DPG Tagung, O27, Phasenübergänge , Berlin 2005

Surface freezing and surface phase transition studied at the liquid eutectic AuSi surface

1. Surface layering at liquid/vapor interface of metals/alloys2. Surface freezing of AuSi above bulk melting temperature3. Solid-solid surface phase transition

Reinhard Streitel, Oleg Shpyrko, Alexei GrigorievSupervisor: Prof. P. Pershan, DEAS, Harvard University

Surface-Induced Layering

Disordered Interface (classical Van-der-Waalsdescription 1873)

Ordered Interface Surface-Induced Layering(perdicted 1984, S. A. Rice)

Liquid-vapor interface

Layering observed in liquid metals and alloys

Experimental measurements (NSLS & APS):

Simple Liquids (Pershan Group, 1995-2004)Gallium, Mercury, Indium, Potassium, Tin

Binary Liquids(Pershan, Rice, Freyland groups;1997-2004)GaIn, BiIn, GaPb, BiPb, HgAu, GaBi,KNa, AuGe, BiSn, AuSi

Scattering geometry

layering peak: (a atomic radii)

a ~ 1.0–1.5Å

qzpeak = /a

qzpeak ~ 2.0-3.0 Å-1

Reflectivity of eutectic AuSi (1)

Eutectic compositionAu0.81Si0.19 (atomic %)TM (bulk) = 361-363°C

Phase diagram AuSi Reflectivity @ different temperatures:

@ T =371°C (about 10 °C above TM (bulk) !) change in reflectivity for high qz

TM(Au)=1063°C TM(Si)=1410°C

Transition at TM + 10 °C

Surface-Normal (Layering) Structure

Normalized X-ray Reflectivity R/RF Electron Density Profile

Ele

ctro

n D

ensi

ty

z, Angstroms

- Reflectivity and structure related through Fourier Transform- Width / Height of quasi-Bragg peak determined by layering decay length

Reflectivity of eutectic AuSi (2)

Surface Phase Transition at 371oC

First-Order TransitionReversible in Temperature

In-Plane Structure: X-GID

Surface freezing and surface phase transition

Surface freezing- Surface freezes above bulk melting point (!): 2D solid surface layer coexists with liquid bulk phase

- Surface freezing in other systems e.g. dilute Pb on liquid Ga (S. A. Rice) BUT surface and bulk chemically different

S.A. Rice et al., Phys. Rev. B 62,

13111-13120 (2000)

liquid Ga

Pb

Structure model: low temperature phase AuSi2

7.4

9.4

12 atoms4 Au, 8 Si

Au

Si

Summary

• surface layering observed in liquid metals and alloys

• liquid AuSi eutectic shows temperature dependence in reflectivity for high qz

• a reversible solid-solid phase transition occurs at about 10 °C above bulk melting temperature

• 2D AuSi2 structure describes high temperature phase GID peaks

future: • reflectivity at even higher temperature• understanding details of structural changes

Acknowledgments

For more info visit http://liquids.deas.harvard.edu

Harvard: Prof. Peter Pershan

Oleg Shpyrko

Alexei Grigoriev

CARS, U. Chicago:

Jim Viccaro, Binhua Lin

Tim Graber, Mati Meron

Jeff Gebhardt

Ben OckoProf. Mosche Deutsch

Research was supported by US DOE grant (DE-FG02-88-ER45379). The work was performed in ChemMatCARSsector of Advanced Photon Source, ArgonneNational Lab. ChemMatCARS at the APSis supported by NSF/ DOE grant numberCHE0087817.

Bar-Ilan, Israel:

Brookhaven: