Tuning second-harmonic generation in Pb-based perovskite ferroelectric single crystals for field sensing

Pb-based ABO3 perovskites have spurred extensive research endeavors, owing to their distinctive electromechanical characteristics that stem from the interactions between A-, B-, and O-site ions. The local dynamics giving rise to the structural symmetry-breaking results in abundant phase transition sequences, e.g., cubic–tetragonal–rhombohedral phase transitions in lead magnesium niobate–lead titanate crystals. In this work, we deliver a concise review of ferroelectric perovskites and the underlying mechanisms governing their ferroelectric and piezoelectric properties. We then present the investigations into the competitions and cooperations of local interactions within such single crystals to comprehend the phase transitions triggered by the “lone pair” and “d0-ness” effects. The two effects manifest local exclusivity on account of the ligand electrons sourced from the equatorial oxygen effecting a selection upon the niobium 4dxy and lead 6pz states. The mixture of nanoregions originating from the two effects can be partially annealed out above the rhombohedral-to-tetragonal phase transition temperature, thereby enhancing the elastic second-harmonic generation. With the aid of a magnetostrictive amorphous alloy bonded to the crystal surface, a low-frequency magnetic field can tune the amplitude of the second-harmonic signals. The mechanism is experimentally verified through a complete chain of modulation-to-demodulation processes, providing a novel tool for sensing low-frequency magnetic fields.