Anomalous Motion of Charged Domain Walls and Associated Negative Capacitance in Copper-Chlorine Boracite

During switching, the microstructure of a ferroelectric normally adapts to align internal dipoles with externally applied electric fields. Dipolar regions (domains), that are favourably oriented, grow at the expense of those in unfavourable orientations and this is manifested in a predictable field-induced motion of the walls that separate one domain from the next. Here, we report the discovery that specific charged 90o domain walls in copper-chlorine boracite move in the opposite direction to that expected, increasing the size of the domain in which polarisation is anti-aligned with the applied electric field. As a consequence, polarisation-field (P-E) hysteresis loops, inferred from optical imaging, show negative gradients and non-transient negative capacitance, throughout the P-E cycle. Switching currents (generated by the relative motion between domain walls and sensing electrodes) confirm this, insofar as their signs are opposite to those expected conventionally. For any given bias, the integrated switching charge due to this anomalous wall motion is found to be directly proportional to time, indicating that the magnitude of the negative capacitance component should be inversely related to frequency (for a given applied ac field). This passes Jonscher’s test for the misinterpretation of positive inductance (associated with an inverse square relationship) and gives confidence that field-induced motion of these specific 90o charged domain walls generates a measurable negative capacitance contribution to the overall dielectric response.