  Anomalously large dielectric aging is found in the high-susceptibility
plateau ferroelectric regime of ~95% deuterated potassium dihydrogen phosphate
(DKDP). Much less aging is found in non-deuterated KDP. Optical images of the
DKDP domain structure show no dramatic change during aging. Small changes in
electric field restore the pre-aged susceptibility, but the previous aging
almost recovers after returning to the aging field. Susceptibility vs. field
can show memory of at least two prior aging fields. Rectifying non-linear
susceptibility develops for fields slightly above or below a prior aging field.
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. Abrupt
random-sign steps in polarization on cooling are accompanied by increases in
susceptibility, indicating competition between large-scale domain-wall
interaction effects and effects of local interactions with disorder.
