Neutron diffraction and polarized single-crystal Raman spectroscopic measurements were made on the high-pressure silicate lawsonite, CaAl2(Si2O7)(OH)2·H2O, from Tiburon Peninsula, California. For the diffraction measurements, intensity reflection data were collected at temperatures of 295, 110 and 20 K using time-of-flight neutron diffraction methods to further examine two reversible, order-disorder type phase transitions occurring at 273 and 155 K [Cmcm (>273 K) → Pmcn (<273 K) → P21cn (< 155 K)]. These data are also used to model the H atom displacements in lawsonite as a function of temperature and to provide better insight into the nature of H bonding. The Raman spectroscopic measurements (2500 to 4000 cm−1 at 4 ⩽ T ⩽ 300 K) were carried out on the same crystal used for the neutron diffraction study. Four OH-related bands are observed between 2700 and 3600 cm−1. The OH groups and H2O molecules, which are linked by hydrogen bonding, build quasi one-dimensional chains in lawsonite, that run parallel to [001] and thus a model consisting of isolated oscillators cannot be used to interpret the spectra at ambient temperature. A notable feature of spectral behaviour at 240–260 K in the vicinity of the CmcmPmcn phase transition is the change-over of strong hydrogen bonding from the OH group to the H2O molecule. The lowest-wavenumber OH(H2O) band at 2780−1 at 4 K is broad and asymmetric, which is related to strong hydrogen bonding, and is characterized by cm strong anharmonicity. This band was deconvoluted into a number of combination modes consisting of an internal-H2O and various external-H2O vibrations.

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