Polarised vibrational spectra of KH2PO3 single crystal

Polarised IR and Raman spectra for KH2PO3 single crystal samples were measured at room temperature. Additionally, the IR spectra for the Xb(Z) sample were also measured at low temperatures (300-14 K). The spectra are discussed on the basis of oriented gas model and group theory. The stretching vOH vibrations of the hydrogen bonds with the O ... O distances of 2.547 and 2.529 Angstrom give characteristic broad ABC-type bands in the LR (polarised parallel to the X and to the b(Z) directions) and Raman (xx, xz and yz) spectra. The Davydov-type (correlation field or factor group) splitting is not observed for the vOH modes. The presence of two independent hydrogen bonds in the crystal is manifested by splitting of the C band into two (C', C ") components and by the different frequencies of the out-of-plane bending gamma OH vibrations. The in-plane bending modes delta OH are strongly mixed/coupled with the stretching vibrations of the PO3 groups. The C bands (C' and C ") change into quite sharp bands on lowering of the temperature. Various simplified models for internal vibrations of the phosphite anions are applied for finding a correlation between the crystal structure and polarised vibrational spectra, The stretching vibrations of the vPH groups manifest their unequivalence in two symmetry-independent hydrogenphosphite anions. (C) 2000 Elsevier Science B.V. All rights reserved.