Coupled engineering of structural, electronic, optical, thermoelectric, magnetic, and piezoelectric responses in Ni-doped ZnFe2O4: A first-principles multifunctional oxide framework

We performed a density functional theory (DFT) study on pristine and Ni-doped ZnFe 2 O 4 to understand how Ni substitution influences their structural, electronic, optical, thermoelectric, magnetic, and piezoelectric properties. The optimized lattice constant of pure ZnFe 2 O 4 is 8.44 Å, which slightly contracts to 8.38 Å upon Ni doping due to the smaller ionic radius of Ni 2+ . The elastic constants increase from C 11 = 265 GPa, C 12 = 145 GPa, and C 44 = 98 GPa in the pure compound to C 11 = 290 GPa, C 12 = 155 GPa, and C 44 = 112 GPa in the doped system, confirming improved stiffness and hardness. The band gap decreases from ∼2.1 eV to ∼1.6 eV after Ni incorporation, enhancing visible-light absorption through Fe–O and Ni–O orbital hybridization. Optical spectra show a red-shift in the absorption edge from 3.0 eV to about 2.0 eV, accompanied by stronger dielectric screening. Thermoelectric results reveal a peak Seebeck coefficient of ∼570 μV/K at 100 K and a power factor of ∼7.5×10 16 (arb. units) near 300 K for pristine ZnFe 2 O 4 , corresponding to a maximum ZT of 0.015 at 50 K. Ni doping suppresses electrical conductivity (<10 22 S/m) and reduces ZT below 4×10 -4 . Magnetic analysis indicates that Ni substitution increases the total magnetic moment from 0.9 μB to 1.4 μB per formula unit by enhancing Ni–Fe superexchange interactions. Importantly, piezoelectric coefficients nearly double upon doping, with d 33 increasing from 4.8 to 8.2 pC/N. Overall, Ni incorporation strengthens mechanical and piezoelectric responses while reducing electronic transport, demonstrating the coupled influence of structural distortion and defect-driven hybridization in designing multifunctional ferrite oxides. • Ni doping tailors ZnFe 2 O 4 ’s structural, optical, and magnetic multifunctional behavior. • First-principles DFT reveals enhanced magnetic and piezoelectric coupling in Ni–ZnFe 2 O 4 . • Electronic structure tuning improves thermoelectric and optical response simultaneously. • Dielectric and absorption spectra show strong correlation with Ni concentration. • Ni substitution enhances multifunctionality, making ZnFe 2 O 4 suitable for smart oxide devices.