Proton dynamics in ferroelectric colemanite

A reexamination of the proton second moment in powdered colemanite, CaB3O4(OH)3∙H2O, over the range 4.2–450 °K has shown that the temperature dependence is in qualitative agreement with earlier work but that the magnitudes of the changes occurring at the two line-width transitions are substantially different. The observed values of the second moment have been compared with values calculated on the basis of models in which the hydrogen-containing molecular groups reorient. The line-width transition at about 130 °K is attributed to a motion in which the water molecules undergo 180° flips about the bisectrix of the H—O—H angle. The activation energy for this motion has been determined as 5.3 ± 0.3 kcal/mole from the temperature dependence of T1. The observed change in the second moment at the ferroelectric transition (270 °K) is in good agreement with the value calculated on the basis of the neutron diffraction model, in which a cooperative motion involving one of the hydrogen atoms of the water molecule and the hydrogen of an adjacent hydroxyl group is active above the transition.