Presentation on theme: "Objectives By the end of this section you should:"— Presentation transcript:
1Objectives By the end of this section you should: recognise a range of basic crystal structuresappreciate that a variety of important crystal structures can be described by close-packingbe able to compare and contrast similar structures
2Why? All crystal structures can be described by: unit cell + symmetry + atomic positionssymmetryUnit cell (cubic)
3Why? All crystal structures can be described by: unit cell + symmetry + atomic positionsIt is also helpful to be descriptive!Can use concepts of close packing, polyhedra, interstitials to compare structuresComplex structures can be related back to basic onesCorrelate with properties?
4Example 1 – Diamond Structure Carbon atoms in all fcc positionsCarbon atoms in half of tetrahedral positions (e.g. T+)Carbon coordinated to 4 other carbon atoms – all are tetrahedralLooking at tetrahedra in the structure helps us see the “diamond shape”
5Diamond StructureSilicon, germanium and -tin also adopt this structure (all group 4 elements)Melting Point (ºC)Conductor?Carbon3550InsulatorSilicon1410SemiconductorGermanium940-Tin230Conductorradius
6Example 2 - Zinc Blende (ZnS: Sphalerite) Sulphur atoms in all fcc positionsZinc atoms in half of tetrahedral positions (e.g. T+)Comparison with DiamondVery important in semiconductor industry (e.g. GaAs)Ball and stick model shows us the 4-fold coordination in both structures
7Example 3 – Fluorite/Antifluorite structure Antifluorite, Na2OOxygen atoms in all fcc positionsSodium atoms in ALL tetrahedral sitesFluorite, ZrO2Zr atoms in all fcc positionsO atoms in ALL tetrahedral sitesNote formulae: blue atoms (fcc) – 4 per unit cellred atoms (tetrahedral) – 8 per unit cell
8Example 4 - Nickel Arsenide (NiAs) h.c.p. analogue of rocksalt structureh.c.p. arsenic with octahedral Nic pointing towards usc pointing upwards
9Coordination of As is also 6 but as a trigonal prism: 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.
10Descriptions of Structures With ccp anion array:Rock salt, NaCl O occupiedZinc Blende, ZnS T+ (or T-) occupiedAntifluorite, Na2O T+ and T- occupiedWith hcp anion array:Wurtzite, ZnS T+ (or T-) occupiedWith ccp cation array:Fluorite, ZrO2 T+ and T- occupied
12Summary of AX structures wurtziteZnS CN = 4 sphaleriteNaCl, NiAs CN = 6CsCl CN = 8General trend is to get higher coordination numbers with larger (heavier) cations. This is seen also with AX2 structures
14Ionic 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: r0 taken as 1.26 Å
15CompromisePX3012 will return to this concept later in the course
16Refs:Krug et al. Zeit. Phys Chem. Frankfurt 4 36 (1955)Krebs, Fundamentals of Inorganic Crystal Chemistry, (1968)
17SummaryMany important structures can be described by close packing with different interstitial sites filledSimilar structures sometimes have similar properties (but see section 7)Comparison of structures can give important information on ionic radii (and trends).