Ferroelectricity in Doped Incipient Ferroelectrics

摘要

We consider the quantum properties of mixed incipient ferroelectrics with small concentrations of fcrroelectricity inducing ions such as KTa_(1-x)Nb_xO_3. We show that the non-uniform strong intra-band electron - phonon interaction leads to the localization of virtual conduction band electrons around the Nb ions and to the off -center displacements of these ions. We consider the off-center displacements as pseudo-spins and show that their interaction with the soft phonon mode of the host lattice (KTaO_3) plays a fundamental role in the phase transition dynamics. The model quantitatively reproduces the basic experimental data in the quantum regime: the temperature and concentration dependence of the inverse dielectric function, the impurity and temperature dependence of the renormalized soft mode frequency and the impurity-concentration dependence of the transition temperature Tc. In particular we show that, the critical concentration for Tc=0 is determined by the ion tunneling frequency, the bare soft mode parameters and the pseudospin - phonon coupling constant.