On the use of Guinier-Hägg film data for structure analysis

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Abstract
From the information contained in one powder photograph obtained in a Guinier-Hägg camera the crystal structure of (NH4)4[(MoO2)4O3] (C4H3O5)2 · H2O, only known in microcrystalline form, has been determined and refined to a moderate degree of accuracy. The work has comprised step-scanning measurement of the intensity profile, indexing with a trial-and-error program, determination of the Mo positions from an incomplete Patterson function derived from resolved integrated intensities, and finally localization of light non-hydrogen atoms and structure refinement by a series of Fourier calculations and profile analyses. The material crystallizes in the monoclinic system, space group C 2, with two formula units in a cell of dimensions a = 14.572(2) Å, b = 10.114(2) Å, c = 11.461(2) Å and β = 121.45(1)°. Since as many as 51 parameters are involved the atomic coordinates obtained are approximate, but the main features of the structure are established. The structure contains discrete [(MoO2)4O3 (C4H3O5)2]4− ions having a twofold axis through an oxygen atom.