Raman spectroscopic study of LiH2PO4

The dielectric constant of polycrystalline LiH2PO4 has been measured between 297 and 17 K. No marked changes were observed over this range, indicating that the room-temperature orthorhombic phase persisted up to 17 K. Raman spectra of polycrystalline LiH2PO4 were also measured at 297, 200, and 70 K in the frequency shift region of 15–4000 cm−1 with Raman-active vibrational modes naively assigned to low-frequency (0–300 cm−1) external and high-frequency (300–4000 cm−1) internal modes. In addition to the internal modes of the PO4 tetrahedra, the internal modes of the LiO4 tetrahedra spectroscopically manifested themselves between 390–500 cm−1. This frequency range overlaps those of ν2ν2 (PO4) and ν4ν4 (PO4). The LiH2PO4O–H vibrational frequencies were in good agreement with crystallographic reports that there are two types of hydrogen bonds: intermediate (long bonds) and strong (short bonds).