Hydrothermal synthesis and characterization of M2Te3O8 (M = Mn, Co, Ni, Cu, Zn):: A series of compounds with the spiroffite structure

Several compounds isotypic to the mineral spiroffite were obtained as high quality single crystals, ia hydrothermal reactions. These materials, which have the general formula M2Te3O8, crystallize in the monoclinic space group C2/c and contain divalent transition metal ions and tellurite building blocks. Typical synthetic conditions for the preparation of these materials are 375 degrees C for 3-5 days in an aqueous NH4Cl solution. This structure type is based on slabs containing Te2O6 groups joined by M cations. These Te2O6 groups are made up of two edge-sharing TeO3+1 polyhedra. The metal tellurite slabs are connected to one another through tellurium(IV) atoms, which are coordinated by four oxygen atoms in a regular TeO4 trigonal bipyramidal configuration. A detailed structural characterization via single crystal X-ray diffraction was performed on four compounds (Co2Te3O8: a = 12.690(1) Angstrom, b = 5.211(2) Angstrom, c = 11.632(2) Angstrom, beta = 98 98(1)degrees, V= 759.7(3) Angstrom(3), R (R-w) = 0.023 (0.046); Ni2Te3O8: a = 12.407(1) Angstrom, b = 5.207(1) Angstrom, c = 11.509(1) Angstrom, beta = 98.723(9)degrees, V= 735.0(2) Angstrom(3), R(R-w) = 0.025 (0.033); Cu2Te3O8: a = 11.8368(8) Angstrom, b = 5.266(2) Angstrom, c = 12.2419(8) Angstrom; beta = 100.316(6)degrees, V= 750.7(2) Angstrom(3), R(R-w) = 0.025 (0.032); Zn2Te3O8: a = 12.681(2) Angstrom, b = 5.200(2) Angstrom, c = 11.786(2) Angstrom, beta = 99.60(1)degrees, V = 766.3(3) Angstrom(3), R(R-w)= 0.031 (0.041)). The manganese analog (a = 12.896(5) Angstrom, b = 5.398(2) Angstrom, c = 11.907(5) Angstrom, beta = 98.14(4)degrees) was identified via powder diffraction patterns. The cobalt, nickel, and copper compounds were observed to have an antiferromagnetic transition by DC magnetic susceptibility measurements. The band gaps were obtained by optical diffuse reflectance spectroscopy and show that these compounds are all wide band gap materials. (C) 1999 Academic Press.