A new mixed-valent molybdenum monophosphate with a layer structure (\mathrm{KMo}{3} \mathrm{P}{2} \mathrm{O}{14}) has been isolated. It crystallizes in the space group (P 2{l} / m) with (a=8.599) (2) (\AA, b=6.392) (2) (\AA, c=10.602) (1) (\AA), and (\beta=111.65(2)^{\circ}). The layers (\left[\mathrm{Mo}{3} \mathrm{P}{2} \mathrm{O}{14}\right}{x}) are parallel to (100) and consist of (\left[\mathrm{MoPO}{8}\right]{\infty}) chains running along (\vec{b}), in which one (\mathrm{MoO}{6}) octahedron alternates with one (\mathrm{PO}{4}) tetrahedron. In fact, four (\left[\left.\mathrm{MoPO}{8}\right|{x}\right.) chains share the corners of their polyhedra and the edges of their octahedra, forming (\left[\mathrm{Mo}{4} \mathrm{P}{4} \mathrm{O}{24}\right]{\times})columns which are linked through (\mathrm{MoO}{5}) bipyramids along (\overrightarrow{\mathrm{c}}). The (\mathrm{K}^{+})ions interleaved between these layers are surrounded by eight oxygens, forming bicapped trigonal prisms (\mathrm{KO}{8}). Besides the unusual trigonal bipyramids (\mathrm{MoO}_{5}), this structure is also characterized by a tendency to the localization of the electrons, since one octahedral site is occupied by (\operatorname{Mo}(\mathrm{V})), whereas the other octahedral site and the trigonal bipyramid are occupied by Mo(VI). The similarity of this structure with pure octahedral layer structures suggests the possibility of generating various derivatives, and of ion exchange properties. 1994 Academic Press, Inc.