A Local Atomic Mechanism for Monoclinic-Tetragonal Phase Boundary Creation in Li-Doped Na_0.5K_0.5NbO₃Ferroelectric Solid Solution

ABO₃ perovskites display a wide range of phase transitions, which are driven by A/B-site centered polyhedral distortions and/or BO₆ octahedral tilting. Since heterogeneous substitutions at the A/B-site can locally alter both polyhedral distortions and/or tilting, they are often used to create phase boundary regions in solid solutions of ABO₃, where the functional properties are highly enhanced. However, the relationships between doping-induced atomistic structural changes and the creation of phase boundaries are not always clear. One prominent example of this is the Li-doped K_0.5Na_0.5NbO₃ (KNNL), which is considered a promising alternative to traditional Pb-based ferroelectrics. Although the electromechanical properties of KNNL are enhanced for compositions near the morphotropic phase boundary (MPB), the atomistic mechanism for phase transitions is not well understood. Here, we combined neutron total scattering experiments and density functional theory to investigate the long-range average and short-range (~10 Å) structural changes in KNNL. We show that the average monoclinic-to-tetragonal (M-T) transition across the MPB in KNNL can be described as an order-disorder-type change, which is driven by competition between a longer-range polarization field of monoclinic structural units and local distortions of the disordered AO₁₂ polyhedra. The current study demonstrates a way to clarify dopant-induced local distortions near phase boundaries in complex solid solution systems, which will be important for the rational design of new environmentally sustainable ferroelectrics.