High Resolution NMR $^{15}$N and $^{31}$P NMR Of Antiferroelectric Phase Transition in Ammonium Dihydrogen Arsenate and Ammonium Dihydrogen Phosphate

Natural abundance $^{15}$N CPMAS NMR has been used to investigate the paraelectric-antiferroelectric phase transition of NH$_4$H$_2$AsO$_4$ (ADA) (T$_{N}\sim 216$K) and of NH$_4$H$_2 $PO$_4$ (ADP) (148K), with a focus on the role of the NH$_{4}^{+} $ ion. Isotropic chemical shift of $^{15}$N for ADA exhibits an almost linear temperature dependence to within T$_{N}\pm$1K, and then changes discontinuously, followed by another almost linear dependence. The spectra of the paraelectric and antiferroelectric phases coexist around the T$_N$. The sharp anomaly around T$_N$ implies that the NH$_{4}^{+}$ ions undergo a displacive transition, whereas the protons in the O$-$H$\cdots$O bonds undergo an order-disorder transition. The $^{15}$N data thus support a mixed order-disorder-displacive mechanism for this transition. The $^{15}$N data on ADP exhibit somewhat different behavior. $^{31}$P CPMAS measurements will also be presented and discussed in terms of the above model.