| Cerium(Ce)is a typical non-radioactive f-electron transition metal,being a research focus of high-pressure physics and strongly correlated electron fields due to its unique electronic structure and condensed matter phase polymorphism.In addition,Ce is considered as an ideal module for study of high pressure properties of heavy actinides because of their similar high pressure properties.There are several controversies about Ce under high pressure,such as phase transition route,phase transition mechanism,phase diagram,and phase boundary according to the experimental and theoretical reports.In this work,high-resolution synchrotron angle-dispersive powder X-ray diffraction technique was used to study the phase transition of Ce.An encapsulation method for the sample in DAC was used for this easily oxidized sample.Two structural transition sequences of Ce were observed around 5 GPa,??????"?? for regular compression and ?????+?"??+?'+?"??'?? for pre-compression.The complex structural transformation was clearly observed from 4.64 to 5.4 GPa for the first time.The pure ?'-Ce phase was observed at 5.4 GPa,far less than the pressure in early work.The phase transition is due to the small grains of ?-Ce in the process of pre-compression.The small grains are conducive to the nucleation of a'-Ce.Meanwhile the isothermal equation of state of Ce up to 90 GPa is B0= 70.85±2.28 GPa,V0=24.04±0.17 A3 and B0'=4,which enriched the data of Ce at high pressure.The structural phase transitions of Ce metal in different pressure mediums up to 90 GPa were studied by using high-resolution synchrotron angle-dispersive powder X-ray diffraction.No chemical reaction at high-pressure between Ce and pressure mediums have been observed.It is found that the c axis of s-Ce is more easily compressed than the other pressure mediumd for the helium is a better hydrodynamic pressure medium.While the compression rates of the a axis in the various pressure mediums are the same.When helium is the pressure medium and the pressure is higher than 53 GPa,the value of c/a is constant 1.6775 ± 0.0003,indicating the compression rate of the c and a axis are close.While in other pressure mediums,the value of c/a fluctuates largly,which may be due to the poor hydrostatic,and the larger shear stress.High-resolution synchrotron angle-dispersive powder X-ray diffraction technique was used to study the phase transition of Ce-10wt.%La,Ce-20wt.%La and Ce-50wt.%La alloys.The structure and stability of the Ce-La alloy were analyzed by combining the CALYPSO structure search program and the first principle calculation.Ce-10wt.%La alloy has the same phase transition sequence with Ce,?+?(ambient)??(1.8GPa)??"(6.9GPa)??(14.0GPa).The pressures of phase transition points are higher than that of Ce,for f electrons play a key role in the phase transition of Ce under high pressure.The addition of La elements causes the f electron proportion in the alloy to decrease,resulting in phase transition requiring higher energy.The phase transition sequence of Ce-20wt.%La alloy is similar to the phase transition behavior of La at high pressure,dhcp(ambient)?fcc(1.98GPa)?dfcc(R3m,7.78GPa)?unknown phase(25.44GPa).The phase transition sequence of Ce-50wt.%La alloy is similar to the phase transition behavior of La at high pressure.The high pressure phase transition sequence is dhcp(ambient)?fcc(2.79GPa)? dfcc(R3m,8.41GPa)?fcc(25.1GPa).Upon pressure release to ambient condition,Ce-La alloy with different components all recover to fcc srtucture.The crystal structure of Ce-50wt.%La alloy in the range of 10-25 GPa is R3m,which is demonstrated by CALYPSO structure search software and ADXRD experiment data.The structural phase transition of Ce and Ce-La alloy at high pressure are analyzed using cluster model.By analyzing the bond length,bond angle and atomic arrangement in the clster model,the transition process of Ce and Ce-La alloy are explained well.A crystal plane named as the base-plane was observed during the phase transition of Ce.The atoms in the base-plane only do the movement in the plane,without the movement outside the base-plane.The crystallography relationship of each phase before and after the transformation is re-constructed by using the base-plane.The phase transition process of Ce and Ce-La alloy under high pressure have been better understood by using this cluster model.A diffractometer with W K?l characteristic X-ray as the light source was designed.This special energy monochrome method based on wave-particle dualism of X-ray using CdTe detector is applied in this diffractometer.The preliminary application is carried out in high pressure phase transformation.The results of high pressure phase transition of Bi are observed,and it is confirmed that the characteristic X-ray diffraction of W K?l can be applied to the phase trasition research under high pressure.The phase and stress characterization of 2024-T351 aluminum alloy friction stir welding zone were carried out by using this diffractometer.For the 2024-T351 aluminum alloy with grain sizes ranging from several microns to several tens of micrometers,the diffraction angle and its half height width are determined by its internal stress,independent of its grain size.This method of diffraction in crystal structure,residual stress,texture and other applications have shown good potential. |
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