Flux synthesis, crystal structure, and characterization of a gamma-vanadate selenite polymorph

Our exploration in vanadate selenites by using boric acid flux method has successfully lead into a new phase, namely, gamma-V2Se2O9. Single crystal X-ray diffraction indicates its a new structure which crystallizes in monoclinic space group P2(1)/c with a lattice parameter of a = 7.4857 (12) angstrom, b = 7.4388 (8) angstrom, c = 13.042 (2) angstrom, beta = 106.619 (9)degrees. Its crystal structure features novel 2D electroneutral [V2Se2O9] layers composed of 1D [V2SeO9](4)-chains that are further bridged by Se (1)O-3 triangular pyramids. [V2SeO9](4-) chain consist corner-sharing of VO6 and tri-dentate Se (2)O-3. Unlike previously reported alpha-and beta-V2Se2O9 with isolated [V4O18](16-)tetramers, corner-sharing of VO6 octahedra lead into a 1D [V2O9](8-)chain in gamma-V2Se2O9. X-ray powder diffraction and thermal behavior analysis were performed to study its phase purity and thermal stability. Optical diffuse reflectance spectra associated with Tauc's fitting indicates a direct bandgap semiconductor with an energy value of 2.34 eV, which consistent with the theoretical prediction. As a compare, the electronic bandgaps of three polymorphs were studied and shows a sequence of alpha-V2Se2O9<beta-V2Se2O9<gamma-V2Se2O9. Density of states were also calculated to reveal the origin of the bandgaps and bond interactions.