Static and dynamic central peaks in the freezing of a structural glass

Combining dielectric, Brillouin, and neutron measurements, two spectral components related to freezing are identified in single crystals of Rb0.38(ND4)0.62D2PO4. The first corresponds to the incommensurate correlations that develop upon cooling and saturate near 80 K. These seem to be mostly static. A second, long-wave dynamic component continues to evolve markedly down to 30 K, developing a broad distribution of slow relaxations. The former may provide the hierarchy of constraints leading to the latter. © 1988 The American Physical Society.