Structural and cation-ordering control of electronic and multiferroic properties in Ni-doped PbTiO 3 : a first-principles study using PBE and HSE12s functionals

Abstract The search for polar semiconductors that combine magnetism, ferroelectricity, and visible-range band gaps is a central challenge in materials science. However, this quest is hindered by a fundamental anticorrelation between these properties. Ni-doped PbTiO 3 (Ni-PTO) has emerged as a promising candidate. However, a unified understanding linking its atomic-scale structure to its functional behavior has remained vague. This study presents a comprehensive first-principles investigation of PbTi 1−x Ni x O 3 , employing both GGA-PBE and HSE12s functionals to unravel the coupled evolution of electronic structure, polarization, and magnetism across a range of compositions and cation ordering motifs. We demonstrate that the specific arrangement of Ni atoms, not just their concentration, is the primary determinant of functional properties. Layered ordering drives metallicity through enhanced Ni–O covalency and extended connectivity. On the other hand, rocksalt ordering preserves a visible-range gap (∼1.88 eV) by isolating magnetic Ni centers. Importantly, we introduce bond population analysis to provide a chemically intuitive framework which reveals that the competition between the host’s polar instability and the dopant’s symmetric preference dictates ferroelectricity. Specifically, the d 8 low-spin state of Ni 2+ in square-planar coordination prefers centrosymmetric geometry for crystal-field stabilization, reducing Ni–O covalency and opposing Ti 4+ -driven polar distortions. Furthermore, anisotropic orderings counteract depolarization at high Ni content restoring large polarization (∼0.90 C m −2 ) and tetragonality.