Magnetic resonance studies of the antiferroelectric phase transitions in the ammonium arsenates NH4H2AsO4 and ND4D2AsO4

The antiferroelectric phase transitions in NH₄H₂AsO₄ and ND₄D₂AsO₄ have been studied through EPR and NMR. The paramagnetic center ${\mathrm{AsO}}_4^{\text{4−}}$ , created by x irradiation of these compounds, was used as a microscopic probe in the EPR studies. Both ⁷⁵As hyperfine structure and the superhyperfine structure from the hydrogen‐bond protons showed changes on passing through the Curie point. Some features of the EPR spectra have been identified with the configurations postulated by Nagamiya in his model of the phase transition in these antiferroelectrics. In conjunction with earlier Raman scattering, Mössbauer effect, NMR, and dielectric studies of ADP, the present EPR studies point to a model of the phase transition where the ${\mathrm{NH}}_4^{+}$ ions play a dominant role. At high temperatures, well above the Curie point (T_c), the ${\mathrm{NH}}_4^{+}$ ions are thought to be executing fast reorientations such that they behave effectively like the spherical ions K⁺, Rb⁺, or Cs⁺. At T_c there is sudden change in the motion of the ${\mathrm{NH}}_4^{+}$ ions, and each ${\mathrm{NH}}_4^{+}$ ion is thought to make a permanent hydrogen bond with an oxygen of an AsO₄ group, leading to a distortion of the group and hence a phase transition. Measurements of the proton nuclear spin‐lattice relaxation times (T₁) support this model. Comparison with earlier results on NH₄H₂PO₄(ADP) suggests that the same mechanism is operative in the phase transition of ADP also. Several anomalies mentioned in earlier studies on KH₂PO₄‐type compounds could also be rationalized on the basis of the present model. Implications of the present work to the Nagamiya model and further experiments to check the conclusions of the present work are also pointed out.