Whole-profile structure solution from powder diffraction data using simulated annealing

Techniques and methods to facilitate the solution of structures by simulated annealing have been developed from the starting point of a space group and lattice parameters. The simulated-annealing control parameters have been systematically investigated and optimum values characterized and determined. Most significant is the inclusion of electrostatic-potential penalty functions in a non-linear least-squares Rietveld refinement procedure. The long-range electrostatic potentials are calculated using a general real-space summation which can be used for all space groups. In addition, a general weighting scheme for penalty functions negates the need to determine weighting schemes experimentally. Also investigated and improved is the non-linear least-squares minimization procedure used in the refinement of structural parameters. The behaviour and success of the techniques have been tested on X-ray diffraction powder data against the known structures of AlVO₄ in P1 with 18 atoms in the asymmetric unit, K₂HCr₂AsO₁₀ in P3₁ with 15 atoms in the asymmetric unit excluding hydrogen, and [Co(NH₃)₅CO₃]NO₃.H₂O in P12₁ with 15 atoms in the asymmetric unit excluding hydrogen.