Drastic modifications of electrical properties of ferroelectric crystal plates with thickness, the case of triglycine sulphate

Abstract

The electrical properties of triglycine sulphate (TGS) single crystal plates are completely modified when the thickness is decreased down to 1 μm. The dielectric constant at the Curie temperature T0 is reduced by two orders of magnitude, coercive field is increased in the same ratio, and hysteresis loops are observed at several degrees above T0. Phenomenologically these observations can be explained in the frame of Devonshire's theory by assuming an internal electric bias Eb ≃ 1 MV/m for the thinnest plates.To explain such a high bias field we have been led to a tentative model with a surface layer where spontaneous polarization is smoothly decreasing from the bulk value PSB to a finite one PSB at the crystal surface. It is in this layer that a high polarizing field is calculated at room temperature. Its values may be slightly reduced when free charges of opposite sign are assumed to be injected by each electrode (Schottky barriers). These free charges will partially stay when temperature is raised above Curie point and lead to a polarizing field in the bulk which could explain the observation of hysteresis loops above the Curie temperature T0. There is, however, experimental evidence that such a dc bias field is small. The spontaneous polarization observed above T0 is finally ascribed to the contribution of surface layers and is a proof that their Curie point is higher than T0.