SYNTHESIS AND CRYSTAL-STRUCTURE OF NA3V(SO4)3 - SPECTROSCOPIC CHARACTERIZATION OF NA3V(SO4)3 AND NAV(SO4)2

Dark green crystals of the compound Na3V(SO4)3 suitable for X-ray structure determination were synthesized by dissolution of V2O5 in NaHSO4 melt at 420-degrees-C under an SO2(g) atmosphere. Slow cooling of the solution from 420 to 320-degrees-C over a period of one week gave a large number of small Na3V(SO4)3 crystals. The compound crystallizes in the rhombohedral (hexagonal) space group R3BAR with a = b = 13.439(1), c = 9.091(1) angstrom and Z = 6. It contains two independent almost perfect VO6 octahedra (within which equal V-0 bond lengths are found) linked by three identical bridging sulfate groups, as an infinite -V-(SO4)3-V- chain parallel to the c-axis. All oxygens coordinated to vanadium belong to the significantly distorted equivalent sulfate groups, which contain four different types of oxygen. The infrared and Raman spectra of polycrystalline Na3V(SO4)3 and NaV(SO4)2 have been recorded and interpreted. The severe distortion of the sulfate groups within Na3V(SO4)3 leads to a more complicated pattern of the vibrational spectra [i.e. IR activity of v1(SO42-) and extensive splitting of v3(SO42-) in the IR and Raman spectra] compared to NaV(SO4)2. Above 1500 cm-1 the Raman spectra, particularly at lower temperatures, exhibit broad features which are interpreted as electronic Raman transitions between the 3E(g) and 3A(g) split levels of the 3T1g (3F) ground state of V3+ (d2) in octahedral oxide fields.