SYNTHESIS AND STRUCTURE STUDIES OF BA2H[ALPHA-FEO4W12O36].26H2O

The heteropolyanion compound Ba2H[alpha-FeO4W12O36] . 26 H2O (1) crystallizes in the tetragonal space group P4n2BAR with the lattice parameters a = 12.398(6), c = 18.721(6) angstrom; Z = 2; D(x) = 4.128 g . cm-3. The structure was solved on a twinned crystal from 1029 observed reflections and refined to an index R of 7.6%. The calculations were done by means of a modified ORFLS-programme by EITEL and BARNIGHAUSEN. The heteropolyanion [alpha-FeO4W12O36]5- has the well known alpha-Keggin structure. The average distance of the four central oxygen atoms to the Fe(III) position (0,0,0) is 1 84 angstrom. The angles angle O-Fe-O are 112.3-degrees (4 x) and 103.9 (2 x), respectively, which leads to an disphenoidal distortion of the FeO4 tetrahedron. The powder and single crystal ESR spectra of I show the anisotropy of the FeIII fine structure transition 1/2 <--> - 1/2. The MoBbauer spectra confirm the tetragonal distortion of the central FeO4 tetrahedron (quadrupole splitting DELTA almost-equal-to 0.50 mm . s-1).