Flexoelectric and electrostatic effects on mechanical properties of CuInP2S6

Ferroelectric polarization is crucial to the development of modern electronic devices. The mechanical properties of ferroelectric materials are however interlinked with its polarization through piezoelectricity and flexoelectricity, as uniform and non-uniform mechanical deformation can bring about changes of polarization. While the mechanical properties of ferroelectric domains and domain walls of conventional ferroelectric oxides have been investigated widely, the mechanical properties of two-dimensional ferroelectrics (van der Waals ferroelectrics) have barely been studied, although they have drawn significant attention due to the need for miniaturization in new-generation low-energy nanoelectronics. Here, we report on variations in mechanical properties of copper indium thiophosphate (CuInP2S6, CIPS), which shows mechanical softening at 180° domain walls, and we discuss flexoelectric and electrostatic effects on the elastic modulus of its ferroelectric domains. In addition, an inhomogeneous mechanical response in mixed-phase CuInP2S6-In4/3P2S6 heterostructures has also been observed. The ferroelectric CuInP2S6 phase appears to be stiffer than the non-polar In4/3P2S6 phase, while the phase boundaries between the two phases, interestingly, display the lowest elastic modulus. Our findings provide new insight into the mechanical properties of two-dimensional ferroelectrics, which may inspire further investigation and applications of these materials.