AB initio crystal structure determination of Cu0.96Bi0.016Rh2O4 from powder diffraction data with TOPAS

Full text: Structure determination by single-crystal X-ray diffraction is a more or less routine procedure in analytical laboratories. For structure solution from powder data a variety of methods can be applied. In recent years the tools and concepts for structure solution have been dramatically increased. Classical approaches were adapted in order to apply them to powder data or more special methods have been developed. Among all of them the most important are Simulated Annealing, Lattice Energy Minimisation, Distance Least Squares, Maximum Entropy and Genetic Algorithm methods. It was observed that a combination of these methods has a higher probability of a successful solution of the structure. Cu0.96Bi0.016Rh2O4 was prepared as powder by solid state reaction from a stoichiometric mixture of high purity grade CuO (99.99%, Heraeus), Bi2O3 (99.999%, Heraeus) and Rh2O3 (99.9%, Heraeus). The sample was ground with acetone in an agate mortar and annealed in an open gold crucible at atmospheric pressure in air at 700 deg C for 5 days and 750 deg C for 4 weeks with several intermediate grindings. X-ray diffraction data were collected on a Siemens D500 Θ/2Θ diffractometer with Bragg-Brentano geometry (Ge-monochromator, scintillation counter) using Cu Kα1 (0.1540598 nm) radiation and Θ/2Θ symmetric step scan mode. For indexing and structure determination the scan was done within a 2Θ-range of 10 deg-140deg with a step width of 0.02 deg 2Θ and a counting time of 38s per step. Indexing was carried out using the programs TREOR90 and ITO. Checking the systematic absences led to an orthorhombic cell with a = 0.307622(4) nm, b = 0.90924(1) nm and c=1.05771(1) nm with only one possible space group P212121 (No. 19). The structure determination was carried out using TOPAS. A combination of Simulated Annealing and Lattice Energy Minimisation was used to solve the structure. The structure determination resulted in an asymmetric unit with 8 different positions (1 Cu, 1 Bi, 2 Rh and 4 O). With a final Rietveld refinement all positional parameters were refined independently, giving a result with very good global R-values, low RBragg factor and reasonable bond distances/angles. In comparison to CuRh2O4 (tetragonally distorted spinel structure) the crystal structure of Cu0.96Bi0.016Rh2O4, although very similar in chemical formula, appears completely different. The structure is build up by double chains consisting of alternating, edge-sharing Rh-O octahedra in direction [100]. These double chains are connected among each other by shared corners and enclose large channels containing the Cu and Bi cations, arranged in an alternating sequence. The Cu-O coordination polyhedron can be described as a strongly deformed - almost planar - tetrahedron with the Cu shifted from centre to one of the basal planes. Copyright (2002) Australian X-ray Analytical Association Inc