In this paper, the lattice vibration modes of ammonium dihydrogen phosphate NH4H2PO4(ADP) and its deuterated analog DADP are studied by using polarized Raman spectra and the first-principles calculation in the framework of the density functional theory. The vibration modes of ADP crystal, in which the basic structural units are the NH+4and H2PO-4groups, have been simulated by using a plane-wave pseudo potential method. Result shows that the peaks of921 and near 3400 cm-1are assigned as the vibrational H2PO-...