(Nb2W4O19), TMA2, Na4(OH2)14(SO4): a new layered structure with Lindqvist heteropolyanions, XAS characterization of the HPAs

The new (Nb2W4O19),TMA2, Na4(OH2)14(SO4) has been evidenced as a minor phase during the Nb2W4O19TMA (tetramethylammonium) salt synthesis. Its crystal structure has been refined from single crystal X-ray diffraction data, system monoclinic, a=10.166(5) Å, b=17.93(1) Å, c=24.81(1) Å, β=93.057(7)°, space group (S.G.) C2/c, Z=4, R1=3.96%, wR1=4.50%. It shows the stacking of cationic and anionic bidimensional layers. The anionic layer of formula [(Nb2W4O19), TMA2 ]2− is formed of isolated Lindqvist HPAs surrounded by TMA groups. The isolated layers adopt a trigonal symmetry that is lost in the crystal by the association of the cationic sheets. These later, of formula [Na4(OH2)14(SO4)]2+ form porous net-like sheets with nearly circular cavities of diameter 7.5 Å. SO43− groups host the available cavities in a disordered manner. The cohesion between the sheets is performed by both electrostatic interactions and a set of hydrogen bonds. In the cationic layers, the highly symmetrical surrounding of HPAs by TMA groups yields a homogeneous electrostatic field at their external surface leading to a statistic Nb/W disorder over the three available independent metallic positions. Then, XAS experiments at the L1/L3-W edge complementarily helped to highlight the preferential cis configuration of (Nb2W4O19)4− anions, help to the strong Nb vs W contrast in their contribution to the backscattering paths. Previously to these experiments, it was of course checked that both the two phases present in the prepared sample contain Nb2W4O19 anions with nearly unchanged geometry.