Evidence for Ferroelectric Nucleation Centres in the Pseudo-spin Glass System Rb_1−x (ND₄) _x D₂PO₄: A ⁸⁷Rb NMR Study

Abstract

The transition region between long-range ferroelectric (FE) order and glass order in the phase diagramof the deuteron glass system Rb_{1− x} (ND₄) _x D₂PO₄ (D-RADP-x) extends from about x = 0.20 tox = 0.32. In this region we have observed phase segregationbetween the two phase states using ⁸⁷Rb NMR. We assume that clusters of FE ordernucleate with different local transition temperatures depending on the accidental Rb content at variousplaces in the crystal. The size of the FE clusters must be above a critical value in order to exhibitindividual transition temperatures (T _c), as wellas individual soft modes. Soft mode behaviour was unambiguously observed in the temperature dependenceof the ⁸⁷Rb spin lattice relaxation (T ₁)well above the average T _c. We expect a clustersize of the order of 5 nm, clearly smaller than the optical wavelength, but well above the observationrange of the Rb nucleus of about 1.4 nm. From our measurements we have determined the total FE andglass phase volume fractions as a function of temperature. The behaviour of the system is discussedfor different crystal compositions in the coexistence region of the two phase states.