Raman spectroscopic study on the micro-structure of NH₄H₂PO₄ and ND₄D₂PO₄ crystals

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 NH₄ ⁺ and H₂PO₄ ^- groups, have been simulated by using a plane-wave pseudo potential method. Result shows that the peaks of 921 and near 3400 cm^-1 are assigned as the vibrational H₂PO₄ ^- and NH₄ ⁺ groups, respectively. In order to investigate the deuteration process, the polarized Raman spectra are obtained in different polarization configurations and recorded in the range of 200-4000 cm^-1, so that we can study the variation of Raman peaks at 921 and 2600-3400 cm^-1. For example, in the scattering geometry X(YY)X, with increasing degree of deuterated, the peak of 921 cm^-1 red shifts and decreases in intensity, while the peaks ranging from 2600 to 3000 cm^-1 decrease in intensity. Moreover, a new broadened line appears in the range of 2000-2600 cm^-1, and the intensity of the new line increases with the degree of deuterated, but no change occurs in the peak position. With Lorentz analysis, it can be inferred that the deuterated of NH₄ ⁺ group is prior to that of H₂PO₄ ^- group. We also study the spectra for other Raman scattering geometry of these crystals, and the result shows that the site symmetries of NH₄ ⁺(ND₄ ⁺) and H₂PO₄ ^-(D₂PO₄ ^-) groups are determined to be C₂, which means that the site symmetry of the two groups in crystals have no change during the deuteration process. These results will be a foundaton for optimizing the growth and property of this kind of material.