A first-principles-derived approach is used to study properties of $\mathrm{Pb}(\mathrm{S}\mathrm{c},\mathrm{N}\mathrm{b}){\mathrm{O}}_3$ alloys that are compositionally modulated along the [001] pseudocubic direction. The direction of the local electrical polarizations is found to be spatially-dependent, and can be adjusted with the modulation parameters. Furthermore, modulated structures with large composition gradients adopt a monoclinic phase—in which the total electrical polarization lies between the [111] and [110] directions—for their ferroelectric ground state, and exhibit large electromechanical responses.

• Received 14 May 2001

DOI:https://doi.org/10.1103/PhysRevB.64.060102