The article presents the results of Mossbauer studies of a stable icosahedral quasicrystal Al63. 5Cu24Fe12. 5,conducted at high pressures up to 12 GPa and at room temperature. Analysis of the Mossbauer spectra provides information on the local electronic structure and dynamics of iron atoms in a quasicrystalline lattice. The obtained data show that with increasing pressure, the distribution of electric field gradients (EPG) on iron nuclei changes, which indicates a rearrangement of the local atomic environment. Possible mechanisms of changes in the electronic structure and local dynamics of iron atoms under pressure are discussed.
Quasicrystals (QCs) are metal alloys with a long – range order, but do not have translational symmetry, unlike ordinary crystals. Their discovery in 1984 by D. Shechtman was a revolutionary event in materials science. Of particular interest are CCS based on aluminum and transition metals (for example, Al-Cu-Fe), which have unique physical properties, such as high hardness, low coefficient of friction, high corrosion resistance, and unusual electronic properties.
The study of the behavior of QCs at high pressures is an important area of research that provides information about their atomic structure, stability, and phase transitions. High pressure can induce structural changes that affect the electronic and magnetic properties of CC. Mossbauer spectroscopy is a sensitive method for studying the local electronic structure and dynamics of iron atoms in various materials, including QCs. The use of Mossbauer spectroscopy at high pressures provides a unique opportunity to study changes in the local environment of iron atoms in QCs under the influence of all-round compression.
A stable icosahedral quasicrystal Al63.5Cu24Fe12. 5 obtained by arc melting followed by annealing to improve the degree of quasicrystalline ordering was chosen as the object of research. The quality of the sample was monitored by X-ray diffraction. The Mossbauer spectra were measured on a constant acceleration spectrometer with a 57Co(Rh) source at room temperature. The pressure was generated in a diamond-type high-pressure chamber. A mixture of methanol and ethanol in a ratio of 4:1 was used as a pressure transfer medium. The pressure was determined by the shift of the R1 luminescence line of rubin. The Mossbauer spectra were processed using a program based on the least squares method, using the distribution of quadrupole splits.
The Mossbauer spectra of Al63. 5Cu24Fe12. 5 at atmospheric pressure are a superposition of several quadrupole doublets, which indicates the presence of different local environments of iron atoms in the quasicrystalline structure. The spectrum parameters (isomeric shift, quadrupole splitting, and line width) are consistent with the data obtained earlier for this compound.
Changes in the shape of the Mossbauer spectra are observed with increasing pressure. In particular, there is a broadening of the lines and a change in the relative contribution of different quadrupole doublets. These changes indicate that pressure affects the electronic structure and local geometry around the iron atoms.
Analysis of the distribution of quadrupole splits (P(EQ)) shows that as the pressure increases, P(EQ) shifts towards higher values of EQ. This may be due to a change in the electron density on the iron core caused by lattice compression and charge redistribution between the Al, Cu, and Fe atoms. In addition, an increase in the average EQ value may indicate an increase in the asymmetry of the local environment of iron atoms.
It is important to note that in the studied pressure range (up to 12 GPa), there are no sharp changes in the Mossbauer spectra that could indicate a phase transition. This indicates a high stability of the quasicrystalline structure of Al63. 5Cu24Fe12. 5 at high pressures.
It is assumed that changes in the local electronic structure and dynamics of iron atoms under pressure are caused by several factors. First, lattice contraction leads to an increase in the overlap of the electron shells of neighboring atoms, which affects the electron density on the iron core. Second, pressure can induce a change in the local geometry around the iron atoms, which also leads to a change in the electric field gradient. Third, it is possible to change the valence state of iron under pressure, which can also affect the parameters of the Mossbauer spectrum.
Mossbauer studies of a stable Al63. 5Cu24Fe12. 5 quasicrystal at high pressures have shown that pressure affects the local electronic structure and dynamics of iron atoms. An increase in pressure leads to a change in the distribution of electric field gradients on iron nuclei, which indicates a rearrangement of the local atomic environment. No phase transitions are observed in the investigated pressure range, which indicates a high stability of the quasicrystalline structure. Further studies, including theoretical calculations of the electronic structure, are necessary for a detailed understanding of the mechanisms of changes in the properties of quasicrystals under high pressure.
Author: Ajay Gupta, Neelima Paul, V. Vijaykumar, B.K. Godwal
Institute: DAEF Inter-University Consortium, University Campus, Khandwa Road, Indore 452 017, India, High Pressure Division, Bhabha Atomic Research Center, Trombey, Mumbai 400 084, India