Polarised FTIR and Raman spectra of betaine phosphite single crystal I. Paraelectric phase

Abstract

Polarised IR and Raman spectra of the betaine phosphite (BPI = Betaine-H 3PO 3) single crystals were measured at room temperature (paraelectric phase). The assignment of the bands is proposed on the basis of the oriented gas model approximation. The stretching vibrations ( vOH) of the hydrogen bonds give strong absorption extending from ca. 3000 cm −1 to 500 cm −1 in the IR spectra polarised parallel to the ferroelectric b(Z) axis (0(4)H(13)...0(4′) (PP2) hydrogen bond) and parallel to the X axis (0(2)H(12)...0(3) (BP) and 0(6)H(15)...0(6′) (PP1)) hydrogen bonds). Those absorptions show characteristic ABC structure with quite weak the A bands. The C bands (ca. 1600 cm −) correspond to the fundamental νOH vibrations. The stretching vibrations of the PO bonds in the phosphite anions show unusual properties which may be explained either by strong coupling with the vibrations (stretching vOH and out-of-plane bending γOH) of the hydrogen bonds or by the disorder of the protons in the inter-phosphite (PP1 and PP2) hydrogen bonds. The betaine molecules are in the form of cations ((CH 3) 3N +CH 2COOH). Some internal vibrations ( vC=O, v sC 3N) strongly interact with the stretching vibrations of the hydrogen bonds. Broad scattering (the broad wing of the Rayleigh line) is observed in the Raman spectrum X(zz)Y below 200 cm −1. It originates either from a low wavenumber overdamped mode or from the disordering in the paraelectric phase.