Discovery And Crystal Structure Of Al-based Natural Quasicrystal Derivatives

Quasicrystals have been received extensive attention for more than 30 years owing to their unique crystal structures.The discovery of natural quasicrystals in the past 10 years has stimulated further enthusiasm in exploring its formation and stability mechanism.In the present work,compounds in two natural quasicrystal systems Al-Ni-Fe and Al-Cu-Fe were synthesized by high-pressure sintering and spark plasma sintering methods.Single crystal X-ray diffraction technique combined with scanning electron microscopy and first-principles calculation were used to solve,refine and study the complex crystal structure of some quasicrystal derivative phases.Firstly,a new hexagonal Al₁₀Ni₃Fe_0.83 phase?Fe atom partially occupied,occupancy factor 0.834?with chemical composition similar to that of natural quasicrystal Al₇₁Ni₂₄Fe₅was found by high-pressure sintering method during simulating the formation process of natural quasicrystal Al₇₁Ni₂₄Fe₅.Although the new alloy phase has been observed by a research group even before quasicrystal discovery,its detailed crystal structure has not been reported.The parameters of the new phase are refined to be:a=7.6981?2??,b=7.6981?2??,c=7.6231?2??,with space group P63/mmc,which is belonging to the Al₅Co₂ type structure.Secondly,the Al₂Cu phase was observed in precipitates of Al₆₂Cu₃₁Fe₇ phase in natural quasicrystal system synthesized by spark plasma sintering.By analyzing several data sets of single crystal X-ray diffraction data,we found the mechanism of why the?-Al₂Cu phase has been incorrectly indexed to be Owen-Al₂Cu phase nearly a century ago.Based on this assumption,the intrinsic relationship between the crystal structures of?-,??-and?-Al₂Cu phases has been systematically analyzed and two additional new related phases have been constructed.Furthermore,the formation energy,mechanical and dynamical stability as well as electronic structures of the both new Al₂Cu phases have been investigated by first-principles calculations.Finally,an Al₇Cu₂FeC_0.5 phase was found in precipitates of Al₆₂Cu₃₁Fe₇ phase in the natural Al-Cu-Fe quasicrystal system synthesized by spark plasma sintering.Single crystal X-ray diffraction data indicates that the C atoms in the structure occupy the position of the distorted cubic space formed by eight Al atoms in the Al₇Cu₂Fe matrix.First-principles calculations show that the C atom does not change the metallic properties of the parent phase although it is connected to the surrounding eight Al atoms.