A synchrotron single crystal X-ray structure determination of (NH sub 4 ) sub 3 Mo sub 4 P sub 3 O sub 16 : A microporous molybdenum phosphate with Mo sub 4 O sub 4 sup 6+ cubes

Reaction of MoO{sub 3}, Mo, (NH{sub 4}){sub 2}HPO{sub 4}, H{sub 3}PO{sub 4}, and H{sub 2}O in a mole ratio of 1.4:1:3.6:6:120 at 360C for 16 hr gives a nearly quantitative yield of black cubes of (NH{sub 4}){sub 3}Mo{sub 4}P{sub 3}O{sub 16} (1). The structure of (1) was solved from data collected on a 30{times}30{times}30 {mu}m{sup 3} crystal at the National Synchrotron Light Source at Brookhaven National Laboratory. The compound is cubic, space group P{ovr 4}3m, with a=7.736(2) {angstrom}, and was refined to residuals of R(R{sub w}) = 0.035(0.049). Phosphate (1) is isotypic with Cs{sub 3}Mo{sub 4}P{sub 3}O{sub 16} and is related to the iron arsenate mineral pharmacosiderite. Unlike the Cs{sup +} compound, (1) can be rendered microporous by thermal removal of the NH{sub 4}{sup +} cations to give ammonia with the charge compensating proton remaining behind the lattice. Water absorption isotherms show the reversible uptake of 5.6 wt% water, which corresponds to over 15 vol% void space in (1) after the NH{sub 3} removal. The framework consists of Mo{sub 4}O{sub 4}{sup 6+} cubes, with six Mo-Mo contacts of 2.570(4){angstrom}, joined together by (PO{sub 4}){sub 6/2} along (100) to form a 3-D network composed of tetramers of triply edge-sharing Mo-centered octahedra and phosphate groups alternating along all (100) directions. The windows and cavities in (1) are large enough that the NH{sub 4}{sup +} cations occupy several different positions in the unit cell.