A NUCLEAR MAGNETIC RESONANCE STUDY OF COLEMANITE

A single crystal of colemanite, CaB3O4(OH)3∙H2O, which is known to be ferroelectric at temperatures below about −2 °C, has been investigated by means of nuclear magnetic resonance (n.m.r.) techniques. The B11 resonances are split because the nuclear Zeeman levels are perturbed by the interactions between the nuclear electric quadrupole moments and the electric field gradients existing at the boron sites. The splittings have been examined in detail at room temperature and at −40 °C. The results have been analyzed and the quadrupole coupling constants, the asymmetry parameters, and the orientations of the principal axes of the electric field gradient tensors existing at the boron sites at room temperature and −40 °C are reported. Selected B11 resonance lines have been examined over the temperature range 40 °C to −120 °C with particular emphasis on the region about 0 °C where a phase transition occurs. The complex proton signal was also studied over the same temperature range. Abrupt broadening of this signal occurred at the phase transition. These studies revealed that the crystal may transform from its centrosymmetrical room-temperature (point group 2/m) form either to a metastable monoclinic form with point group 2 or to a triclinic form with point group 1. It is not clear whether two transitions, separated by only about 3 °C, are involved or whether there is only one transition with two alternative arrangements, differing only slightly in activation energy, available to the structure. The transition or transitions are of the second-order displacive type. Where possible, the results have been interpreted in terms of the crystal structure.