Debye Temperature at the Fivefold- and Threefold-Symmetry Surface of the Al-Pd-Mn Quasicrystal.
- PDF / 2,244,285 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 115 Downloads / 186 Views
LL8.7.1
Debye Temperature at the Fivefold- and Threefold-Symmetry Surface of the Al-Pd-Mn Quasicrystal. R. Lüscher, T. Flückiger, M. Erbudak, and A. R. Kortan1 Laboratorium für Festkörperphysik, ETH Zürich, CH-8093 Zürich, Switzerland. 1 Guest Scientist. ABSTRACT Specular-beam intensity profiles are measured in low-energy electron diffraction as a function of temperature for the fivefold- and threefold-symmetry surfaces of the icosahedral quasicrystal Al-Pd-Mn. The resulting shifts of the Bragg-peaks to lower energies are accounted for mainly by a lattice expansion perpendicular to the surface and allow us to determine the thermal expansion coefficient of atomic layers to be (24.5 ± 2.0) × 10-6 K-1 for both surfaces. As a result of several measurements, a surface Debye temperature of 298 ± 7 K is found for the pentagonal and 301 ± 10 K for the threefold-symmetry surface. This similarity implies comparable bonding forces for the atoms at these surfaces. Additionally, the derived thermal expansion coefficient supports a layer-based structural model for both surfaces investigated here.
INTRODUCTION Quasicrystals (QC’s) form an intermediate state between ordinary crystals and disordered amorphous structures. They lack periodicity, yet possess long-range orientational order of crystallographically forbidden symmetries. Many of them exhibit icosahedral symmetries possessing twofold- (2f), threefold- (3f) and fivefold-symmetry (5f) directions. Because of their unique physical and chemical properties such as low friction and adhesion coefficients QC’s have become promising coating materials and have attracted much interest over the last years. These observations involve processes and phenomena occurring at the surface and are not essentially equal for different surfaces of the same material. Shen and coworkers have investigated the three high-symmetry surfaces of the Al-Pd-Mn in terms of structural stabilities, showing that they are all compatible with quasicrystallinity, when annealed at sufficiently high temperatures [1]. A related question is whether all surfaces are bulk-terminated. Papadopolos et al. have demonstrated the bulk termination for the pentagonal surface using high-resolution STM combined with a geometrical structure model [2]. The 3f surface has been investigated by means of STM by Rouxel and coworkers, revealing a rougher yet similar terrace-like structure as observed on the pentagonal surface [3]. Recently, a maximum density rule as an extension of Bravais rule has also been introduced in order to obtain densest bulk-terminated surface layers for all high-symmetry surfaces of Al-Pd-Mn [4]. It could be theoretically shown that the 5f termination is the densest and most stable termination, followed by 2f and 3f . In agreement with this, several groups have reported the evolution of facets on these surfaces [3]. Another quantity associated with the structural stability is the Debye temperature θD. In the Debye theory for crystals, θD corresponds to the highest normal mode of vibration. It reflects the stre
Data Loading...
