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Structure Type Assignments

Structure Type Assignments

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Structure Type Assignments

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  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