SPATIAL CORRELATION OF DEUTERON ORDERING IN THE GLASS PHASE OF RB0.5(ND4)0.5D2PO4

We have investigated the extent of the spatial correlation of deuteron ordering in the solid solution Rb1-x(ND4)xD2P04 (D-RADP-X, X = 100x) by means of quadrupole-perturbed Rb-87 NMR. In contrast to the usual a pproach where glass transitions are discussed in terms of a departure from complete disorder, we are treating the ordering in D-RADP-X as a departure from the long-range-ordered ferroelectric (FE) and antiferro electric (AFE) phase states. The composition parameter x is a paramete r for the competing FE and AFE interactions which reduces the spatial correlation length of the ordered phases to the nanometric region (0.3 5 < x < 0.56). This approach allows us to make use of techniques emplo yed in the classical distortive phase transitions and to expand them t o short-range correlated systems. The results of this analysis clearly show that the glass state of D-RADP-50 consists of short-range-ordere d AFE clusters of an average size 6 to 8 nm3: i.e., a correlated order of about 150-200 O-D ... 0 bonds. This size is too small to exhibit f eatures of the long-range-ordered AFE phase state, such as a phase tra nsition with a well-defined transition temperature. Nevertheless, this local order can be distinguished unambiguously from a random freeze-o ut of the deuterons in the O-D ... 0 bonds.