Both real and imaginary parts of the dielectric constants are measured along a- and c-axes in the crystal samples of $Rb_{1-x}(NH_4)_xH_2PO_4$ (x=0.43) and $Rb_{1-x}(N-D_4)_xD_2PO_4$(x=0.4) in the temperature range from room temperature to 11K. In the case of $Rb_{1-x}(NH_4)_xH_2PO_4$(x=0.43) sample, there is no observable anisotropy in dielectric susceptibility between the two axes. But in $Rb_{1-x}(ND_4)_xD_2PO_4$ (x=0.4), we observed temperature difference of about 4 ~ 8K between glass transition temperatures of the two axes depending on frequency which ranges from 50Hz to 5MHz.
Phase transitions of $K_{1-x}(NH_4)_xH_2PO_4$ mixed pressed pellet samples are investigated by dielectric measurements and Raman scattering experiments over the whole range of x between x=0 and x=1. From the results of dielectric constant measurements, we observed phase coexistences for x of the phase boundary layer in the samples of $K_{1-x}(NH_4)_xH_2PO_4$. Temperature dependences of the $PO_4$ internal vibrational modes and $NH_4$ librational mode give significant informations about glass formation in addition to ferroelectric and antiferroelectric transitions. Behaviour of temperature dependence of the $\nu_2$(A$_1$) mode of $PO_4$ internal modes in $K_{1-x}(NH_4)_xH_2PO_4$ is very similar to that of completely deuterated $Rb_{1-x}(ND_4-)_xD_2PO_4$ sample.
