The low-temperature behaviour of the pyroelectric coefficient

The results of both theoretical and experimental investigations of the temperature dependence of the pyroelectric coefficient γδ (T) are given. It is shown that in ideal crystals the γδ (T) may be represented by the sum of Debye's and Einstein's functions, which describe the contributions of acoustic and optical modes, respectively. Experimentally obtained γδ (T) dependences for different kinds of linear pyroelectrics and ferroelectrics are listed. The experimental data for linear pyroelectrics (except the region of the lowest temperatures) are described well by the mentioned sum of Debye's and Einstein's functions. In ferroelectrics the acoustic modes contribution to the γδ (T) is not found out in the frames of the accuracy of measurements. The simple estimations of the coefficient of anharmonicity and of the nonlinear dipole moment show that in ferroelectrics the relative contribution of the optical modes is greater than of the acoustic ones to the extent of the ratio (ω D /ω oj )2, where ω D is the Debye's frequency of the crystal, and ω oj is the frequency of the optically active mode. In order to find out the acoustic modes contribution, it is necessary either to rise the accuracy of measurements or to measure the γδ (T) dependences at lower temperatures (T < 5 K). The brief discussion of the influence of defects on the γδ (T) is given.