Raman scattering investigation of disorder and of hydrogen bonds versus temperature in sodium 4-nitrophenolate dihydrate (NPNa)

Sodium 4-Nitrophenolate dihydrate (NPNa), an optically highly non-linear material, has been synthesized in methanol in form of 6 X 9 X 6 mm3 transparent single crystals. X-rays precession measurements confirmed the values of the unit cell parameters of symmetry Ima2 equals C222v. Polarized Raman scattering experiments have been performed, as a function of temperature, between 10 and 300 K in a wide spectral range. We present the results in the low wavenumber region [10 - 200] cm-1 in which we studied in detail four peaks: 57, 55, 178 and 189 cm-1 at 20 K. The two former are lattice modes, the two latter are hydrogen bond vibrations corresponding to two different O...O distances, 2,00 angstroms and 2.789 angstroms respectively. In addition, a relaxor behavior has been observed and analyzed in relation with a possible disorder in the substance. The analysis of the behavior versus temperature of the wavevector, damping and intensity of the three vibrational modes provide new results with respect to the dynamical properties of the substance, compared to its structure.