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M: Metallphysik

M 8: Quasikristalle II

M 8.3: Vortrag

Montag, 24. März 2003, 12:15–12:30, IFW B

Atomic scale observation of phase transitions in complex solids: Quasicrystals — •W. Sprengel¹, K. Sato¹, F. Baier¹, R. Würschum², and H.-E. Schaefer¹ — ¹Institut für Theoretische und Angewandte Physik, Universität Stuttgart, 70550 Stuttgart — ²Institut für Technische Physik, Technische Universität Graz, 8010 Graz, Österreich

In the present paper we demonstrate that order-disorder phase transitions in complex solids as decagonal Al_71.5Ni₁₄Co_14.5 quasicrystals can be studied specifically on an atomic scale by making use of positrons as local probes for chemical surroundings. Positrons are trapped in the structural vacancies of quasicrystals as demonstrated by positron lifetime studies. The chemical surroundings of these vacancies in Al-Ni-Co quasicrystals are shown to change drastically with temperature in two reversible stages at 650 K and 1140 K as evidenced by the coincident Doppler broadening of the positron-electron annihilation photons. These studies show that at ambient temperature preferably transition metal atoms are neighboring the vacancies as expected for the central cluster of the ordered quasi-unit-cell decagons. Near the melting temperature, Al atoms appear to surround the vacancy as anticipated for the disordered structure. The 1140 K stage can be quantitatively correlated to the structural order-disorder phase transition recently detected by diffuse neutron scattering. The present investigations are of importance for future atomic-scale studies of phase transitions in complex systems and are therefore of wide interest.