We report a systematic study of the elastic and dynamic properties of mixed Rb1-x(NH4)xH2AsO4 (RADA) crystals using high-resolution Brillouin light scattering over a broad temperature range (40–300 K). By varying the ammonium concentration, we reveal how \(\:\text{N}{\text{H}}_{4}^{+}\) substitution alters the elastic stiffness tensor, phase transition temperatures, phonon damping, and order parameter relaxation dynamics. The results show a clear evolution from ferroelectric to antiferroelectric and proton-glass behavior as the ammonium content increases. We observe enhanced elastic softening, increased phonon scattering, and reduced relaxation times with doping, indicating a growing degree of structural disorder and frustration in the hydrogen-bond network. The observed changes in phonon thermal conductivity correlate with these elastic anomalies. Our findings highlight the potential of chemical substitution in tuning the mechanical and thermal properties of hydrogen-bonded ferroelectric crystals, with implications for designing adaptive low-temperature optical and memory materials.
