An inelastic neutron-scattering study of the improper ferroelectric transition in copper chlorine boracite

Abstract

The improper ferroelectric copper-chlorine boracite (Cu3B7O13Cl) has been studied in its high-temperature (cubic) phase using inelastic neutron-scattering techniques. A preliminary survey showed normal behaviour of the acoustic branches and no low-lying underdamped optic branches to account for the structured X-ray diffuse scattering found by P. Felix (1973). No evidence could be found for any underdamped soft modes at the X-point of the cubic-F Brillouin zone boundary (ZB), where the standard theory of the improper cubic-orthorhombic transition predicts a pair of degenerate soft modes. However, quasielastic critical scattering was found at the X-points, and apparently around some cubic-phase Bragg points, too, similar in distribution to the structured X-ray diffuse scattering. The intensity of the ZB scattering was measured for a number of X-points at Tc+10K (386K). The observed intensities are in qualitative agreement with calculated dynamical intensities, obtained by using the antisymmetric part of the known static displacements at the transition as a reasonable approximation for AB mode displacements. However, a strikingly improved agreement is obtained in terms of a model in which some 5-10% of the crystal volume (at Tc+10K) is composed of microdomains a few unit cells across within which the structure is that of the orthorhombic phase. The plausibility and implications of this conclusion are discussed.