Dielectric constant and ferroelectric hysteresis of Rb2ZnBr4

The low frequency dielectric constant of Rb2ZnBr4 is measured in the normal phase, the modulated phase, and in that one with a tripled c-axis. In the latter phase a ferroelectric hysteresis loop is observed. The spontaneous polarization (of about 0.14 μC/cm2) is very small as well as the anomalies in the dielectric constant. Rb2ZnBr4 belongs to the family of improper ferroelectrics since only the cube of the primary order parameter couples to the polarization which is treated as a secondary order parameter. This kind of coupling is in agreement with the observed direction of the spontaneous polarization. A Landau expansion of the free energy is constructed on the basis of the coupling between primary and secondary order parameter. In the modulated phase this model predicts a polarization wave which has the periodicity of the third harmonic of the primary lattice distortion. When the modulation wave vector locks-in to the commensurate value of 1/3c* the polarization wave becomes the macroscopic polarization.