Face Six Top Press Ultrasound Measurements And Dac Loaded Cerium Phase Transition Under High Pressure

This dissertation includes two topics:1.developing a pressure calibration technology in a large-volume cubic press under hydrostatic environment;and 2.studying on theγ-αphase transition of cerium,a typical f-electron transition metal,at room temperature.The cubic press is a large volume high pressure apparatus with six anvils.In traditional high pressure ultrasonic measurements on the cubic press,solid materials are commonly used as the pressure media;the produced non-hydrostaticity will affect all of the measurements including the velocity determination in this study.Even in the cubic press with a solid-liquid hybrid pressure medium for ultrasonic measuring reported recently,the traditional phase transition point's technique was still used as the pressure calibration.In order to determine the pressures in a wider range,the extrapolation or interpolation through a few of calibration points has to be used;such a pressure calibration curve is therefore not very reliable.In order to change such a situation,by measuring the sound velocity of a single crystal at high pressure combining with the high pressure Brillouin scattering data,we have explored the precise pressure calibration technique in the cubic press with hydrostatic loading and then employed it in our ultrasonic measurement in this study.Cerium is a typical f-electron transition metal.Due to its f-electrons play an important role,Ce has similar phase transitions and compression properties with many heavy actinides. With an element of non-radioactive and no chemical poison,Ce was used as a model for those materials to be investigated for a long time.However,even about its phase diagram,there are a number of controversies,especially on the position of phase boundaries and phase structures. By using the synchrotron radiation X-ray diffraction(ADXRD) method and the diamond anvil cell(DAC) high pressure technology,we have made a further study on theγ-αphase transition of cerium.The main conclusions are as following:1).By measuring the sound velocity of single crystal MgO at high pressures and using the high pressure Brillouin scattering data,we have calibrated the pressures point by point in a wide pressure range in the cubic pressure cell under a hydrostatic environment,and furthermore,have established a new pressure calibration technique for the large volume pressure cell.This new calibration method is more reliable than the traditional one which only depends on 2 or 3 phase transition points.As an example,the measured sound velocity on LY12 Al under this hydrostatic environment loading is in good agreement with references, and it further confirmed the new pressure calibration method. Furthermore,we have programmed a computer code to control the oscilloscope to collect data synchronously.This program control technique can keep oscilloscope to take samples in step with setting pressure-control curve within a time error inside of 2 seconds. This time error is far small than those of manual operation.Moreover,the distortion of ultrasonic echoes from different reflecting boundaries has been discussed.2).A possible new phase of Ce(γ-Ce) at room temperature and high pressures has been suggested by indexing weak diffraction peaks from background.At room temperature,theγ-αphase transition begins at about 0.69 GPa.As pressure up to 0.89 GPa,the diffraction intensities ofγ-Ce andα-Ce are about in the same.At about 1.15 GPa,the transition ofγ-Ce toγ'-Ce was observed.Althoughγ-Ce toγ'-Ce have the same fcc structure,their equation of states or P-V curves are distinctly different with each other.Theγ'-Ce coexist withα-Ce up to the highest pressure of this run,4.27 GPa.By unloading,theγ'-Ceγ-Ce,andα-Ce recovered to their initial state along with the same P-V curve as pressure loaded up.