Dynamics of Phason Fluctuations in thei−AlPdMnQuasicrystal

Abstract

We report on the study of the dynamics of long wavelength phason fluctuations in the i-AlPdMn icosahedral phase using coherent x-ray scattering. When measured with a coherent x-ray beam, the diffuse intensity due to phasons presents strong fluctuations or speckles pattern. From room temperature to 500 degrees C the speckle pattern is time independent. At 650 degrees C the time correlation of the speckle pattern exhibits an exponential time decay, from which a characteristic time tau is extracted. We find that tau is proportional to the square of the phason wavelength, which demonstrates that phasons are collective diffusive modes in quasicrystals, in agreement with theoretical predictions.

Comparative study on internal friction in an Al-Pd-Mn icosahedral quasicrystal and its crystal approximants

“…16 Experimentally, the internal friction of icosahedral quasicrystals was first measured in i-Al-Pd-Mn single-crystal samples by Feuerbacher et al 17 They observed two major peaks, which were attiributed to two different phason relaxation mechanisms, i.e., those involving local individual phason jumps and collective correlated phason jumps, respectively. 12,17 It has been shown that the activation parameters in the latter agree well with those evaluated for the relaxation processes of the phason wall in i Al-Pd-Mn by Feuerbacher et al 12 and of the phason waves in i Al-Pd-Mn by Francoual et al 11 Crystal approximants [18][19][20] to a quasicrystal are the crystalline phases whose local structure is similar to that of the quasicrystal. These crystal approximants have played important roles in the investigations of the physical properties of quasicrystals.…”

“…These parameters indicate that peak B originates in a relaxation process with a collective atomic motion involving many atoms, as pointed out by Feuerbacher et al 17 Collective and correlated phason jumps are most plausible as the mechanism for it. Phason fluctuations accompanied by collective and correlated phason jumps in i Al-Pd-Mn were investigated by Francoual et al 11 using coherent x-ray scattering and time-correlation spectroscopy. They observed the relaxation of phason waves with characteristic times ranging from 20 to 70 s at 923 K, and they evaluated the activation enthalpy to be 3 Ϯ 1 eV.…”

“…In fact, phason waves or phason fluctuations in quasicrystals are accompanied by collective atomic jumps and their relaxation proceeds with a slow diffusive process involving thermally activated atomic jumps. Francoual and her co-workers 11 have investigated the dynamics of phason modes in i Al-Pd-Mn by coherent x-ray scattering and time-correlation spectroscopy. They observed the expected -dependent decay of phason waves with decay times ranging from 20 to 70 s at 923 K and estimated the activation enthalpy to be 3 Ϯ 1 eV.…”

Structure determination of quasicrystals

“…The diffuse scattering measurement alone can not indicate however if phason modes are really dynamical diffusive modes or just quenched in fluctuations. Using coherent X-ray diffraction it was possible to demonstrate in the i-AlPdMn quasicrystal that phason modes are diffusive modes for temperature larger than 500°C [42], whereas fluctuations are quenched in for temperature below 500°C.…”

Elasto-Dynamics of Quasicrystals

“…This idea originated from Bak [12,13] who indicated that the phason describes particular structure fluctuations (or structure disorders) in quasicrystals, and it can be viewed as a six-dimensional space description; (2) The phason field w is diffusion rather than wave propagation, so phasons play different roles to phonons in the hydrodynamics of quasicrystals. This idea can be found in Lubensky et al [52], Francoual et al [53] and Socolar et al [54], etc. They claimed that the phason field w represents diffusion rather than wave propagation; thus, phasons play different roles than phonons in the hydrodynamics of QCs.…”