Control of Artificial Relaxor Behaviors in Ferroelectric/Dielectric Superlattices

Piezoelectric materials are an important class of functional materials, with applications ranging from sensors and actuators to energy harvesters and magnetoelectric devices. Understanding the structure-property relations and tuning the response of piezoelectrics is, thus, of great technological importance. Relaxor ferroelectrics such as (1-<i>x</i>)PbMg_1/3Nb_2/3O₃-(<i>x</i>)PbTiO₃ and BaZr<i>_x</i>Ti_1-<i>x</i>O₃, in particular, have drawn considerable attention as high-performance piezoelectric materials due to their ultrahigh piezoelectric coefficients, but the complex structure-property relation in relaxor ferroelectrics remains underexplored. In general, it is thought that the chemical disorder of the cations generates random fields which are essential to the formation of relaxor-like polar order. Thus, manipulation of the chemical order should provide a pathway to control the relaxor-like polar order, and hence the dielectric and piezoelectric response of relaxors. In conventional relaxors based on solid solutions, however, the deterministic control of chemical order is challenging due to the inherent randomness, which calls for a new platform for unveiling the structure-property relations of relaxors.<br/><br/>Recently, fabrication of artificial relaxors based on superlattices has been proposed as a novel pathway to achieve the chemical-order control at unit-cell precision [1]. Here, [BaTiO₃]<i>_m</i>/[BaZrO₃]<i>_n</i> superlattices, wherein the nanoscale-polar structures are confined in the ferroelectric BaTiO₃ layers, while the dielectric BaZrO₃ layers tune the spacing between neighboring layers of nanoscale-polar structures, are proposed and used as a model system to explore the effect of chemical-order control. [BaTiO₃]<i>_m</i>/[BaZrO₃]<i>_n</i> superlattices (<i>m</i>, <i>n</i> = 4-12 unit cells) were fabricated via pulsed-laser deposition and characterized by X-ray diffraction and atomic-resolution microscopy. These studies confirm the production of high-quality heterostructures, wherein the BaTiO₃ and BaZrO₃ layers are coherently strained to each other with relatively sharp interfaces in between. Dielectric permittivity measurements revealed significantly enhanced in-plane dielectric-maximum temperature <i>T</i>_m^IP in the superlattices compared with the solid solution BaZr_0.5Ti_0.5O₃ (from 173 K to ≥433 K), and a trend of higher <i>T</i>_m^IP in superlattices with larger BaTiO₃-layer thickness (from 433 K to ≥573 K as <i>m</i> increases from 4 to 12). Subsequent measurements of hysteresis loops and third-harmonic nonlinearity revealed relaxor-like behavior in the superlattices with the smallest BaTiO₃-layer thickness and ferroelectric-like behavior in the superlattices with larger BaTiO₃-layer thickness, with the crossover appearing between <i>m</i> = 4 and <i>m</i> ≥ 8. Piezoelectric measurements along the out-of-plane direction revealed that the effective piezoelectric coefficient <i>d</i>₃₃ is enhanced in superlattices with the smallest periodicity (<i>m</i> = <i>n</i> = 4, <i>d</i>₃₃ = 7.8 pm/V) compared to both superlattices with larger periodicities (<i>m</i> = <i>n</i> = 12, <i>d</i>₃₃ = 4.8 pm/V) and the solid solution (5.4 pm/V). Higher BaTiO₃ composition in the superlattice further enhances the piezoelectric response (<i>m</i> = 12, <i>n</i> = 4, <i>d</i>₃₃ = 12.3 pm/V).<br/><br/>Altogether, these results suggest that an artificial relaxor phase can be achieved in ferroelectric/dielectric superlattices such as [BaTiO₃]<i>_m</i>/[BaZrO₃]<i>_n</i>, wherein the chemical order can be deterministically controlled and the relaxor behavior can be manipulated. Considering both the dielectric and piezoelectric response can be tuned by the chemical order, this provides a novel platform for exploring the complex structure-property relation in relaxor ferroelectrics, and opens up future opportunities for the design of high-performance piezoelectric superlattices.<br/><br/>REFERENCES<br/>[1] Z. Tian, M. Xu, J. Kim, H. Pan, D. Lou, X. Huang, J. M. LeBeau, and L. W. Martin. Tunable Artificial Relaxor Behavior in [BaTiO₃]<i>_m</i>/[BaZrO₃]<i>_n</i> Superlattices. <i>Submitted.</i>