Establishing the morphotropic phase boundary in van der Waals ferroelectrics

The formation of morphotropic phase boundaries (MPBs) is a pivotal strategy in piezoelectric ceramics and crystals, primarily used to enhance the electromechanical coupling. However, the application of this strategy in van der Waals (vdW) piezoelectrics and ferroelectrics has been limited, largely due to challenges in achieving phase coexistence and enabling possible polarization rotation. In this study, we address this gap by synthesizing a Selenium doped vdW ferroelectric, CuInP₂(S_1−x Se _x )₆, with a doping range of 0 ⩽ x⩽ 0.15, to create an MPB. Our findings indicate the presence of an MPB near x = 0.05, situated between monoclinic and trigonal phases. This boundary was confirmed using x-ray diffraction and transmission electron microscope techniques. Remarkably, the composition at x = 0.05 shows a high dielectric constant (ϵ_r = 13.8) and an impressive local effective piezoelectric coefficient (d _33eff = 51 pm V⁻¹) at 80 K. Additionally, an unusual softening of the Young’s modulus was observed near MPB. These results are crucial for enhancing electromechanical coupling in vdW layered materials and herald new avenues for the design and optimization of piezoelectric and electromechanical properties in these materials.