Defects and Domain Walls in Soft Ferroelectric CsGeX₃ (X = Cl, Br, I)

Conductive domain walls (DWs) hold promise for nanoscale, energy-efficient multi-level diodes and neuromorphic circuitry. In CsGeX₃, caesium and halogen vacancies are intrinsic point defects that can induce electrons and holes, respectively. Controlling the formation and position of vacancies can in principle give switchable local n-type or p-type conductivity at DWs and enable new concepts for DW-based circuitry. This requires a material where mobile point defects of both positive and negative charge can accumulate at DWs. CsGeX₃ is here chosen as our model system due to the possibility of having mobile caesium and halogen vacancies.<br/> <br/>We use density functional theory (DFT) to study intrinsic point defects and DWs in CsGeX₃. Using <i>hybrid</i> DFT we investigate electronic structure and defect formation energies in bulk, showing highly mobile holes and free electrons, as well as relatively shallow defect levels. Additionally, a study on point defect mobility in bulk, reveals exceptionally mobile anion vacancies with migration barriers comparable to Li vacancies in the best solid-state electrolytes. Furthermore, we investigate possible DWs and show that parallel and head-to-tail Y-type 71-degree DWs are the most stable DWs. Moreover, the mobility of DWs, with and without point defects in their vicinity, has been studied, showing extremely mobile DWs, implying that polarisation can easily be switched. Finally, the possibility of reversible p- and n-type conductivity in DWs in CsGeX₃ and other similar materials is discussed.