Vibrational spectra of Mg(OH)2 and Ca(OH)2 under pressure

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Abstract
The effects of compression on the structure and bonding of Mg(OH)2 brucite and Ca(OH)2 portlandite are documented to pressures of 34 and 24 GPa, respectively, by way of infrared spectroscopy of the O–H stretching vibration at 300 K. The frequency of the infrared‐active (A2u) mode of both hydroxides decreases with pressure, by −0.6 cm−1/GPa for Mg(OH)2 over the pressure range studied, and by −3.5 cm−1/GPa for Ca(OH)2 to 10.6 GPa. An additional mode observed in Mg(OH)2 is due to an optically induced hot band (A2u X A2u overtone): with increasing pressure, its intensity grows and its frequency decreases (−3.7 cm−1/GPa). The negative pressure dependencies of these modes (negative Grüneisen parameters) are explained in terms of compression increasing the hydrogen bond strength within the hydroxide structure. On increasing pressure above 12.8 GPa, the full width at half‐maximum of the O–H vibration in Ca(OH)2 rapidly broadens by a factor of 4. The infrared data suggest that Ca(OH)2 undergoes pressure‐induced amorphization at 11.7(±1.1) GPa at 300 K, consistent with independently collected x‐ray diffraction patterns. In the amorphous phase, a distribution of O–O bond lengths ranging between 2.8 and 3.3 Å is inferred from the breadth of the vibrational band. On decompression, the infrared spectra show a full return to the crystalline form at 2.8 (±1.4) GPa. Neither upon amorphization with increasing pressure nor upon recrystallization with decreasing pressure are there any prescursory phenomena evident in the spectra. In contrast with Ca(OH)2, the isomorphous Mg(OH)2 appears to be stable throughout the pressure range of this study.

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