M. Naveed-Ul- Haq

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.

Multiferroics with strong magnetoelectric coupling can be created by improving the magnetostriction of the ferrite phase and combining it with the best-known piezoelectric. Herein, the ferromagnetic CoFe₂O₄ was modified with Al incorporation further improve the resistivity, magnetic, and magnetostrictive properties and then combined with (0.5)Ba(Zr_0.2Ti_0.8)O₃−(0.5)(Ba_0.7Ca_0.3)TiO₃ (BCZT) with different weight percentages. XRD results revealed the presence of a mixed spinel cubic phase for CAF (Fd-3m), whereas BCZT crystallized in a tetragonal structure (Amm2) in composite systems. The dielectric response of the materials was studied using impedance spectroscopy and it displayed a typical Maxwell–Wagner polarization of two-phase composites. The magnetic response of the CAF sample improved significantly which is attributed to the preference of Al³⁺ ions on tetrahedral sites. The magnetization of composites increased as the nonmagnetic BCZT component increased and the (0.75)CAF-(0.25)BCZT system had the highest saturation magnetization. We anticipate that these systems will have significantly improved magnetoelectric characteristics than unmodified CFO-BCZT systems.

The structure, electric permittivity, impedance, electronic structure, magnetic, and optical properties of Fe₂TiO₅ and Co-doped Fe₂TiO₅ are reported.