Complex antipolar [radical]2 x 4 x 2[radical]2 structure with Pnma symmetry in BiFeO sub(3) and BiFe sub(1/2) Sc sub(1/2) O sub(3): First-principles calculations

First principles calculations are done for a [radical]2 x 4 x 2[radical]2 Pnma structure, which has been recently discussed in several attempts to describe experiments in complex magnetoelectric perovskites and which experimentally was shown to compete with several ferroelectric phases. This makes these materials extremely attracting as switchers, starters, field-stimulated capacitors, high-voltage converters, transmitters, etc. The relative energies of the [radical]2 x 4 x 2[radical]2 Pnma structure have been calculated from first principles and analyzed as a function of pressure in BiFeO sub(3). The stability of two polymorphs of the [radical]2 x 4 x 2[radical]2 Pnma structure has been studied for solid solution BiFe sub(1/2) Sc sub(1/2) O sub(3). The main distortions and relative energies of these two polymorphs in BiFe sub(1/2) Sc sub(1/2) O sub(3), in terms of Pm3m parent symmetry, have been calculated from first principles as well.