For example, Fm3m is a cubic space group, and the 2-folds and 4-folds present are not listed in the symbol. Only the so called full symbol ( F 4/m -3 2/m) shows these additional elements. At this point it is worthwhile to recall that protein structures are restricted to non-mirror and non-inversion symmetry operations.
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This structure contains a simple cubic arrangement of anions and the cations in cubic holes (or vice versa). If all the atoms are the same, the lattice results. Unit cell For CsCl a=412 pm 8 unit cells Space group Fm3m (225) Fluorite, CaF 2 The cations lie in a face centered cubic arrangement and the anions occupy all of the tetrahedral holes. In the anti-fluorite structure, the positions of the cations and anions are reversed, as in Li 2O. Unit cell For CaF 2 a=546 pm 8 unit cells Space group Fm3m (225) An alternate view of fluorite has the anions in a simple cubic arrangement and the cations ions in half the cubic holes. Unit cell Zinc blende or sphalerite, ZnS The anions form a face centered cubic arrangement with the cations occupying half the tetrahedral holes (or vice-versa). If all the atoms are the same, the lattice results.
Unit cell For ZnS a=541 pm 8 unit cells Space group F43m (216) Wurtzite, ZnS The anions form a hexagonal close packed lattice and the cations occupy half of the tetrahedral holes. Unit cell For ZnS a=382 pm and c=626 8 unit cells Space group P6 3mc (186) Rutile, TiO 2 The structure of rutile contains titanium atoms at the corners and center of the unit cell. Each titanium atom is surrounded by an approximate octahedron of oxygen atoms, and each oxygen atom is surrounded by an approximate equilateral triangle of titanium atoms.
Unit cell Space group P4 2/mnm (136) a=459; c=296 pm 8 unit cells Rhenium trioxide, ReO 3 This structure has the rhenium atoms lying at the corners of a cube and the oxygen atoms at the center of each edge. Unit cell NiAs This structure can be considered as a close packed on involving both anions and cations on the same close packed lattice.
Both nickel and arsenic have six nearest neighbors. Unit cell For NiAS a=362 and c=503 pm Space group P6 3/mmc (194) CdI 2 This structure is arranged in layers with two layers of anions between layers of cations. Unit cell Space group P3m (164) a=424; c=684 pm Extended structure showing hexagonal symmetry Quartz, SiO 2 Each silicon atom is surrounded by a tetrahedron of oxygen atoms. Unit cell Space group P3 121 (152) a=491; c=541 pm 4 unit cells, a fragment large enough to show all of the oxygen atoms (red) around 4 of the silicon atoms (golden) NaTl Each sodium atom is surrounded by 4 sodium atoms arranged tetrahedrally and 4 thallium atoms arranged tetrahedrally.
Fm3m Space Group
Together the 8 surrounding atoms form a cube. Each thallium atoms is similarly surrounded by 4 thallium atoms and 4 sodium atoms forming a cube.
Fm3m Space Group Names
Four of each type of cube combine to make the unit cell. Smaller cube having sodium atom at its center Unit cell For NaTl a=749 pm Space group Fd3m (227) Calcium carbide, CaC 2 Calcium carbide is an acetylide containing units of C 2 2. The calcium and acetylide ions are arranged in a manner similar to the sodium and chloride ions in the.