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Behaviour of H₂O and OH in lawsonite: a single-crystal neutron diffraction and Raman spectroscopic investigation

¹ Institute of Inorganic Chemistry, Russian Academy of Science, Lavrentiev prosp. 3, Novosibirsk 630090, Russia
² Department of Geography and Geosciences, University of Louisville, Louisville, KY, 40292, USA
³ Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL, 60439, USA

^* Corresponding author, e-mail: kolesov{at}che.nsk.su

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 Cmcm Pmcn 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.

Key-words: lawsonite, high-pressure silicates, H₂O molecule, hydroxyl groups, hydrogen bonding, Raman spectroscopy, neutron diffraction.

This article has been cited by other articles:

				E. Libowitzky Comment on "Behaviour of H2O and OH in lawsonite: a single-crystal neutron diffraction and Raman spectroscopic investigation" by B.A. Kolesov et al. European Journal of Mineralogy, August 1, 2009; 21(4): 915 - 918. [Full Text] [PDF]

				B. A. Kolesov Reply to the comment by E. Libowitzky on "Behaviour of H2O and OH in lawsonite: a single-crystal neutron diffraction and Raman spectroscopic investigation" European Journal of Mineralogy, August 1, 2009; 21(4): 919 - 921. [Full Text] [PDF]