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(a) Schematic of the field-temperature phase diagram for a uniaxial ferroelectric with the II order phase transition. The solid line is the line of the I order phase transition corresponding to the ferroelectric switching. The solid point shows the position of the critical point corresponding to the ferroelectric. The dashed lines are the so-called Widom lines. On crossing these lines (e.g., along the dotted lines) the lattice contribution to the dielectric susceptibility passes through a maximum, as shown in (b). (b) Schematic of the temperature dependence of the lattice contribution to the differential susceptibility of a ferroelectric with the II order: (1) no dc bias applied, (2) under a small dc bias , and (3) under a dc bias that is larger than . The position of the maximum in curves (2) and (3) as function of the dc bias corresponds to the Widom line shown in Fig. 1(a).
(a) A model nonpolar structure with a ferroelectric instability with respect to displacements (in the plane of the drawing) of cations shown with black circles. Anions are shown with open circles. (b) A model structure obtained from (a) by adding the atoms shown with gray circles. Now the environment of the ferroelectric black cations can be viewed as weakly polar. The structure shown in (b) may exhibit the ferroelectriclike response when characterized in field directed as shown in Fig. 1(b).
(a) A hysteresis loop. (b) Loop calculated as the half-sum of two loops obtained from (a) by -axis shifts of and . (c) Loop calculated as the half-sum of two loops obtained from (a) by -axis shifts of and .
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