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) over bar , 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 C-2, 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.