The effects of pressure on the structure of ice XI were investigated using density-functional theory. The structure of ice Ih consists of a hexagonal diamond lattice of oxygen atoms with one hydrogen atom placed at random in one of the two energy minima existing in between every pair of oxygens. A parallel was drawn with the behavior of ice Ih and of α-quartz, systems that undergo pressure-induced amorphizations. The results show that pressure induces a mehanical 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.
Bibliographical noteFunding Information:
S. B. thanks Paolo Giannozzi for his pristine interest in pressure-induced amorphization in silica and for the work done together on this subject. This research was sup?>ported by NSF Grants No. EAR-0135533 and No. EAR-0230319, by MIUR under the PRIN-2000 initiative, and by INFM through the MRC Forum project and Iniziativa Trasversale Calcolo Parallelo . Computations were performed at the Minnesota Supercomputing Institute and at the CINECA supercomputing center in Bologna.