⁸⁷Rb NMR study of the paraelectric-antiferroelectric phase transition in Rb_0.52(ND₄)_0.48D₂AsO₄

⁸⁷Rb 1/2→-1/2 NMR line shape and spin-lattice relaxation time (T₁) data demonstrate the presence of a first-order transition from a paraelectric (PE) to an antiferroelectric (AFE) phase in substitutionally disordered Rb_0.52(ND₄)_0.48BD₂AsO₄ where the PE and AFE lines overlap between 140 and 135 K. The ⁸⁷Rb site symmetry is compatible with the I4¯2d symmetry above T_c and with the P2₁2₁2₁ symmetry of pure ND₄D₂AsO₄ below T_c. Superimposed on the glassy order that produces an inhomogeneous broadening of the ⁸⁷Rb NMR lines is a dynamic disorder of the deuterons in the O-D...O bonds above T_c and an antiferroelectric long-range order below T_c. The O-D...O dynamics is driven by an antiferroelectric soft mode which freezes out at T_c and determines the ⁸⁷Rb T₁.