2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/25044The boundary conditions, customarily used in the Landau-type approach to ferroelectric thin films and nanostructures, have to be modified to take into account that a surface of a ferroelectric (FE) is a defect of a ``field'' type. The surface (interface) field is coupled to a normal component of polarization and, as a result, the second order phase transitions are generally suppressed and anomalies in response are washed out, as observed experimentally.5 pages, 1 figure. This contains much expanded first part of cond-mat/0402100 with detailed derivation of nonlinear effective equation of state for the net polarization, inhomogeneous part of polarization, and a fit to the data. (v2) extended discussion of the small characteristic length in the problem, relevant reference addedMaterials Sciencetext