Raman spectra of an Rb_0.5(ND₄)_0.5D₂PO₄ (DRADP-50) mixed crystal have been carefully investigated in the 15-300 K temperature range for all fundamental polarization geometries. The spectra have been interpreted in terms of vibrational modes of principal structural units and critically discussed in comparison with published data on analogous (deuterated and protonated) compounds. Analysis of the temperature dependence of peak positions and halfwidths has made it possible to estimate two characteristic temperature regions ( approximately 200-220 K and approximately 100-120 K) where incipient cluster formation and freezing of individual structural units takes place in this compound on cooling through the dipolar glass transition.