AbstractThe Raman spectra of oriented single crystals of7LiH2PO3and6LiH2PO3at 293 K and 83 K are reported and compared.6Li/7Li substitution was used to identify the translational modes of the lithium atoms. Bands with a lithium isotopic shift of 3–6% are observed in the 300–500 cm−1region. The spectra of the6LiH2PO3single crystal indicate a coupling between the lithium vibrations and the deformation modes δ(PO3) of the HPO2OH−ion. Assignments are proposed for internal modes of the crystal. Polarized infrared reflection spectra at 293 K and longitudinal Raman spectra at 293 K and 83 K in the 400–2500 cm−1region are reported for7LiH2PO3single crystal. An analysis in terms of Drude oscillators leads to a better understanding of the dielectric properties of the monocrystal and also to the determination of the dispersion parameters of the transverse and longitudinal optical modes. The values obtained for the longitudinal frequencies and for the oscillator strengths in Raman scattering and infrared reflection are in go