Identifying protonated decavanadate polyanions

The decavanadate anion, [V (sub 10) O (sub 28) ] (super 6-) , is a common constituent in vanadate minerals and synthetic materials, and both protonated, [H (sub x) V (sub 10) O (sub 28) ] (super (6-x)-) , and mixed-valent, [(V (super 4+) (sub x) V (super 5+) (sub 10-x) )O (sub 28) ] (super (6+x)-) , varieties also occur. Here we examine recent structure refinements containing protonated decavanadates in order to develop simple criteria to help identify specific OD atom(s) belonging to (OH) groups of the decavanadate polyanion. Non-protonated O atoms on the surface of the decavanadate polyanion (i.e., [1]-, [2]-, and [3]-coordinated O atoms) have incident bond-valence sums from V atoms in the range approximately 1.7-2.0 vu. Protonated O atoms on the exterior have incident bond-valence sums from V atoms in the range approximately 1.2-1.5 vu and are either [3]-coordinated or [2]-coordinated by V atoms. All hydrogen bonds are directed toward a neighboring decavanadate polyanion in which the OA (acceptor) anions are 2.61-3.08 Aa from their respective OD (donor) anions on the adjacent polyanion, and there is a linear relation between the OD...OA distance and the incident bond-valence received by OD from the bonded V atoms. For well-refined structures, the incident bond-valence sums from V atoms and the associated OD...OA distances suffice to identify protonation of the decavanadate polyanion and the specific anion(s) that is (are) protonated. These criteria show that one structure reported as protonated is not protonated, and one structure reported as containing a mixed V (super 4+/5+) protonated decavanadate polyanion contains only V (super 5+) and is not protonated, and another structure also reported as containing a mixed V (super 4+/5+) protonated decavanadate polyanion contains both V (super 4+) and V (super 5+) but is not protonated.