Dipole freezing in pure and defective BCCD

Abstract

We have studied the complex dielectric constant of nominally pure BCCD in the low temperature region at hydrostatic pressures up to 300 MPa. Several strong anomalies occur in the dielectric loss factor tanδ_b, within the ferroelectric phase accompanied by a large frequency dispersion and sudden decreases in the dielectric constant ϵ′_b. We interpret this by several distinct freezing processes in the ferroelectric dipole system of BCCD. The results are compared to those of defective BCCD crystals which show similar dielectric anomalies.In Br doped BCCD the tanδ_b anomaly enters the region of modulated phases at low pressures, where the transition to the ferroelectric phase is suppressed and modulated structuresremain stable down to lowest temperatures, i.e., a freezing process appears within incommensurate and commensurate dipole configurations.

Key Words:

• Commensurate and incommensurate structures
• dielectric loss
• dipole freezing