Rb87NMR study of the paraelectric-antiferroelectric phase transition inRb0.22(ND4)0.78D2PO2

${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ (1/2→- 1) / 2 NMR line-shape and spin-lattice relaxation data demonstrate the presence of a first-order phase transition from a paraelectric (PE) to an antiferroelectric (AFE) phase for the mixed crystal ${\mathrm{Rb}}_{0.22}$(${\mathrm{ND}}_4$ ${)}_{0.78}$ ${\mathrm{D}2\mathrm{PO}}_4$ around $T_c$=150 K. The transition is driven by an antiferroelectric soft mode as in pure ${\mathrm{NH}}_4$ ${\mathrm{H}}_2$ ${\mathrm{PO}}_4$. Between 150 and 140 K the PE and the AFE phases overlap, and a continuous decrease of the paraelectric volume fraction or an increase of the antiferroelectric one, respectively, was observed. The determination of the ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ electric-field-gradient (EFG) tensors in both phases revealed that the average symmetry relations of the EFG tensors are consistent with the space groups I4¯2d (PE phase) and P$2_1$ $2_1$ $2_1$ (AFE phase), respectively, of the pure ${\mathrm{NH}}_4$ ${\mathrm{H}}_2$ ${\mathrm{PO}}_4$ crystal. The increase in the inhomogeneous broadening of the NMR lines with decreasing temperature indicates an incipient glassy-type ordering before the AFE long-range ordering sets in.