NUCLEAR MAGNETIC RESONANCE STUDY OF COLEMANITE

Abstract

A single crystal of colemanite, which is ferroelectric at temperatures below about -2.0° C., has been investigated by means of nuclear magnetic resonance (n.m.r.) techniques, over a temperature range 52° C. to -136° C. The splitting of the B11 n.m.r. signal in colemanite, caused by the perturbation of the nuclear Zeeman levels by the interactions between the nuclear electric quadrupole moment and the electric field gradients existing at the boron sites at room temperature and at -40C., has been fully analysed, using the procedure developed by Volkoff and coworkers. The quadrupole coupling constants, the asymmetry parameters and the orientations of the principal axes of the electric field gradient tensors at room temperature and -40° C. are given in Tables XXV - XXVII. A selected set of B11 n.m.r. lines has been examined over the temperature range 52 C. to -136° C. This investigation suggests that the ferroelectric transition is second-order and not the order disorder or martensitic type. The transition temperature itself has been found to depend upon the history of the crystal, but is apparently about 2° C. higher than previously reported temperatures of about -2.0° C In addition, a previously unknown phase in colemanite is reported. A large temperature hysteresis loop is associated with the transition to the new phase on cooling, the transition takes place at about -80° C, whereas on heating, the transition takes place at about -35° C. The point groups for the three phases have been determined as 2/m., 2 and 1 going from room temperature to -80° C., respectively. The results of the n.m.r. investigation have been interpreted, as far as possible at present, in terms of the crystal structure.