Aging dynamics in ferroelectric deuterated potassium dihydrogen phosphate

Anomalously large dielectric aging is found in the high-susceptibility plateau ferroelectric regime of nominally $\ensuremath{\sim}95%$ deuterated potassium dihydrogen phosphate (DKDP). Much less aging is found in nondeuterated KDP throughout most of its plateau region. Optical images of the DKDP domain structure show no dramatic change during aging. Small changes in electric field restore the preaged susceptibility, but the previous aging almost recovers after returning to a respective aging field. Susceptibility vs field can show memory of at least two prior aging fields. Aging effects are not fully erased even by brief heating above the Curie point, indicating a role for diffusion of hydrogen to the domain walls, leaving changes in disorder that can survive temporary absence of domain walls. Asymmetrical nonlinear susceptibility develops for fields slightly above or below a prior aging field and during field sweeps, with the dependence of the second-harmonic magnitude on sweep rate giving a characteristic time comparable to the time for hydrogen to diffuse a domain-wall width.