damaged titanite is also of our research interest.The Two-stage Light Gas Gun Technique was utilized to perform high-pressure experiments on synthetic crystalline titanite and radiation damaged titanite.The shocked titanites were then characterized by X-ray diffraction,infrared and Raman spectroscopy.This study has four main objectives:1.Exploring the effects of high pressure impact on structural changes of synthetic crystalline titanite and natural radiation damaged titanite;2.Investigating the similarities and differences of amorphous structure and spectroscopic characteristics in titanite induced by high-pressure shocking and radiation damaged;3.Comparing high-pressure shock and high-temperature annealing effects on radiation damaged titanite and their similarities and differences of crystal structure changes;4.Studying the behavior of hydrogen in natural radiation damaged titanite under high-pressure shocking.The results show that:(1)Although the two extreme conditions(shock and radiation)can both produce crystal lattice damage of crystalline titanite,as seen by peak intensity decreasing,line boarding and losing spectral details in X-ray diffraction patterns,infrared and Raman spectra,there are distinct differences between their specific processes and damage mechanism.On one hand,the main peak of Ti-O stretching vibration shifts down,which is opposite to that of the radiation damage.On the other hand,the reduction of unit cell parameters of a,b,c and cell volume V is in contrast to the radiation damage process.(2)X-ray diffraction spectrum of shocked damaged titanite indicates that high pressure shocking causes further impacts on the atomic long-range order of the metamict state of titanite.However,Raman and infrared analysis shows a shocking-induced partial recrystallization in radiation damaged titanite,as evidenced by an increase in band intensity,line sharpening and more spectral details.Especially,the damaged Ti-O stretching vibration in infrared spectrum are also evidently recovered,which is unachievable by high-temperature anneal,This observation,further indicates that the high-pressure induced partial recovery of the damaged titanite is different from that of high temperature in terms of the process and mechanism.The Raman Ti-O stretching vibration shifts from 646 cm-1 in metamict state to 605 cm-1 in crystalline state,which also proves that the high pressure shock causes radiation damaged titanite to recrystallization.Based on the changes of infrared band intensity and the Raman band shift of Ti-o stretching vibrations,it can be concluded that the pressure which causes the damaged titanite to recrystallize is between 23.4 and 27.2 GPa.At higher impact pressure of 49.1 GPa,Raman spectra appear to have some additional unknown peaks,which are not present in titanite of P21/a and A2/a phases.This could be indicative of shock-induced unknown new phase or phases.(3)This present work found that the high pressure impact process may cause some H in amorphous areas migrate to crystalline areas locating in O1 site and,forming a dipole vibrating along the[100]direction.To some degree,this is similar to the high-temperature tempering process.However,the total amounts of H involved in the diffusion process in high pressure shocking is very small according to infrared data,This could be due to the fact that the shock process(in ms level)is too fast for most H to migrate.The effect of high-pressure shock on titanite is a rather complicated process which needs further investigations.The physical mechanism for the partial structural recovery or lattice repair of the damaged titanite during the high pressure impact needs more in-depth research.