7 - Ferroelectric properties of niobium and tantalum fluoride compounds

Abstract

The main properties of ferroelectric materials include relatively high dielectric permittivity, ferroelectric-paraelectric phase transition that occurs at a certain temperature called the Curie temperature, piezoelectric effect, pyroelectric effect, nonlinear optic property,including the ability to multiply frequencies, ferroelectric hysteresis loop, and electrostrictive, electro-optic and other properties. The main source of spontaneous polarization in crystals is the relative “freedom” of cations that fit loosely into the crystal's octahedral cavities. The number of degrees of freedom of the octahedrons affects the spontaneous polarization value and hence influences the crystal’'s ferroelectric properties. According to Abrahams and Keve, ferroelectric materials can be classified into three structural categories according to their atomic displacement mechanisms: one-dimensional, two-dimensional, and three-dimensional. When all of the atomic displacement vectors are parallel to a polar axis of the crystal structure, the compound belongs to the one-dimensional category. Compounds that belong to the two-dimensional category undergo polarization reversal due to atomic displacement in a plane that contains a polar axis. In crystals that belong to the three-dimensional category, reorientation of the polarization occurs due to displacements that appear to be relatively equal in all the three directions.