Crystal structure of potassium tetramolybdate, K2Mo4O13, and its relationship to the structures of other univalent metal polymolybdates

K2Mo4O13 crystallises in the triclinic system, space group P, with Z= 2 in a unit cell of dimensions a= 7·972, b= 8·352, c= 10·994 Å, α= 119·4, β= 62·7, and γ= 109·8°. Its structure, determined by three-dimensional Patterson and Fourier methods and refined by least-squares techniques, contains subunits of eight distorted MoO6 octahedra sharing edges, joined by a common edge to form infinite chains, with adjacent chains having no common oxygen atoms. The potassium ions occupy irregular eight-co-ordinate interchain sites. Rb2Mo4O13 is isomorphous with K2Mo4O13. The tetramolybdate chain structure is compared with those of some other anhydrous polymolybdates(VI), and the similarity of the subunit of eight octahedra to the basic units found in a number of complex molybdates, and to the hepta- and octa-molybdate polyanions, is discussed. The possible existence of polyanions in molybdate melts is considered, and the ability to predict polymolybdate structures from i.r. absorption spectra critically assessed.