Bonding Between the Decavanadate Polyanion and the Interstitial Complex in Pascoite-Family Minerals

The decavanadate isopolyanion, [V10O28]6–, is a constituent of pascoite-family vanadate minerals and synthetic materials, and both protonated, [HxV10O28](6–x)–, and mixed-valence, [V4+xV5+10–x)O28](6+x)–, varieties have been described. Here we analyze the interaction between the interstitial complex and the decavanadate structural unit using the principle of correspondence of Lewis acidity-basicity. The Lewis base strengths of the decavanadate polyanions vary from 0.054 to 0.154 vu and [V10O28] structures can form from the simple cations Cs+, Rb+, K+, Tl+, and Na+; simple cations with higher Lewis acidities are too acid to form structures. Cations may bond to transformer (H2O) groups to form polyatomic cations that have lower Lewis acidities than the corresponding simple cation. The occurrence of the polyatomic cation {(V5+O2)Al10(OH)20(H2O)18}11+ in caseyite shows the potential for decavanadate phases to incorporate large heteropolycations into their structures. In turn, this suggests that the [V10O28] polyanions may be used to induce co-crystallization of large aqueous polyatomic cations, thus facilitating their structural characterization. There is an inverse relation between the amount of (H2O) in the interstitial complex and the number of bonds between interstitial simple cations and the O2– ions of the vanadate units, and there is a strong correlation between the unit-cell volume per decavanadate unit and the number of (H2O) groups.