Synthesis, crystal structure, and magnetic measurement of two new diselenites: M(2)(Se2O5)(3) with M=Fe(III), Cr(III)

Two new diselenite compounds have been synthetized by solid state reaction and their structures have been determined: Fe-2(Se2O5)(3) (I) and Cr-2(Se2O5)(3) (II). Both compounds crystallize in the monoclinic system, with the same space group P2(1)/n and moreover the unit cell dimensions are similar: a 11.325(1) Angstrom, b = 11.038(1) Angstrom, and c = 11.563(2) Angstrom, beta = 102.56(1)degrees, P2(1)/n, Z = 4, and V = 1411.0(8) Angstrom(3) for (I); a = 10.904(1)) Angstrom, b = 11.369(2) Angstrom, and c = 11.673(2) Angstrom, beta = 95.11(2)degrees, P2(1)/n, Z = 4, and V = 1441.1(6) Angstrom(3) for (II). In both compounds, the structure is built up from isolated octahedra sharing all their oxygen atoms with Se2O52- groups, thus forming a three-dimensional network. Each diselenite group acts as tetramonodentate ligand. Iron octahedra are more distorted than chromium ones. Some differences appear in the thermal behavior studies of these compounds. The chromium compound presents only a one step process instead of two for the iron compound giving an intermediate phase Fe-2(SeO3)(3). Magnetic measurements reveal a weak antiferromagnetic coupling for both compounds with a transition to a long range magnetic ordering at low temperature T = 32 K for (I). (C) 1996 Academic Press, Inc.