Newsletter 107 - October 2011 - (pdf - 0.6 MB) - Psi-k

Large variation of vacancy formation energies in the surface of
crystalline ice
M. Watkins 1,2,3 , D. Pan 4 , E. G. Wang 5 , A. Michaelides 1,2,3 , J. VandeVondele 6
& B. Slater 1,3
1 Department of Chemistry, Christopher Ingold Building, 20 Gordon Street,
University College London, London WC1H 0AJ, UK
2 London Centre for Nanotechnology, University College London,
London WC1H 0AJ, UK
3 TYC@UCL, University College London, London WC1H 0AJ, UK
4 Institute of Physics, Chinese Academy of Sciences,
PO Box 603, Beijing 100190, China
5 School of Physics, Peking University, Beijing 100871, China
6 Institute of Physical Chemistry, University of Zurich,
Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
Abstract
Resolving the atomic structure of the surface of ice particles within clouds, over the temperature
range encountered in the atmosphere and relevant to understanding heterogeneous
catalysis on ice, remains an experimental challenge. By using first-principles calculations,
we show that the surface of crystalline ice exhibits a remarkable variance in vacancy formation
energies, akin to an amorphous material. We find vacancy formation energies as
low as 0.10.2eV, which leads to a higher than expected vacancy concentration. Because a
vacancys reactivity correlates with its formation energy, ice particles may be more reactive
than previously thought. We also show that vacancies significantly reduce the formation
energy of neighbouring vacancies, thus facilitating pitting and contributing to pre-melting
and quasi-liquid layer formation. These surface properties arise from proton disorder and
the relaxation of geometric constraints, which suggests that other frustrated materials may
possess unusual surface characteristics.
(Published in Nature Materials )
Contact person: b.slater@ucl.ac.uk
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