The size dependence of the ferroelectric properties of nanowires is studied from first principles. We show that the ferroelectric distortion along the wire axis disappears below a critical diameter of about . This disappearance is related to a global contraction of the unit cell resulting from low atomic coordinations at the wire surface. It is shown that a ferroelectric distortion can be recovered under appropriate tensile strain conditions.
Similar calculations were performed for selected nonstoichiometric wires with full Ba–O and full coverage, yielding results similar to Fig. 2 except that the crossing between and curves was slightly shifted to larger (respectively, smaller) cell parameters for BaO (respectively, ) coverage.
To test the possible influence (especially in reason of the lateral dipole) of the artificial replica induced by the periodic boundary conditions, we repeated some calculations enlarging the vacuum regions up to , without significant change in the results.