Neutron scattering study of freezing inRb1−x(ND4)xD2PO4

Crystals of the type ${\mathrm{Rb}}_{1\mathrm{-}\mathrm{x}}$(${\mathrm{NH}}_4$ ${)}_{\mathrm{x}}$ ${\mathrm{H}}_2$PO ${}_4{}^{}{}_{}{}^{}$ are frustrated due to the competing interactions of rubidium and ammonium which favor the ferroelectric and antiferroelectric configurations, respectively, within the hydrogen-bond system. The temperature, wave-vector, and frequency dependences of diffuse scattering are reported for a fully deuterated crystal of this type with an ammonium concentration of 62 mol %. Diffuse scattering streaks are observed along the 〈100〉 directions of the tetragonal crystal which peak at a wave vector that corresponds neither to the ferroelectric nor the antiferroelectric ordering of the pure crystals. Their intensity pattern in reciprocal space displays systematic extinction according to the odd representation of the group of this wave vector. The energy-resolved diffuse scattering has been analyzed as a superposition of an elastic, a quasielastic, and a very broad inelastic contribution. The quasielastic width decreases rapidly on cooling and cannot be resolved below 110 K.