This fact neces-sitates a search for alternative mechanisms of nonlinear re-sponse not studied so far, and calls for a refinement andgeneralization of the static and dynamic SRBRF model.The mechanism of nonlinear response originally proposedwithin the SRBRF model in Ref. 5 ~to referred to here asmechanism I! has been derived under the assumption thatpolar clusters are embedded in a rigid lattice; the nonlinearityeffects are thus entirely due to the intrinsic nonlinearity ofthe rigid SRBRF model. A possible alternative mechanism,characteristic of the generalized SRBRF model and exploredin the present work, is based on the fact that polar clusterscan be displaced from their equilibrium positions by the ap-plied electric field; this then leads to the field modulation ofthe intercluster coupling ~mechanism II!. A similar idea wasused in connection with the field-temperature (E,T) phasediagram of PLZT,