The continuous-wave deuteron magnetic resonance of N${\mathrm{D}}_4$${\mathrm{D}}_2$P${\mathrm{O}}_4$ (ADP) was investigated above and below the antiferroelectric phase transition temperature (228°K). Principal coordinate systems of the deuteron-electric field gradient tensor were determined. A large line splitting occurs below $T_c$ owing to the localization of deuterons on phosphate tetrahedra. Smaller line splittings also occur which are due to distortions of the OD bond directions. Proton relaxation times of N${\mathrm{H}}_4$${\mathrm{H}}_2$P${\mathrm{O}}_4$ exhibit a minimum at 170°K and a discontinuity near the transition temperature ($T_c=148\mathrm{}°$K). The discontinuity is accompanied by a change in activation energy for the N${\mathrm{H}}_4^{+}$ ion reorientation from 3.7 kcal ${\mathrm{mole}}^{-1\mathrm{}}$ ($T>\mathrm{}{T}_c$) to 4.3 kcal ${\mathrm{mole}}^{-1\mathrm{}}$ ($T<\mathrm{}{T}_c$). ADP was doped with arsenic and the electron paramagnetic resonance of the $\gamma$-irradiated material was investigated from 77 to 330°K. A transition of the triplet-proton hyperfine structure to a quintet was observed in the vicinity of 300°K. The measurements performed show that the proton orientations and dynamics are very similar to those observed for potassium dihydrogen phosphate (KDP) and that the main difference in dielectric behavior between ADP and KDP results from the reversal of Slater energies of proton configurations about the phosphate tetrahedra. The energy reversal appears to be due entirely to the ammonium ions.

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