07-12E

The paper presents the results of group-theory analysis and lattice dynamics simulation in three temperature phases of the monoclinic RbD$_2$PO$_4$. Combining the methods of group theory with the analysis of crystal structure peculiarities in different structural phases, it is shown that the lowest temperature phase should reveal a strict antiferroelectric state with P2$_1$ symmetry rather than the ferrielectric state reported repeatedly. Lattice dynamics simulation was performed in the quasi-phenomenological approach comprising the Coulomb, short range, covalent, van der Waals and hydrogen bonded interactions.