New High-Pressure Phase and Pressure-Induced Amorphization of Ca(OH)2: Grain Size Effect

Abstract

In situhigh-pressure Raman spectroscopy was used to investigate the phase stability of powder and single crystal portlandite, Ca(OH)2, up to 22 GPa. Our results show that the room temperature, high-pressure behavior of this compound is strongly influenced by grain size. Compression of fine grained powder Ca(OH)2(<500 Å in thickness) leads to pressure-induced amorphization at around 11 GPa, as shown by extreme broadening of the Raman features. This is in agreement with previous studies of powder samples of Ca(OH)2. However, when single crystal samples (thickness ∼10 μm) are used, new Raman peaks replace the portlandite spectrum at 6 GPa, indicating a crystal-to-crystal phase transformation. The new Raman spectrum does not match either of the two known phases, portlandite or the baddeleyite form. A comparative Raman study shows that the new phase resembles Sr(OH)2. The new form of Ca(OH)2eventually undergoes pressure-induced amorphization around 20 GPa, suggesting that it is not thermodynamically stable at this condition. Under decompression, both single crystal and fine grained samples completely revert back to portlandite. These observations suggest that small grain size can stabilize the low-pressure phase in the powder samples, thus allowing the 6 GPa phase transformation to be by-passed and amorphization to occur.