SYNTHESIS, CRYSTAL-STRUCTURE, AND MAGNETIC MEASUREMENT OF 2 NEW DISELENITES - M(2)(SE2O5)(3) WITH M=FE(III), CR(III)

Two new diselenite compounds have been synthetized by solid state reac tion and their structures have been determined: Fe-2(Se2O5)(3) (I) and Cr-2(Se2O5)(3) (II). Both compounds crystallize in the monoclinic sys tem, with the same space group P2(1)/n and moreover the unit cell dime nsions 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 ox ygen atoms with Se2O52- groups, thus forming a three-dimensional netwo rk. Each diselenite group acts as tetramonodentate ligand. Iron octahe dra 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 rev eal a weak antiferromagnetic coupling for both compounds with a transi tion to a long range magnetic ordering at low temperature T = 32 K for (I). (C) 1996 Academic Press, Inc.