New mixed valent monophosphates (A_{3} \mathrm{O}{2}\left(\mathrm{MoO}{4}\right){4}\left(\mathrm{PO}{4}\right){4}) with an intersecting tunnel structure have been isolated for (A=\mathrm{Rb}, \mathrm{Tl}). The structure of the (\mathrm{Rb}-) phase has been solved from a single crystal X-ray diffraction study. It crystallizes in the space group (C 222{1}) with (a=14.222(1) \AA, b=14.223(1) \AA), and (c=19.227) (4) (\AA). The structure is in fact closely related to that of the Mo(V) monophosphate (\mathrm{K}{2} \mathrm{O}(\mathrm{MoO}){2}\left(\mathrm{PO}{4}\right){2}). It consists of similar double layers (\left[\mathrm{Mo}{2} \mathrm{P}{2} \mathrm{O}{11}\right]{\infty}) parallel to (001) and it is deduced from this (\mathrm{Mo}(\mathrm{V})) monophosphate by a rotation of one double layer out of two of (180^{\circ}), so that it can be described as a chemical twin of the phosphate (\mathrm{K}{2} \mathrm{O}(\mathrm{MoO}){2}\left(\mathrm{PO}{4}\right){2}). The geometry of the (\mathrm{MoO}{6}) octahedra is characteristic of (\mathrm{Mo}(\mathrm{V})) in spite of the mixed valent (\mathrm{Mo}(\mathrm{V})-\mathrm{Mo}(\mathrm{VI})). The bond valence calculations suggest a delocalization of the electron between the two octahedra of the bioctahedral units (\mathrm{Mo}{2} \mathrm{O}_{11}). 1994 Academic Press, Inc.