Study On The High Pressure Structural Phase Transition Of BeB₂,and High Pressure Synthesis Of Al_0.5Fe_0.5

In this paper, ab initio evolution simulations have been done with the USPEX(Universal Structure Predictor: Evolutionary Xtallography) code to explore all possiblemetastable compounds of BeB2. Four new phases have been uncovered with space groupof Cmcm, Pmma, Amm2and P31m. Among them, the new phase with Cmcm symmetryhas the lowest enthalpy than all known stable phases when the pressure is below13GPa.When the pressure exceeds13GPa, Cmcm phase transition into the known phase F43mphase. The formation enthalpy, phase transition, elastic constants and the phonon spectrumhave been calculated and confirming the stability of these BeB2phases on thethermodynamics and kinetics. While the Imma3, Fdm and P6/mmm phases mentioned inprevious works are found to be dynamic unstable by the phonon spectrums. The hardnessof Cmcm and Pmma phases is predicted to be36.8GPa and38.9GPa, respectively, by asimple empirical hardness formula. These hardness values are approaching the criticalvalue of superhard material40GPa. The calculated band structures and density of statesreveal that these new phases are metallic. Further analysis on the Mulliken population andcharge density difference maps reveal that the electron transfer from Be atom to B atomand B-B bonds, which maybe the origin of calculated high hardness for Cmcm phase andPmma phase. The calculated formation enthalpy showed that the increasing pressure is notfavourable for synthesis of new phase Cmcm. The Vickers hardness of Cmcm phase wascalculated using three empirical models. By analyzing the Crystal Orbital HamiltonPopulation (COHP) diagrams, it was determined that the total interaction of the Be-Bbonds is stronger than that of the B-B bonds, indicating a very complex bonding feature inthe new phase. The superconductivity has been studied as well, it was predicted that thenew Cmcm phase is nearly absent of superconductivity.Conducting the high temperature and high pressure experiment by using cubic press,obtained the optimal process conditions of the preparing bulk material by constantlyadjusting experiment parameters, such as temperature, pressure and time, and finallysuccessfully synthesized the bulk with main phase of Al0.5Fe0.5, and the organizationstructure is homogeneous and compact. The composition and microstructure of the synthetic phase is determined by test means including metallographic analysis, XRD,SEM and TEM. The hardness testing is performed to measure micro hardness andRockwell hardness of the synthetic phase, which was found to be549HV and46HRV inthe testing. And also, the friction and wear testing was carried out, found that the syntheticphase has a certain friction and wear resistance.