Large enhancement of properties in strained lead-free multiferroic solid solutions with strong deviation from Vegard's law

Efforts to combine the advantages of multiple systems to enhance functionalities through solid-solution design present a great challenge due to the constraint imposed by the classical Vegard's law. Here, we successfully navigate this trade-off by leveraging the synergistic effect of chemical doping and strain engineering in the solid-solution system of (1-x)BiFeO₃-xBaTiO₃. Unlike bulks, a significant deviation from Vegard's law accompanied by enhanced multiferroism is observed in strained solid-solution epitaxial films, where we achieve a pronounced tetragonality (∼1.1), enhanced saturated magnetization (∼12 emu/cm³), substantial polarization (∼107 μC/cm²), and high ferroelectric Curie temperature (∼880°C), all while maintaining impressively low leakage current. These characteristics surpass the properties of their parent BiFeO₃ and BaTiO₃ films. Moreover, the superior ferroelectricity has never been reported in corresponding bulks (e.g., P ∼5 μC/cm² and T_C ∼300°C for bulk, with x = 0.5). These findings underscore the potential of strained (1-x)BiFeO₃-xBaTiO₃ films as lead-free, room temperature multiferroics.