engleski

Different refinements of [(Ta_6Cl_12)Cl(H_2O)_5][CdBr_4]• 6H_2O

The compound [(Ta_6Cl_12)Cl(H_2O)_5][CdBr_4]• 6H_2O contains the paramagnetic [Ta_6Cl_12]^3+ cluster unit possessing one unpaired electron (S = 1/2) in HOMO. Single crystal analysis and magnetic susceptibility data were reported in the literature [Vojnović, M. et.al., Matter. Res. Bull. (2001) 36, 211]. Structural data [Fd3m space group with a = 19.974 (1) &Aring ; ] show that cluster units are arranged in the diamond type lattice, with four nearest neighbours at the distance of 8.676 (1) &Aring ; . Magnetic susceptibility data were interpreted in a quasi one-dimensional model with two magnetic interactions, J = 68 K and J’ = 21 K (K = 1.38 x 1023 J). Structural analysis also shows that terminal ligand positions of cluster units are disorderly occupied by one Cl^- anion and five H_2O molecules. Assuming that Cl^- anion is responsible for emerging of two different magnetic interactions, a hypothetical structure of the title compound in the lower space group Imma and without disorder on terminal ligand positions was modelled. This space group is the highest subgroup of Fd3m having a property that some of the terminal ligand positions of a particular cluster unit are unique and symmetry independent. Refinements on freshly collected data (T = 200 K) in the space group Imma has not revealed assumed position of Cl^- anion ; the disorder was still necessary in the interpretation of the observed electron density at terminal ligand positions. Special kind of refinement was made assuming that the crystal is twinned by pseudo-merohedry with six different domains of Imma symmetry. Only by artificial fixing of chlorine and oxygen structural parameters (constituents located on terminal ligand positions) it was possible to refine the structure in this model and to obtain some reasonable values for the volume fractions of twin domains. All models were refined by Jana2000 crystallographic computing system. Crystal data for different models in of [(Ta_6Cl_12)Cl(H_2O)_5][CdBr_4]• 6H_2O (MoKα ): 1. model (disorder): space group Fd3m (No. 227), a = 20.170 (3), V = 8205 (2), Z = 8, p(Cl2) = 1/6 ; p(O2) = 5/6, R = 0.0413, Rw = 0.1225, No. of ref. = 330 2. model (disorder): space group Imma (No. 74), a = b = 14.262 (3), c = 20.170 (3), V = 4102.5 (2), Z = 4, p(Cl1, Cl2, Cl3) = 1/6 ; p(O1, O2, O3) = 5/6, R = 0.0452, Rw = 0.1303, No. of ref. = 1296 3. model (twin): space group Imma (No. 74), a = b = 14.262 (3), c = 20.170 (3), V = 4102.5 (2), Z = 4, α _1 = 0.17 (1), α _2 = 0.22 (1), α _3 = 0.10 (1), α _4 = 0.14 (1), α _5 = 0.17 (1), α _6 = 0.20 (1), R = 0.0412, Rw = 0.1094, No of ref. = 9002

x

nije evidentirano