Sign of the nonlinear dielectric susceptibility of amorphous and crystalline SrTiO3 films

Capacitance variation with electrical field and temperature of SrTiO3 films based on metal-insulator-metal structures is shown to be opposite before and after crystallization. This motivates us to investigate the relationship between the crystallinity and the nonlinear dielectric susceptibility sign. Structural data (Extended X-Ray-Absorption Fine Structure and X-Ray Diffraction) are presented together with capacitance measurements as a function of static electric field and temperature. A statistical model shows that the nonlinear dielectric susceptibility sign results from the balance between short-range repulsive and long-range attractive interactions. The short-range interaction contribution to the nonlinear susceptibility depends on the anharmonic third-order term in the interionic potential energy, whereas the long-range contribution depends on the transverse effective charge logarithmic derivative against interionic distance. This last is found to be an increasing function of the transverse effective charge itself. The sign reversal between crystalline and amorphous SrTiO3 is explained by the predominance of short-range contribution in the amorphous case and by the major long-range contribution reflected by the high transverse effective charge in the crystalline case. The consequences of the model are discussed in the cases of NaCl, CsCl, and the Curie temperature and constant of BaTiO3.