Phase transition of $Rb_{1-x}(NH_4)_xH_2AsO_4$ mixed crystal for x=0, 0.27,0.31,0. 41 and 0.75 was investigated by Raman scattering dependence on temperature. We have observed spectral changes corresponding to structural glass transition for samples of x=0.27, 0.31 and 0.41, while antiferroelectric phase transition for the x=0.75 sample. We could not observe any significant anisotropy in Raman susceptibility dependence on temperature between $B_2$-mode and E-mode low frequency Raman bands with respect to the proton glass freezing effect. Dielectric anisotropy of recent report may thus seem to be correlated with the proton configurational excitation of order-disorder fluctuations (below $20cm^{-1}$) rather than the proton tunneling modes.
