DETAILED POWDER DIFFRACTION STUDY OF - Cu(NH3 )2 Br2 STRUCTURE
Vladimír Jorík, Dušan Mikloš, Michal Dunaj - Jureo, Milan Hrubo
Department of Inorganic
Chemistry ,Slovak Technical University Bratislava, 812 37 Bratislava, Slovak
Republic
jorik@cvt.stuba.sk
Keywords: crystallochemistry, powder
diffraction, Cu-complexes, Rietveld refinement
In the past a lot of time and effort was spent to give a satisfactory explanation of " a,b - isomerism" of Cu(NH3)2Br2 [1,2]. A few various reaction schemes of Cu(NH3)2Br2 preparation exist, which yield powder preparations of the so-called - isomer. The X-ray powder diffraction (XRD) seems to be an appropriate technique for such a study due to the lack of suitable single crystals. Indeed, the crystal structure of the " a- isomer" was solved from powder diffraction data in 1959 and classified as the monoclinic a- modification [3]. There were, however, discrepancies observed in powder diffraction patterns of samples originated from various preparations, and simulated diffraction pattern using the monoclinic structural model [3] differed from the experimental diffraction patterns, too. This motivated us to perform a detailed XRD study of the a-modification of powder Cu(NH3)2Br2 prepared by different ways.
A Rietveld refinement, based on XRD data, of three powder samples of the expected a- form of Cu(NH3)2Br2 resulting from different preparations, was carried out to clear up the discrepancies stated above. Tha data were collected with Stoe Stadi-P transmission diffractometer, equipped with Cu- or Co-tube and curved Ge(111) primary beam monochromator and a linear PSD. The initial estimates of the atomic coordinates were taken from [3]. All three powder patterns were analyzed mostly by the GSAS [4] system of programs, considering the C 2/m space group. The use of these parameters, however, causes the refinement progress to stop at unsatisfactory value of Rwp in case of all three samples. No routine procedure of Rietveld refinement changed this status. The mechanical mixture of pure monoclinic Cu(NH3)2Br2 and cubic NH4Br allowed the refinement to converge with final Rwp of about 8 % for the sample prepared by thermal decomposition. This result was confirmed by chemical analysis, too, which yields chemical composition of the sample not corresponding to the pure Cu(NH3)2Br2 compound. To obtain satisfactory refinements of the samples prepared from water or acetone solutions it was necessary to change the structural model. We have added a small fraction of the Cu-atoms into the positions, corresponding to the Cu-sites in the assumed cubic b- modification [5] of Cu(NH3)2Br2 in agreement with the Fourier difference map. The refinement ended with Rwp of 8.4 % and 7.5%, respectively.