Na2SeO4.H2SeO3.H2O crystals are orthorhombic, space group Cmc2(1), with a = 9.615(6)angstrom, b = 11.193(12) angstrom, c = 9.615(10) angstrom and Z = 4. The crystal structure was determined from three-dimensional X-ray diffraction data taken on an automatic diffractometer with MoK-alpha radiation and refined by least-squares techniques to R = 0.049 for 775 non-zero reflections. The selenate anions and selenious acid molecules occupy C(s) sites and form infinite chains through hydrogen bonds with an 0...0 distance of 2.622(15) angstrom. Water molecules of C(s) symmetry are attached to the selenious acid molecules by weak hydrogen bonds with an 0...0 distance of 2.957(25) angstrom. The second proton (H(61) ) of the water molecule interacts with two pairs of oxygen atoms belonging to the neighbouring chains. The stretching vibration of the strong hydrogen bonds (2.622 (15) angstrom) shows a characteristic feature containing two strong bands at almost-equal-to 2800 cm-1 (A) and 2400 cm-1 (B) and a very weak band at almost-equal-to 1500 cm-1 (C), appearing in both IR and Raman spectra. The frequencies of the A and B bands are different in the IR spectra for the X(a) and Z(c) polarizations as well as in the polarized Raman spectra. This is due to a strong interaction/coupling between the nu-OH vibrations of the nearest hydrogen bonds in the same chain. The transition dipole moment of the nu-OH is almost parallel to the Se(1)...Se(2) direction. The delta-15-OH vibration gives bands in the region 1300-1260 cm-1, while gamma-OH appears at almost-equal-to 790 cm-1. The internal vibrations of the selenate anions and selenious acid molecules are discussed in terms of a site and factor group effect. An assignment of their bands is proposed in terms of the approximate type of motions using the "oriented gas" model approach. The stretching vibrations of the water molecules appear, in fact, to be stretching vibrations of the hydrogen bonds. It is spectroscopically proved that the H(61) protons of water molecules interact with as many as four oxygens.
