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Objectives By the end of this section you should: recognise a range of basic crystal structures appreciate that a variety of important crystal structures.

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Presentation on theme: "Objectives By the end of this section you should: recognise a range of basic crystal structures appreciate that a variety of important crystal structures."— Presentation transcript:

1 Objectives By the end of this section you should: recognise a range of basic crystal structures appreciate that a variety of important crystal structures can be described by close-packing be able to compare and contrast similar structures

2 Why? All crystal structures can be described by: unit cell + symmetry + atomic positions symmetry Unit cell (cubic)

3 Why? All crystal structures can be described by: unit cell + symmetry + atomic positions It is also helpful to be descriptive! Can use concepts of close packing, polyhedra, interstitials to compare structures Complex structures can be related back to basic ones Correlate with properties?

4 Example 1 – Diamond Structure Carbon atoms in all fcc positions Carbon atoms in half of tetrahedral positions (e.g. T + ) Carbon coordinated to 4 other carbon atoms – all are tetrahedral Looking at tetrahedra in the structure helps us see the diamond shape

5 Diamond Structure Silicon, germanium and -tin also adopt this structure (all group 4 elements) Melting Point (ºC)Conductor? Carbon3550Insulator Silicon1410Semiconductor Germanium940Semiconductor -Tin 230Conductor radius

6 Example 2 - Zinc Blende (ZnS: Sphalerite) Sulphur atoms in all fcc positions Zinc atoms in half of tetrahedral positions (e.g. T + ) Comparison with Diamond Very important in semiconductor industry (e.g. GaAs) Ball and stick model shows us the 4-fold coordination in both structures

7 Example 3 – Fluorite/Antifluorite structure Antifluorite, Na 2 O Oxygen atoms in all fcc positions Sodium atoms in ALL tetrahedral sites Fluorite, ZrO 2 Zr atoms in all fcc positions O atoms in ALL tetrahedral sites Note formulae:blue atoms (fcc) – 4 per unit cell red atoms (tetrahedral) – 8 per unit cell

8 Example 4 - Nickel Arsenide (NiAs) h.c.p. analogue of rocksalt structure h.c.p. arsenic with octahedral Ni c pointing towards usc pointing upwards

9 In the c-direction, the Ni-Ni distance is rather short. Overlap of 3d orbitals gives rise to metallic bonding. The NiAs structure is a common structure in metallic compounds made from (a) transition metals with (b) heavy p-block elements such as As, Sb, Bi, S, Se. Coordination of As is also 6 but as a trigonal prism:

10 Descriptions of Structures With ccp anion array: Rock salt, NaClO occupied Zinc Blende, ZnST + (or T - ) occupied Antifluorite, Na 2 OT + and T - occupied With hcp anion array: Wurtzite, ZnS T + (or T - ) occupied With ccp cation array: Fluorite, ZrO 2 T + and T - occupied

11 Summary (see web pages)

12 Summary of AX structures wurtzite ZnS CN = 4 sphalerite NaCl, NiAsCN = 6 CsClCN = 8 General trend is to get higher coordination numbers with larger (heavier) cations. This is seen also with AX 2 structures

13 Summary of AX 2 structures SiO 2, BeF 2 silica structureCN = 4 : 2 TiO 2, MgF 2 rutile structureCN = 6 : 3 CdCl 2, CdI 2 layer structureCN = 6 : 3 PbO 2, CaF 2 fluorite structureCN = 8 : 4 Compare:1) Be, Mg, Ca fluorides 2) Si, Ti, Pb dioxides

14 Ionic radii and bond distances Ionic radii cannot be accurately measured - estimated from trends in known structures or from electron density maps (crystallography) (reference - Shannon, Acta Cryst. (1976) A32 751) Oxygen ion: r 0 taken as 1.26 Å

15 Compromise PX3012 will return to this concept later in the course

16 Refs: Krug et al. Zeit. Phys Chem. Frankfurt 4 36 (1955) Krebs, Fundamentals of Inorganic Crystal Chemistry, (1968)

17 Many important structures can be described by close packing with different interstitial sites filled Similar structures sometimes have similar properties (but see section 7) Comparison of structures can give important information on ionic radii (and trends). Summary


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