Berlin 2008 – wissenschaftliches Programm
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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 33: Molecular Structure
CPP 33.4: Vortrag
Donnerstag, 28. Februar 2008, 15:45–16:00, C 230
Effect of compression rate on re-crystallization of hexagonal ice — •Marion Bauer1, Erwin Mayer1, and Thomas Loerting2 — 1Institute of General, Inorganic and Theoretical Chemistry, University of Innsbruck — 2Institute of Physical Chemistry, University of Innsbruck
We studied the re-crystallization kinetics of ice upon compression under isothermal conditions (170 - 230 K) and characterized the recovered phases by powder X-ray diffraction at 80 K. A competition between crystallization to the thermodynamically stable and a metastable phase was found. The Ice Ih --> Ice II and the Ice Ih --> Ice III transitions are the two competing parallel reactions in the pressure range 0.2 - 0.4 GPa. Stable Ice II is formed at compression rates up to 0.01 GPa/min, predominantly metastable Ice III is formed at compression-rates exceeding 0.1 GPa/min and mixtures of Ice II and Ice III are formed at compression-rates in-between. That is, Ice Ih has two energetically similar ways of adapting the hydrogen bonded network as a reaction to the application of external pressure. Similarly, high density amorphous ice (HDA) shows two parallel ways of crystallization upon heating under isobaric conditions [1]. For both the compression-rate dependent case observed here and the heating-rate dependent case observed earlier [1], the stable ice polymorph shows a slower, albeit earlier growth compared to the metastable polymorph.
[1] Salzmann, Mayer, Hallbrucker, PCCP 6, 5156-5165 (2004).