INVESTIGATION OF THE SURFACE COMPOSITION AND STRUCTURE OF AN AlPdMn QUASICRYSTAL
USING MEDIUM ENRGY ION SCATTERING
T.C.Q. Noakes*, P.
Bailey*, C.F. McConville$, C.R. Parkinson$, J. Ledieu&
, R. McGrath&
* CLRC Daresbury Laboratory,
Daresbury, Warrington, WA4 4AD
$ Department of Physics, University of Warwick, Coventry,
CV4 7AL
& Surface
Science Research Centre, University of Liverpool, Liverpool, L69 3GH
^
Ames Laboratory, Iowa State University, USA
Work is reported on
the analysis of a Al0.71Pd0.20Mn0.09 5-fold
symmetric quasicrystal using the technique of medium energy ion scattering
(MEIS). This technique can provide both compositional and structural data
with near monolayer resolution in the surface (1-2 ML) and near surface(10-20
ML) region. Previous STM studies of AlPdMn quasicrystals have shown that
long anneal times at temperatures close to the dissociation temperature are
required to produce a flat well ordered surface[1]. Complimentary MEIS data
have been recorded to study the effect of temperature on this material and
determine details of the surface structure obtained. MEIS experiments are
normally carried out using a beam of energetic ions (in this case 100keV
He+) directed along a known crystallographic direction. In order
to determine suitable directions, VEGAS simulations were carried out using
a 14,000 atom slab with appropriate stoichiometry and a face centred icosahedral
(FCI) structure. Several large channels were identified which could be located
experimentally and were subsequently used for the acquisition of data. Compositional
analysis of the sample after annealing showed a progressive decrease in the
near surface Pd content with a coincident increase in the Mn signal until
for the highest anneal the composition was at the limit of the stoichiometry
consistent with the FCI structure. Simultaneous structural analysis indicated
a decrease in the crystallinity of the near suface region even though the
surface itself retained its order as evidenced by the LEED pattern. Compositional
data for the very surface showed Al enrichment at all temperatures. MEIS
data indicated that the surface was isostructural with the bulk, but with
small changes in the angular positions of the blocking dips. These indicate
a significant contraction in the top layer spacing consistent with previous
LEED [2] and SXRD [3] measurements.
[1] J. Ledieu et al,
Surf. Sci. 433-435 (1999) 666
[2] M. Gierer et al,
Phys. Rev. Lett. 78 (1997) 467
[3] M.J. Capitan et
al, Physica B 283 (2000) 79