THE STRUCTURAL BASIS OF FERROELECTRICITY IN COLEMANITE

Abstract

The complete structure of colemanite, CaB3O4(OH)3∙H2O, in both its nonpolar and ferroelectric phases, has been determined by neutron diffraction. This work shows that one of the hydrogen atoms of the water molecule and the hydrogen of an adjacent hydroxyl group, which are in a state of dynamic disorder in the nonpolar phase, settle into ordered noncentric positions in the ferroelectric phase. The ordering of the hydrogen atoms is accompanied by small displacements of the other atoms from the positions they occupy in the nonpolar phase. Assuming reasonable charge assignments, the magnitude of the spontaneous polarization calculated from the observed positional changes is of the same order of magnitude as the experimental value. On the basis of these results, a model for the atomistic mechanism of the transition to the ferroelectric state is advanced.