1 / 8

Structure Type Assignments

Structure Type Assignments. Suri Kabekkodu. Structure Type Notation. LPF uses a longer notation: Structure Type Formula, Pearson Symbol, Space group number Examples: Cu3 Au,cP4,221; Cu3 As,cI64,220; NaCl,cF8,225. Structure Type requirements. Must crystallize in the same space group

zanthe
Télécharger la présentation

Structure Type Assignments

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Structure Type Assignments Suri Kabekkodu

  2. Structure Type Notation • LPF uses a longer notation: Structure Type Formula, Pearson Symbol, Space group number • Examples: Cu3 Au,cP4,221; Cu3 As,cI64,220; NaCl,cF8,225

  3. Structure Type requirements • Must crystallize in the same space group • Similar cell parameter ratio • Same Wyckoff positions (Wyckoff sequence) • When all the above 3 conditions satisfy, we have similar atomic environments

  4. Wyckoff sequence Wyckoff notation of a given space group changes with • Choice of origin • Alternative setting Hence it is required to standardize the crystal structure prior to the “atomic environment” comparison

  5. Standardization of Crystal Structure • First proposed by Parthe and Gelato (Acta Cryst. (1984), A40, 169-183) • The method calculated standardization parameter  based on • Shift of origin • Rotation of the coordinate system • Inversion of the basis vectors The smallest  represents the standard description

  6. Standardization • Program STRUCTURE TIDY (Parthe and Gelato) to standardize the structure • Standardization procedure criteria Space group setting Cell parameters Origin of the coordinate system Order of the atom sites

  7. Standardization Criteria (-Contd) • Additional constraints • Triclinic space groups: Niggli reduced cell • Monoclinic with b-axis unique • Orthorhombic a b  c (when not fixed by the space group) • Trigonal space group with R lattice (triple hexagonal cell) • Choice of origin: at the inversion center • Enantiomorphic space groups: smallest index on the relevant screw axis

  8. Special Cases • In general standardization procedure can directly compare candidates for isotypic compounds. However there are some special cases: • Refinable coordinate very close to zero

More Related