Mixed thio/oxo orthovanadates Na3[VSxO4-x] (x=2, 3):: Synthesis, crystal structures, properties

Mixed sodium thio/oxo orthovanadates(V), dark red Na-3[VS3O] and orange red Na-3 [VS2O2], were synthesized via reactions in the melt starting from V, Na, Na2S, Na2O and sulfur. The structure of the low temperature phase of Na3[VS3O] (space group Pnma, a = 589.5(3), b = 962.8(5), c = 1186.6(6) pm, Z = 4, R1 = 0.0494) contains anions [VS3O](3-) almost identical to those known from the high temperature form, beta-Na-3[VS3O] (space group Cmc2(1), a = 968.4(4), b = 1194.6(4), c = 590.5(2) pm, Z = 4, R1 = 0.0291). The second order phase transition between these two forms at 536 degrees C was studied by temperature dependent powder diffraction and explained on the basis of a comparison of the anion packing in the two related structures. The packing of the dithiodioxovanadate anions in Na-3[VS2O2] (space group Pbca, a = 1162.7(2), b = 592.71(12), c = 1766.7 (4) pm, Z = 8, R1 = 0.0312) is also closely related. The chemical bonding in the anions [VS3O](3-) and [VS2O2](3-) of approximately ideal C-3v and C-2v symmetry is discussed on the basis of FP-LAPW band structure calculations and force constants obtained from Raman spectroscopy. The decrease of the calculated band gaps with increasing S content x in Na-3[VSxO4-x] is in accordance with the optical properties showing a gradually deepening of the crystal and solution colour. Discernible trends in the chemical bonding in this series of mixed thio-oxo anions also include the amount of pi bonding of the V-O and V-S bonds and the corresponding variation of force constants and V-ON-S distances.