THE POSITIONS OF THE HYDROGEN ATOMS AND THEIR ROLE IN THE FERROELECTRIC BEHAVIOR OF COLEMANITE

Abstract

neutron diffraction study of the hydrated borate mineral, colemanite (CaB3O4(OH)3 * H2O has been carried out at room temperature and at -20°C, primarily to determine the positions of the hydrogen atoms above and below the ferroelectric transition temperature of -2.5°C. Most of the hydrogen bonds in colemanite are of quite normal character and do not change appreciably through the transition. However, one of the hydrogen atoms of the water molecule and the hydrogen atom of an adjacent hydroxyl group, which are in a state of dynamic disorder at room temperature, are found to settle into ordered, non-centrosymmetric positions below the Curie point. Some of the other atoms are found to undergo small, but in some cases significant, displacements from their room temperature positions. Assuming a reasonable distribution of charges, the magnitude of the spontaneous polarization calculated from the observed positional changes is comparable to the measured value. These results have been used to develop a qualitative theory of the mechanism of the transition from the atomistic point of view.