NEW HIGH-PRESSURE PHASE AND PRESSURE-INDUCED AMORPHIZATION OF CA(OH)(2) - GRAIN-SIZE EFFECT

In situ high-pressure Raman spectroscopy was used to investigate the p hase stability of powder and single crystal portlandite, Ca(OH)(2), up to 22 Cpa, Our results show that the room temperature, high-pressure behavior of this compound is strongly influenced by grain size, Compre ssion of fine grained powder Ca(OH)(2) (<500 Angstrom in thickness) le ads to pressure-induced amorphization at around 11 Cpa, as shown by ex treme broadening of the Raman features, This is in agreement with prev ious studies of powder samples of Ca(OH)(2). However, when single crys tal samples (thickness similar to 10 mu m) are used, new Raman peaks r eplace the portlandite spectrum at 6 Cpa, indicating a crystal-to-crys tal phase transformation. The new Raman spectrum does not match either of the two known phases, portlandite or the baddeleyite form. A compa rative Raman study shows that the new phase resembles Sr(OH)(2), The n ew form of Ca(OH)(2) eventually undergoes pressure-induced amorphizati on around 20 GPa, suggesting that it is not thermodynamically stable a t this condition. Under decompression, both single crystal and fine gr ained samples completely revert back to portlandite, These observation s suggest that small grain size can stabilize the low-pressure phase i n the powder samples, thus allowing the 6 GPa phase transformation to be by-passed and amorphization to occur. (C) 1996 Academic Press, Inc.