The angular dispersion of polar phonons with wave vectors k(k >> ( o l c ) ) rotating in the planes of symmetry of the orthorhombic LiHzPOl single crystal has been observed with Raman scattering experiments at T = 293 K in the regions 2 5 0 6 0 0 cm-' and 850-1350 cm-'. The agreement of the experimental data with the existing theories of angular dispersion is very satisfying.
The Raman spectra of oriented single crystals of 7LiH2PO3 and 6LiH2PO3 at 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−1 region. The spectra of the 6LiH2PO3 single 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−1 region are reported for 7LiH2PO3 single 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 good agreement.
Using the relations between nonlinear optical coefficients and Raman scattering efficiencies of LO and TO phonons in LiH,PO, single crystal, numerical values of d,, and d,, coefficients, in relative units, have been determined. The variations of the Raman scattering efficiencies have been studied with the wave vector k rotating in the xOz and the y Ot planes. Theory and experiment are found to be in good agreement.
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