Twofold surface of the decagonal Al-Cu-Co quasicrystal

Abstract

We have investigated the atomic structure of the twofold surface of the decagonal Al-Cu-Co quasicrystal using scanning tunneling microscopy and low-energy electron diffraction. We have found that most of the surface features can be interpreted using the bulk-structure model proposed by Deloudi and Steurer (S. Deloudi, Ph.D. thesis, ETH, Zürich, 2008). The surface consists of terraces separated by steps of various heights.Step heights and steps sequences match with the thickness and the stacking sequence of blocks of layers separated by gaps in the model. These blocks of layers define possible surface terminations consisting of periodic atomic rows which are aperiodically stacked. These surface terminations are dense (∼10 at./nm2) and are of three types. The first two types are pure or almost pure Al while the third one contains 30-40 at. % of transitionmetal atoms. Experimentally, we observe three different types of fine structures on terraces, which can be interpreted using the three possible types of bulk terminations. Terraces containing transition metals exhibit a strong bias dependency and present a doubling of the basic 0.42 nm periodicity, in agreement with the 0.84 nm superstructure of the bulk. In addition, a high density of interlayer phason defects is observed on this surface that could contribute to the stabilization of this system through configurational entropy associated with phason disorder. We have investigated the atomic structure of the twofold surface of the decagonal Al-Cu-Co quasicrystal using scanning tunneling microscopy and low-energy electron diffraction. We have found that most of the surface features can be interpreted using the bulk-structure model proposed by Deloudi and Steurer ͑S. Deloudi, Ph.D. thesis, ETH, Zürich, 2008͒. The surface consists of terraces separated by steps of various heights.Step heights and steps sequences match with the thickness and the stacking sequence of blocks of layers separated by gaps in the model. These blocks of layers define possible surface terminations consisting of periodic atomic rows which are aperiodically stacked. These surface terminations are dense ͑ϳ10 at. / nm 2 ͒ and are of three types. The first two types are pure or almost pure Al while the third one contains 30-40 at. % of transition-metal atoms. Experimentally, we observe three different types of fine structures on terraces, which can be interpreted using the three possible types of bulk terminations. Terraces containing transition metals exhibit a strong bias dependency and present a doubling of the basic 0.42 nm periodicity, in agreement with the 0.84 nm superstructure of the bulk. In addition, a high density of interlayer phason defects is observed on this surface that could contribute to the stabilization of this system through configurational entropy associated with phason disorder.

Weak bonding of Zn in an Al-based approximant based on surface measurements

“…(2) a Zr-Ni-Cu-Al metallic glass [20], and (3) the two-fold surface of a quasicrystal, decagonal Al-Cu-Co [21]. However, the roughness of the two Al-Pd-Zn surfaces behave differently.…”

Variation of growth morphology with chemical composition of terraces: Ag on a twofold surface of a decagonal Al-Cu-Co quasicrystal

“…Both Al-rich types of terrace are found at the surface but it has been difficult to assign a model termination unambiguously 18 because their compositions and densities are very close, according to the bulk model. Elsewhere, we will present distinct evidence that the Al-rich #1 terraces can be assigned as the 85%-Al termination and the Al-rich #2 terraces as the 100%-Al termination.…”

“…The surface investigated is normal to a twofold ͓10000͔ direction, using the same notation as Duguet et al 18 The sample has a bulk one-dimensional periodicity of 0.84 nm, two times the basic 0.42 nm periodicity of decagonal phases. This is observed in selected area electron-diffraction patterns along ͓00110͔ showing diffuse interlayer lines that reflect the 0.84 nm superstructure.…”

“…21 Additional details about crystal growth and surface preparation can be found elsewhere. 18,20 Benchmark DFT calculations are performed using the plane-wave-based Vienna ab initio simulation package ͑VASP͒ to assess the adhesion of an fcc slab of Ag to fcc slabs of different metal substrates ͑Al, Cu, Co, and Ag͒. The details of our DFT calculations ͑e.g., use of the Perdew-BurkeErnzerhof form of the generalized gradient approximation and of the projector augmented wave approach to describe electron-ion interactions͒ are similar to our previous calculations for Ag films and have been described in detail previously.…”

“…In a recent paper, we elucidated the atomic structure of the surface by comparing experimental low-energy electron diffraction and STM data 18 with a structural model derived from a five-dimensional Patterson analysis of x-ray diffraction data from Deloudi et al 19 Three surface terminations were found to exist, based on different structures present on different terraces. From the bulk model, the surface atomic density of the selected terminations is close to that of Al͑111͒ and the compositions range from 50-60 at.…”