Excess dielectric nonlinearity of incipient ferroelectrics

Moscow State University, 119899, Moscow, Russia

Evaluations of anharmonicity constants b of free energy expansion made from dielectric measurements for SrTiO₃ and KTaO₃ [1] show the values obtained are dependent on the field strength of electrical bias in regular manner. This peculiarity is in contrary with fundamental states of thermodynamic theory.

Two mechanisms leading to spurious field dependence of anharmonicity coefficients are considered. Because, according to [2], the surface layer of ferroelectric acts as Shottky barrier, electroded samples have to be considered as three layer structures, with surface layers possessing the nonlinear capacity and nonlinear conductance. In this conditions the rise of effective values of nonlinearity coefficients in relatively weak bias fields takes place necessarily.

In strontium titanate additional inhomogeneity at temperatures lower 105 K is caused by breakup of crystal in structural domains with different orientations of tetragonal axes, each of them being with anisotropic dielectric permittivity. For model mentioned calculations of field dependences of effective permittivity and nonlinearity coefficients were made in the framework of effective medium approximation [3]. It is shown the formal evaluation of b coefficient in multidomain medium brings to its field dependence with overestimated amounts in weak field region.

The general conclusion is based, that anharmonicity constants measured in strong bias field approach thermodynamic values for homogeneous crystal in spite of structural domains and surface layers effect. Moreover, study of dielectric nonlinearity characteristics in small bias fields enables to obtain the information about inhomogeneity of samples and the surface layers characteristics.

1. I.V. Ivanov et al. Izv.Vyssh.Uchebn.Zaved.,Fiz. (in Russian) 24, No. 8, 6 (1981).
2. D. Kahng, S.H. Wemple. J.Appl.Phys. 36, 2925 (1965).
3. Electrophysical properties of percolation systems (ed. by A.M. Lagar'kov). Moscow, 1990.