06-21U

The research of the physical properties of K(H$_{1-x}$D$_x$)$_2$PO$_4$\ crystals was performed in the framework of proton ordering model with tunneling in cluster approximation. The KD$_2$PO$_4$'s\ model parameters were derived under condition of acceptable description of the experimental data for crystal's thermodynamic characteristics. On the basis of the introduced procedure deriving parameters for K(H$_{1-x}$D$_x$)$_2$PO$_4$\ crystals, the thermodynamic properties of these crystals was calculated and the results were compared to the experimental ones. The adequacy of the models with and without tunneling to the description of K(H$_{1-x}$D$_x$)$_2$PO$_4$\ crystals was compared. The problem of applying the mean crystal approximation to analysis of physical characteristics K(H$_{1-x}$D$_x$)$_2$PO$_4$\ crystals was discussed.