Quasielastic Mossbauer scattering in stable icosahedral Al-Cu-Fe quasicrystals

Quasicrystals can show a new type of localised defect termed a "phason". These defects are important for the understanding of both the structure and the atomic motion (diffusion). These defects lead to short-ranged atomic motion without involving vacancies and confined in space. We have studied icosahedral (Al62CU25.5Fe12.5)-Fe-57 using quasielastic Mossbauer spectroscopy for T less than or equal to 1100 K. We fmd that the Debye Waller factor f(T) of the central elastic (quadrupole-split) line decreases abruptly below the Debye result f(D)(T) for T > 600 K. But there is an additional quasielastic signal which correlates with the appearance of this anomaly. This is due to atomic motion of the Fe atoms from phason jumps. The electric field gradient shows a different dependence on temperature, decreasing only slowly up to about 800 similar to K, but then much faster. These results give the temperatures where phason motion (greater than or equal to 600 K), and then vacancy motion (greater than or equal to 800 K), strongly affect the Mossbauer spectrum.