STRUCTURE DETERMINATION FROM POWDER DATA USING SYMMETRY ADAPTED FUNCTIONS: SIMREF22. J. Ihringer and H. Ritter, Institut fur Kristallographie der Universitat Tubingen, Charlottenstr.33, D-72070 Tubingen.

The novel method presented here determines and refines an unknown structure (S^R), which is the result of a commensurate or incommensurate perturbation of a known idealized structure (S^I). In such cases, the difference vectors between the atomic positions of S^R and S^I are the sum of a few symmetry adapted shift vectors of S^I. The latter are irreducible representations of Si, their magnitude may be understood as order parameter [[eta]]_[[nu]] in the sense of the Landau theory for a phase transition leading from S^I to S^R. When the appropriate representations are found, the full profile refinement program SIMREF22 applied to the diffraction pattern of the unknown structure refines, besides the coordinates of I, the order parameter [[eta]]v for each active representation. The symmetry of determines the symmetry of S^R, therefore, the careful analysis of line splitting in highly resolved powder patterns is the first step to select an order parameter of proper symmetry. Further restrictions for its selection arise from the assumption, that certain molecules or parts of the lattice remain rigid. The program package consists of the following parts:

1. Visual choice of the atomic setting for S^I

2. Computation of all irreducible representations of S^I

3. Visualisation of the atomic shift vectors with display of its symmetry S^R for each irreducible representation of S^I

4. Full profile refinement of S^I and, [[eta]]_[[nu]] with SIMREF22.

An example for the application of the method is the structure determination for the four phases of Pb₅Al₃F₁₉. Financial help by the DFG for project Ih 9/3-3 is gratefully acknowledged.