Electron Paramagnetic-Resonance and Electron-Nuclear Double-Resonance Observations of Domains in the Irradiated Ferroelectrics K${\mathrm{H}}_2$As${\mathrm{O}}_4$ and K${\mathrm{D}}_2$As${\mathrm{O}}_4$

A direct observation of ferroelectric domains in x-irradiated K${\mathrm{H}}_2$As${\mathrm{O}}_4$ and K${\mathrm{D}}_2$As${\mathrm{O}}_4$ using electron paramagnetic resonance (EPR), and in the case of K${\mathrm{H}}_2$As${\mathrm{O}}_4$ also using electron-nuclear double-resonance (ENDOR), is reported. The nature of the observed domain splittings and consequently the effects of an externally applied electric field on the EPR and ENDOR spectra are explained. Moreover, the higher resolution possible with the ENDOR technique, has, for the first time, made it possible to use protons as microscopic probes and to identify in general lines from individual domains in all directions.