Brillouin-scattering results from modes coupling linearly to the polarization are presented for ammonium concentrations x=0.15, 0.20, 0.25, 0.72, 0.75, and 0.78. For the two lowest and two highest concentrations, structural transitions occur on cooling, while the middle two represent both extremes of the range of x for which structural glasses form in this system. In combination with dielectric and birefringence measurements, the following information is extracted from the data: the anharmonicity of the uncoupled acoustic modes, the strength of the piezoelectric coupling, the growth of quasistatic electrostriction effects below 100 K in the glass-forming range, as well as that of dynamic effects also related to electrostriction. The strength of the latter two increases markedly with x. Excellent fits to the Brillouin spectra are obtained with a model of polarization fluctuations that assumes a broad distribution of relaxation times. Various integrals on that distribution are extracted from the spectra. The structural transitions near the glass boundaries of the phase diagram are complex. On the Rb-rich side, unusually strong polarization fluctuations remain below the transition. © 1987 The American Physical Society.