Microwave and millimetre-wave dielectric response of Rb_1-x(ND₄)D₂PO₄ dipolar glass

The microwave and millimetre-wave dielectric response of Rb_0.5(ND₄)_0.5D₂PO₄ dipolar glass has been studied around the frustration temperature T_f≈120 K. It was found that above the frustration temperature T_f the soft relaxational deuteron mode is responsible for the whole dielectric dynamics below the phonon range. Its frequency decreases from that of submillimetre waves to that of microwaves following the classical law ν_R = A(T-T_f), with A = 0.75 GHz K^-1, but in the vicinity of T_f the soft mode gradually shifts towards the glass-like dispersion predominantly related to the diffusion of Takagi defects according to the Vogel-Fulcher law. From the experimental results, the distribution of the relaxation times and of the local polarization at various temperatures is calculated. It is shown that the local polarization distribution function obtained from the dielectric response ε*(ν,T) coincides well with that obtained from the NMR results.