Raman-scattering study of LiH2PO4

The structural evolution of polycrystalline LiH2PO4 has been studied between room temperature and 13 K by using dielectric measurement and Raman-scattering study. No significant change is observed over the temperature range, indicating that the room-temperature orthogonal phase persists to 13 K. Assignments of vibrational modes are qualitatively proposed: low-frequency (0 - 300 cm-1) external modes and high-frequency (300 - 4000 cm-1) internal modes. In addition to the internal modes of PO4 tetrahedra, the internal modes of LiO4 are spectroscopically found. This is a peculiar structural aspect of LiH2PO4, compared to other MX2RO4-type (M = K, Rb, Cs, NH4, Tl; X = H, D; R = P, As) crystals without MO4 tetrahedra. The modes in the frequency range of 390 - 500 cm-1 are assigned as the internal modes of LiO4. This frequency range overlaps with those of ν2(PO4) and ν4(PO4), and may complicate the assignment of internal modes of LiH2PO4. The LiO4 tetrahedron shares oxygens with the four neighbor PO4 tetrahedra. The frequencies of O-H vibration of LiH2PO4 indicate that there are two types of hydrogen bonds, including an intermediate one and a strong one.