Raman Scattering Study of Rb$_{1-x}$(ND$_4$)$_x$D$_2$PO$_4$ Mixed Crystal


Raman-scattering spectra of a 71.4% deuterated crystal of Rb$_{0.52}$(ND4)$_{0.48}$D$_2$PO$_4$ were studied in a wide range of temperatures down to 5 K. At low temperatures the system forms a so-called glass state, with no long range order transition to either a ferroelectric or antiferroelectric phase.We have concentrated our study on the vibrations related to the internal modes of the molecular groups (phosphate and ammonium). Special attention is focused on the $\nu_2$ (symmetric deformation mode) of the PO$_4$ and the libration mode of the ammonia, because they are very sensitive to the ordering of the protons, and to the ferroelectric- or antiferroelectric-like ordering for the former and the glass-like arrangement for the latter.

Ferroelectrics 76, 23–32 (1987)