Hydrous potassium and ferric iron sulphate (Maus's salt)

Maus's salt is reported in the literature as having the chemical formula KsFea(SO4)6(OH)2.nH20 where n=8,9 . Hexagonal, P63/m, a=9.71 (1), c= 18.96 (2) A, Z = 2 , Dm=2"412 (Scharizer, Z. Kristallogr. (1923). 58, 420-444), Dx = 2.437 g cm -a. The structure consists of groups Fe30(H20)3(SO4)6, built up by three FeOs(H20) octahedra and six SO4 tetrahedra. Linkages formed by one kind of K atom give a stratiform arrangement with inter-sheet connexions provided by other K atoms. There is evidence of (K, H20) substitution, while the presence of oxonium and hydroxyl groups is suggested by symmetry considerations. Introduction. Short prismatic crystals of the above compound were obtained by evaporation at 70-80°C of an aqueous solution of ferric iron sulphate and potassium sulphate. Unfortunately these crystals are very unstable and quickly deteriorate if removed from the equilibrium solution. For the X-ray study a crystal (,-~0"2x0.2x0.3 mm) with its mother liquor was sealed in a Lindemann-glass capillary tube. This avoids deterioration of the crystal but has the disadvantages of X-ray absorption by the solution and the growth of secondary crystals. Weissenberg photographs revealed the crystals to be hexagonal, belonging to the space groups P63/m or P6a, thus confirming the previous results (Beran, 1971). Unit-cell parameters were obtained from zerolevel Weissenberg photographs and refined (using a least-squares method) from high-0 reflexions (Mo Kc~ radiation, automatic diffractometer). Intensities were collected on a Philips PW 1100 four-circle automatic diffractometer by ~-20 scans (scan speed 0.04 ° s -~, scan range 1.2 °, 2°< 0< 26 °) using Mo Ke radiation and a graphite monochromator. A total of 1059 independent reflexions were measured. Of these, 168 weak reflexions with (Itop-2/]/~op) < Ibck were omitted (/top is the intensity in counts/sec measured at the maximum of the reflexion during the first scan; Ibck is the mean of two 5 s preliminary background measurements). To this group of unobserved reflexions a conventional Fo value identical with the minimum measured Fo was attributed. Intensities were corrected for Lorentz and polarization effects. The tangent formula of Karle & Karle (1966) was used to locate the heavy atoms. The structure was refined in the space group P6a/m by the fullmatrix least-squares method (Busing, Martin & Levy, 1962). In the last cycles of refinement anisotropic thermal parameters were taken into account, while four reflexions (002, 0,0,10, 4,1,10, 0,0t16 ) were excluded because of secondary extinction effects. The final conventional R index was 0.086 for 887 non-zero reflexions and 0-099 including the unobserved reflexions. The atomic scattering factors used in this work are those given by Cromer & Waber (1965) with anomalous dispersion correction for Fe after Cromer (1965).