REAL-SPACE STUDY OF MECHANICAL INSTABILITY OF ICE XI ON A 'BOND-BY-BOND' BASIS

摘要

In this work we investigate by means of first principles computations the pressure induced behavior of ice XI and VIII. Ice XI is the low temperature, low pressure, ferroelectric, and hydrogen-ordered form of common ice Ih, and ice VIII is the low temperature, high pressure, anti-ferroelectric, and hydrogen-ordered form of ice VII, We show that ice XI transforms continuously into ice VIII under compression in a manner that retains the ice XI electric dipole ordering. The phase found here, 'ice VIII-like', has not been observed experimentally yet and it is unlikely to, but both ice XI and ice VIII are expected to amorphize inside each others stability field. Therefore, the present study is relevant to understanding amorphization itself. It provides a method for interpreting the high pressure amorphization process as intermediate phases produced in a stepwise transformation process between IHBN and 2HBN hydrogen ordered structures. We followed the procedure of Umemoto and Wentzcovitch (see Figure 1), for one leg of a hysteresis loop from 0-90 kbar and our results agree with theirs. (In addition we obtain an additional data point at 85 kbar that is consistent with Fig. 1 of Umemoto and Wentzcovitch's work, Summarizing the relevant results of Umemoto and Wentzcovitch's work; near 35 kbar, an incommensurate phonon instability develops, followed by the collapse of the lowest acoustic branch. This multitude of unstable phonons, each one leading to a slightly different metastable structure, is what is referred to as amorphization. Up to 90 kbar, adjacent layers perpendicular to the [001] direction shift laterally with respect to each other along the [010] direction. Then, ice XI transforms to a 2HBN structure, i.e., an 'ice VIII-like' phase. Similarly to ice XI, 'ice VIII-like' is also ferroelectric, instead of antiferroelectric like ice VIII. This transition is parallel to the case of disordered ice Ih; paraelectric ice Ih transforms under pressure to paraelectric ice VII preceded by an intermediate amorphous phase. The conclusions of Umemoto and Wentzcovitch's work were that a more drastic molecular reorientation is required to produce an anti-ferroelectric ice VIII structure which must involve a higher energy barrier than the ice XI-ice VIII-like transition.