Impact of Crystallite Size and Phase Boundaries on Magnetic and Thermoelectric Properties of Cu-Added BiFeO₃

In this study, bismuth ferrite (BFO) and copper-added BFO were synthesized using the coprecipitation method. The incorporation of copper into the BFO lattice led to a reduction in the phase percentage of BFO due to the early formation of CuBi₂O₄. X-ray diffraction analysis revealed a decrease in crystallite size up to 0.1 CBFO, followed by an increase. This reduction in crystallite size causes an imbalance between the spins of the sublattices, resulting in an antiferromagnetic core/ferromagnetic shell (AC/FS) structure. The uncompensated spins generated by the decreasing crystallite size weaken the ferromagnetic properties with the addition of Cu. Additionally, the reduction in crystallite size leads to decreased electrical conductivity due to carrier scattering, with the maximum conductivity observed in BFO attributed to its volatilization. The Seebeck coefficient enhancement in 0.1 and 0.15 CBFO indicates an energy filtering effect caused by barriers at the phase boundaries. The introduction of Cu into the BFO matrix also results in reduced lattice thermal conductivity due to active centers for phonon scattering created by Cu-induced defects. The lowest lattice thermal conductivity was observed in 0.1 CBFO, which is attributed to the significant reduction in crystallite size and the presence of phase boundaries enhancing phonon scattering. The highest thermoelectric figure of merit (zT) was achieved in thermally unstable BFO due to Bi³⁺ volatilization, which was mitigated by the formation of CuBi₂O₄ in CBFO.