Impurities and ferroelectric properties of semiconductors

Within the framework of the vibronic theory of ferroelectric semiconductors the influence of impurity carriers on the ferroelectric properties of the crystal is investigated with the width of the allowed electron bands taken into account. A possibility of a significant shift of the Curie point and a modification of the form of the dependence of the low-symmetry lattice distortion and soft mode frequency in the ferroelectric phase on temperature is predicted. It is demonstrated that ferroelectric ordering may also arise as a result of the vibronic mixing of a narrow impurity band with the host band. The principal characteristics of such a system are calculated.