Anisotropic Ion Migration and Electronic Conduction in van der Waals Ferroelectric CuInP

Van der Waals (vdW) thio- and seleno-phosphates have recently gained considerable attention for the use as "active" dielectrics in two-dimensional/quasi-two-dimensional electronic devices. Bulk ionic conductivity in these materials has been identified as a key factor for the control of their electronic properties. However, direct evidence of specific ion species' migration at the nanoscale, particularly under electric fields, and its impact on material properties has been elusive. Here, we report on direct evidence of a phase-selective anisotropic Cu-ion-hopping mechanism in copper indium thiophosphate (CuInP