Koichiro Umemoto and Renata Wentzcovitch
University of Minnesota
wentzcov@cems.umn.edu
The effects of pressure on the structure of ice XI - an ordered form of the
phase of ice (Ih) which is known to amorphize under pressure - are investigated
theoretically using density-functional theory. We find that pressure induces
a
mechanical instability which is initiated by the softening of an acoustic phonon
occurring at an incommensurate wavelength, followed by the collapse of the entire
acoustic band and by the violation of the Born stability criteria. It is argued
that phonon collapse may be a quite general feature of pressure-induced
amoprphization. The implications of our fndings for the amorphization of ice
Ih
are also discussed.