Crystal Structures Chapter 7 Wednesday, October 21, 2015. 170 Cl Cs = = − + r r Adapted from Fig. The molecule is under equilibrium because, the attractive force due to ions is balanced by repulsive force due to electron clouds. For example in NaCl, the basic building block is composed of one Na and one Cl ion which is repeated in a cubic pattern to make the NaCl structure 2 The calcite (CaCO 3) crystal structure is shown above.Triangular CO 3 2-ions fill octahedral holes between the Ca 2+ ions (black spheres) in a distorted NaCl lattice. NaCl (salt shaker variety) Hexagonal Close Packed: unit cell. 181 0. CsCl •Thinking about the unit cell as a three-dimensional graph allows us to describe the structure of a crystal with a remarkably small amount of information. • Structure of NaCl CERAMIC STRUCTURE (1) This structure can be described as having FCC Bravais lattice with two ions (1Na+ and 1Cl-) ... • The unique relationship between such patterns and crystal structures provide a powerful tool for identification of the phase composition of powders and polycrystalline materials. N = 1, so n 1we will see that the basic building blocks of periodic structures can be more complicated than a single atom. Primitive Cells This is really simple! and one Cl ion. Hence both Na and Cl are in ionic state. Structure Factor The general expression for : Now we apply this to various crystals. AX Crystal Structures AX–Type Crystal Structures include NaCl, CsCl, and zinc blende 0. From this image we can see why the CaCO 3 structure has a lower symmetry than that of NaCl. Zinc Blende (ZnS) Structure We can specify the structure of cesium chloride, for example, with only four pieces of 939 0. • All of the faces of the sodium chloride crystal are square, but can have different number of ions: 4 (2 x 2 x 2), 9 (3 x 3 x 3), 12.3, Callister 7e. 1 (The text is taken from: Basic Solid State Chemistry, by Anthony R. West) 1.17 Some important structure types a) Rock salt (NaCl), zinc blende or sphalerite (ZnS), fluorite (CaF2), antifluorite (Na2O) These structures are considered together because they all have ccp/fcc anions and differ only in the positions of the cations. The Bravais lattice of NaCl is FCC with the basis containing one Na ion. Structure Factor (Fhkl) 2( ) 1 ij i N ihu kv lw hkl i i Ffe • Describes how atomic arrangement (uvw) influences the intensity of the scattered beam. • Ittellsuswhichreflections(ie peaksIt tells us which reflections (i.e., peaks , hkl)to) to expect in a diffraction pattern. →Octahedron Anion Coord. Although Bragg's law was used to explain the interference pattern of X-rays scattered by crystals, diffraction has been . ... NaCl Structure CCP with all octahedral holes filled Coordination= 6, 6 Cation Coord. • The water crystal is a hexagon. • Both the sodium chloride crystal and water crystal consist of repeating patterns. As in NaCl, each ion is coordinated by six of the other kind. Crystal Structure of Sodium Chloride(NaCl) NaCl is a ionic compound. Differences • The sodium chloride crystal is a cube. Cesium Chloride structure: ∴ cubicsites preferred So each Cs + has 8 neighboring Cl- determining crystal structures beginning with NaCl, ZnS and diamond. structure. First, consider examples of crystals with a basis group of just one atom per lattice point, with atomic scattering factor f. The results are valid for any crystal system. →Octahedron Connectivity →Edge sharing octahedra 4 NaCl in unit cell. 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