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Structure determination of incommensurate phases

Structure determination of incommensurate phases. An introduction to structure solution and refinement Lukas Palatinus, EPFL Lausanne, Switzerland. Outline. This tutorial will cover: introduction to incommensurate structures (very briefly) determination of the symmetry structure solution

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Structure determination of incommensurate phases

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  1. Structure determination of incommensurate phases An introduction to structure solution and refinement Lukas Palatinus, EPFL Lausanne, Switzerland

  2. Outline This tutorial will cover: • introduction to incommensurate structures (very briefly) • determination of the symmetry • structure solution • structure refinement • validation of the structure

  3. Incommensurate structures Aperiodic structure is a structure that lacks periodicity, but exhibits a long-range order Three main classes: Modulated composites quasicrystals structures

  4. Incommensurate structures Incommensurately modulated structure has a basic 3D periodicity that is perturbed by an incommensurate modulation. Modulated structure Composite

  5. Incommensurate structuresreciprocal space Reciprocal space is discrete despite of the aperiodicity

  6. Incommensurate structuresreciprocal space

  7. Incommensurate structuresreciprocal space -120 120 1-10

  8. Incommensurate structuresreciprocal space -1301 -1200 1200 1-102 1-100 1-10-3

  9. Incommensurate structuresreciprocal space Most current diffractometer softwares allow for indexing of an aperiodic diffraction pattern. However, the q-vector can be only refined, not found automatically. The result is indexing of the pattern by 4 integers: -6 -2 4 2 1970.51001 80.49380 -4 -2 2 0 116733.00000 327.45499 -4 -2 1 -1 280.85901 56.31390 -4 -2 1 -2 156.37300 51.69950 -4 -2 4 -2 135.81400 42.38190 -4 -2 1 0 50292.10156 214.59900 -4 -2 1 -3 21.82130 23.57890 -6 -2 -1 0 1678.30005 69.71670 -4 -2 1 1 372.96399 53.42990

  10. Incommensurate structures

  11. Superspace

  12. Superspace Construction of superspace in reciprocal space

  13. Superspace Construction of superspace in reciprocal space

  14. Superspace Construction of superspace in reciprocal space a*s4 b1 q a*s1

  15. Superspace Embedding of the structure into superspace R3

  16. Superspace Embedding of the structure into superspace R3

  17. Superspace Structure model of a modulated structure consists of: • Structure model of basic structure • Modulation functions for the parameters of the basic structure: • Modulation of position • Modulation of occupancy • Modulation of displacement parameters Modulation functions are most often modeled by a Fourier series:

  18. Superspace

  19. Superspace symmetry

  20. Symmetry The symmetry is described by a (3+d)-dimensional space group. A 4D superspace group must be 3+1 reducible = the internal and external dimensions cannot mix together. General form of a symmetry operation: Example of superspace group operations: x1, -x2, 1/2+x3, -x4 -x1, -x2, x3, 1/2+x4

  21. Symmetry How can the symmetry be determined? The first three rows are the components of the basic space group. The sign of RI depends on the action of the symmetry operation on the q-vector: 2-fold: -x1, x2, -x3, -x4 2-fold: -x1, x2, -x3 mirror: x1, -x2, x3, x4 mirror: x1, -x2, x3

  22. Symmetry The translational part is determined from the extinction conditions in complete analogy to the 3D case: in general: hR = h, h. = integer c-glide: x1, -x2, 1/2+x3: h0l, l=2n “superspace c-glide” with shift along x4: x1, -x2, 1/2+x3, 1/2+x4: h0lm, l+m=2n

  23. Symmetrysuperspace group symbol C2/m(0)0s

  24. Symmetrysuperspace group symbol C2/m(0)0s Herman-Mauguin symbol of the basic space group Symbol of the q-vector Definition of the intrinsic shifts in the fourth dimension s=1/2; t=1/3 q=1/4; h=1/6 Generators: -x1, x2, -x3, (1/2)-x4 x1, -x2, x3, 1/2+x4 Centering: 1/2 1/2 0 0

  25. Symmetry The search for the superspace group is facilitated by the space group test of Jana2000

  26. Symmetry Rational part of the q-vector

  27. Symmetry Rational part of the q-vector

  28. Symmetry Rational part of the q-vector  Centering vector: 0 1/2 0 1/2

  29. Superspace symmetry

  30. Structure solution

  31. Structure solution Structure solution means finding a starting model that is good enough to be refined by least-squares. Two cases: 1) small to medium modulations (weak to moderately strong satellites) 2) strong modulations = satellites comparable to or stronger than main reflections

  32. Structure solution Case 1 - small modulations: a) Solve the average structure from main reflections b) Refine the modulations from small starting values

  33. Structure solution Case 2 - large modulations: no reasonable average structure exists The structure can be solved by two methods: • superstructure approximation: the components of a q-vector are approximated by commensurate values and the structure is solved as superstructure: q=(0.345, 0, 0.478) ==> q(1/3, 0, 1/2) => 6-fold supercell • directly in superspace by charge flipping (lecture tomorrow, 13:30). Both the average structure and modulation functions can be obtained at the same time.

  34. Structure solution In Jana2000 you can: • Directly call Sir97/Sir2004. The data are prepared, sent to Sir2004, and the model is imported back. • Manually export data into SHELX format, solve the average structure by SHELX and import the structure back to Jana2000. • Prepare input files for the charge flipping calculation with Superflip and EDMA. Superflip returns the density map and a list of structure factors in Jana2000 format, EDMA can provides a structure model of the average structure.

  35. Structure solution

  36. Structure refinement

  37. Structure refinement Two step procedure: • Refine the average structure against the main reflections using standard crystallographic methods. • Refine the modulation parameters of the atoms, namely: • Occupational modulation (1 function) • Positional modulation (1 function for the x, y and z components) • Modulation of ADP’s (1 function per parameter = up to 6 functions)

  38. Structure refinement Initial modulation refinement cookbook Recommended: • Start with the heaviest atoms or with atoms with largest modulation • If you suspect strong occupational modulation of some atoms, start with occupational modulation, otherwise refine positional modulation first.

  39. Structure refinement Initial modulation refinement cookbook Recommended II: • Watch the R-values of the satellites AND the Fourier maps of the modulation functions

  40. Structure refinement Initial modulation refinement cookbook Discouraged: • Don’t use more modulation waves than you have satellite orders Reason: The contribution of the higher harmonics to low-order satellites is negligible. If it were there, high-order satellites would be observed.

  41. Structure refinement Initial modulation refinement cookbook Discouraged II: • Don’t switch off automatic refinement keys and automatic symmetry restrictions of Jana2000 unless you are sure it is necessary. For temporary fixing of some parameters use Refine commands/Fixed commands

  42. Structure refinement Initial modulation refinement cookbook Discouraged III: • Don’t refine the ADPs in the initial stages of the refinement unless you see the evidence in the difference Fourier map

  43. Structure refinement Special functions

  44. Structure refinement Special functions Sawtooth function Crenel function (block wave)

  45. Structure refinement Special functions + harmonic modulation Harmonic functions are mutually orthogonal on the interval <0; 1>. Shorter interval leads to severe correlation between the parameters.

  46. Structure refinement

  47. Evaluation of the structure

  48. Evaluation of the structureFourier maps Fourier maps are indispensable: Check, if the modulation functions match the shape of the electron density:

  49. R3 Evaluation of the structuret-plots

  50. Evaluation of the structuret-plots

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