Phase competition and effect of chemical ordering in ferroelectric relaxor PbSc_0.5Nb_0.5O₃ from first principles

ABSTRACT

We present results of first principles calculations for ferroelectric relaxor PbSc_0.5Nb_0.5O₃. An ordered supercell with Sc and Nb atoms alternating in all directions (NaCl-like superstructure) has a ferroelectric ground state with R3 spacegroup. It is characterized by a tilt system a⁻a⁻a⁻ as well as different sizes of adjacent oxygen octahedra. Analysis of phonon instabilities in cubic $Fm\bar{3}m$ reveals similarity with an antiferroelectric PbZrO₃. This is further confirmed by the fact that possibly antiferroelectric P2₁/b structure of PbSc_0.5Nb_0.5O₃ (symmetry lowered from Pbam due to Sc/Nb superstructure) is energetically very close to the ground state. The ferroelectric order is shown to be less sensitive to deviations in the B-site distribution than the antiferroelectric one.

KEYWORDS:

• Relaxor ferroelectrics
• soft modes
• antiferroelectricity
• first principles

Acknowledgments

We thank S. Leoni for fruitful discussions. This work was supported by the Czech Science Foundation as well as by the Australian Research Council through its Discovery Projects program. The computational part of this research was undertaken on the NCI National Facility in Canberra, Australia, which is supported by the Australian Commonwealth Government.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1. Being aware that antiferroelectricity is a term that has a deeper meaning than just an antiparallel polar shifts, we keep this notation throughout the article to emphasize the analogy with PZO. It should be read as a ’possibly AFE’ state.

Additional information

Funding

The Czech Science Foundation [grant number 15-04121S]