Applicability of van der Waals density functionals in the calculation of materials properties applied to ferroelectrics

The applicability of van der Waals density functionals in the calculation of materials properties of normal and relaxor ferroelectric materials is explored using density functional theory (DFT). The accuracy in the calculation of structural, mechanical, and electronic properties of BaTiO₃ (BT), PbTiO₃ (PT), and Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) are investigated using different DFT exchange-correlation (XC) functionals including various forms of the van der Waals density functionals such as vdW-DF-C09, vdW-DF-cx, and vdW-DF-ob86, as well as the LDA, PBE, and PBEsol functionals for comparison. In general, the results obtained indicate that the van der Waals density functionals vdW-DF-C09 and vdW-DF-cx provide an overall better description when considering the full spectrum of the structural, mechanical, and electronic properties of BT, PT, and PMN-PT in their different crystal phases. The improved performance in materials properties was rationalized by the behavior of the enhancement factor in the small regime of the reduced gradient observed with the van der Waals density functionals. Additionally, the applicability of the vdW-DF-C09 functional in the calculation of the structural and electronic properties of BT under different types of strain is described, indicating a clear structure-property correspondence between the displacement of the Ti atom and the change in the magnitude of polarization.