Fig. 1. (a) Visualization of the wolframite-type MWO4 structure. The 3d-metal atoms (M) are orange, tungsten atoms are grey and oxygen atoms are red. The structure consists of chains of edge-sharing metal–oxygen octahedra, and the chains are linked with each other by corner-sharing. (b) Illustration of a sandwich-type cluster, consisting of two Keggin fragments and a 3d metal (M) bridging layer. Each Keggin fragment is missing a triad (i.e. a group of three edge-sharing [WO6]10− octahedra) which is replaced by the bridging layer, and is cradling a heteroatom (X), which in some cases is the same element as in the bridging layer. The bridging layer connects to the fragments with a rotational angle typical for (c) β-Keggins, 60° compared to the α-Keggin. Structures are drawn with VESTA.24.
