Pressure-dependent anharmonic coefficient of OH in portlandite by NIR-IR spectroscopy with DAC

Abstract

NIR and IR absorption spectra of portlandite were measured to investigate the behavior of hydroxyls in portlandite under pressure with DAC. The IR absorption peak of fundamental OH stretching motion of portlandite at 3645 cm⁻¹ shifted to the lower wavenumber at the rate of −2.1 cm⁻¹/GPa, and disappeared at 16.6 GPa. The secondary broad IR absorption peak at 3400 cm⁻¹ appeared over 8.3 GPa and its absorption was enhanced with applying pressure. The peak broadening of OH together with the absence of H₂O molecules (5200 cm⁻¹) during compression suggests pressure-induced amorphization of portlandite. The NIR absorption peak due to overtone of OH stretching motion of portlandite at 7075 cm⁻¹ shifted to the lower wavenumber at the rate of −7.5 cm⁻¹/GPa, which indicates more clearly increasing hydrogen bonding between layers in portlandite than the red shift of the IR peak. Assuming the Morse function as anharmonic potential energy of OH vibration, anharmonic coefficient of OH bond of portlandite against pressure was obtained from the pressure-dependent peak shift of the fundamental and overtone modes. The anharmonicity of OH bond in portlandite is enhanced with increasing pressure.