Quasicrystals, materials with long-range order that is not translationally invariant, are a unique class of solids that challenge traditional notions of crystal structure. Understanding their atomic structure is a fundamental challenge in materials science and solid-state physics. Positron annihilation studies (PAS) provide valuable information about the electronic structure and defects in quasicrystals, helping to develop more accurate structural models.
IAP is a sensitive method that allows probing the electron density and the presence of vacancies or other defects in a material. Positrons embedded in a quasicrystal annihilate with electrons, emitting gamma quanta. Analysis of the angular distribution or lifetime of these gamma quanta allows obtaining information about the local electron structure and defects.
Our study is devoted to the analysis of the influence of the results obtained by the positron annihilation method on the understanding of the structural models of icosahedral quasicrystals. In particular, Al-Mn, Al-Mn-Si, Al-Cu-Li and Al-Cu-Fe alloys were studied. A common feature for all these icosahedral quasicrystalline phases is the two-component nature of the positron lifetime spectrum.
Analysis of these spectra allows us to estimate the sizes of vacancies, varying from the sizes of a single vacancy to complexes of six vacancies. The concentration of such vacancies in the studied quasicrystalline alloys is in the range from 1 to 8 ppm, which is three orders of magnitude less than in metallic glasses.
It is interesting to note that in metastable quasicrystals such as Al-Mn and Al-Mn-Si, the vacancy clusters disappear upon crystallization, leading to single-component lifetime spectra. However, in stable quasicrystalline alloys, the two-component spectra indicating the presence of vacancy clusters persist even after prolonged annealing.
The obtained data apparently contradict models based on filling space with Penrose or random mosaics, and rather confirm cluster models of the structure of quasicrystals.
Studies using the IAP method have shown that quasicrystals are characterized by a complex electronic structure with a pseudogap at the Fermi level, which explains their low electrical conductivity. The presence of vacancies and other defects affecting mechanical and thermal properties has also been detected.
Comparison of experimental IAP data with the results of theoretical calculations based on various structural models allows us to evaluate the adequacy of these models. Models that best agree with IAP data are considered the most likely candidates for describing the real structure of the quasicrystal.
Thus, positron annihilation studies play an important role in the construction and verification of structural models of quasicrystals, contributing to a deeper understanding of their unique properties and application prospects.
Author: R. Chidambaram, M. K. Sanyal, P. M. G. Nambissan, P. Sen †
Institute: Bhabha Atomic Research Centre, Bombay, 400 085, India, Sahi Institute of Nuclear Physics, Kolkata 700 064, India