HYDROGEN ATOM LOCALIZATION IN LAWSONITE USING SINGLE CRYSTAL PMR DATA. Kozlova S.G., Gabuda S.P., Armbruster T.*,Libowitzky E.*, Institute of Inorganic Chemistry, Novosibirsk, Russia; * University of Bern, Bern, Switzerland

The phase transition at 273 K in lawsonite is characterised by the loss of the mirror plane --m due to the rotation of water molecules and OH groups [Libowitzky, Armbruster, Am. Miner, 1995, 90]. This transition may be displacive or of order-disorder type. In the latter case the Cmcm phase, stable above 273K, must be orientationally disordered (either dynamically or statically). Proton magnetic resonance (PMR) spectroscopy was applied for the characterisation of the Cmcm hydrogen sublattice at 295K.

A lawsonite crystal, 2x2x4mm³ in size was oriented with respect to its morphology as well as by optical and X-ray methods.The PMR spectra were measured at 12 MHz. The spectra revealed a resolved fine structure, caused by the dipole-dipole interactions in multi-spin system [OH-H₂O-OH-OH-H₂O]. The mean square widths (M) of the spectra recorded parallel to the crystallographic axes are as follows: M[100]=19,6; M[010]=11.7; M[001]=50.3 G² (error is 5%).The analysis of the fine structure of spectra and of the M values provide evidence that the hydrogen sublattice of the Cmcm lawsonite structure is disordered. In a structural sense this disorder can be interpreted as a time averaged dynamic disorder of the H₂O and OH groups oscillating between two equivalent sites similar to the ordered H positions of the Pmcn lawsonite structure. The corrected for libration motion H-H distance is R(H₂O)=1.52A, and mean-square angle of H-H librations is 0.18 rad². Libration motion leads to distortion of X-ray and neutron diffraction data [Busing, Levy, Acta Cryst. 1963, 17, 142]. Using obtained values we calculated the "uncorrected" H-H distance, which will be observed in diffraction study: R(H₂O)=1.38A, that is in excellent agreement with X-ray value 1.34(4)A.