A STUDY OF THE MECHANISM OF THE FERROELECTRIC TRANSITION IN MONOCLINIC K2ZNBR4

Structural investigations and dielectric measurements in the range from audio to microwave frequencies have been performed in order to reveal the static and dynamic nature of the ferroelectric transition of potassium tetrabromozincate. In the ferroelectric phase a tilt of the tetrabromozincate tetrahedra and a shift of the two independent potassium ions in the same direction constitute the main structural changes at the transition and account almost quantitatively for the observed spontaneous polarization. In the paraelectric region two bromine atoms of each tetrahedron are disordered. They are assumed to move in double wells and to be responsible for the Debye-like dielectric relaxation that was found in the microwave region. The potassium ions and the other two bromine atoms are much less disordered, if at all, but contribute substantially to the condensing mode of A(u) symmetry. The characteristic frequencies of these latter atoms are beyond 10 GHz. The transition may be classified as partially order-disorder and partially displacive.