1 / 21

Metals

Metals. Ionic Compounds. anion. cation. Ceramics. Radius Ratio Rules. sites occur within close-packed arrays. common in ionic compounds. if r c is smaller than f R A , then the space is too big and the structure is unstable. Summary: Sites in HCP & CCP.

una
Download Presentation

Metals

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Metals

  2. Ionic Compounds anion cation Ceramics

  3. Radius Ratio Rules sites occur within close-packed arrays common in ionic compounds if rc is smaller than fRA, then the space is too big and the structure is unstable

  4. Summary: Sites in HCP & CCP 2 tetrahedral sites / close-packed atom 1 octahedral site / close-packed atom sites are located between layers: number of sites/atom same for ABAB & ABCABC

  5. Common Ionic Structure Types • Rock salt (NaCl) sometimes also ‘Halite’ • Derive from cubic-close packed array of Cl- • Zinc blende (ZnS) • Derive from cubic-close packed array of S= • Fluorite (CaF2) • Derive from cubic-close packed array of Ca2+ • Cesium chloride (CsCl) • Not derived from a close-packed array • Complex oxides • Multiple cations

  6. Example: CaF2 (Fluorite) • F- ~ 1.3 Å; Ca2+ ~ 1.0 Å; • rc/RA = 0.77 • Ca2+ is big enough for CN = 8 • But there are no 8-fold sites in close-packed arrays • Consider structure as CCP cations • F- in tetrahedral sites • RA /rc> 1  fluorine could have higher CN than 4 • Ca:F = 1:2  all tetrahedral sites filled • Places Ca2+ in site of CN = 8 • Why CCP not HCP? - same reason as NaCl

  7. Fluorite Ca2+ F- CN(F-) = 4 CN(Ca2+) = 8 [target]

  8. CsCl • Cl- ~ 1.8 Å; Cs+ ~ 1.7 Å; • rc/RA = 0.94 • Cs+ is big enough for CN = 8 • But there are no 8-fold sites in close-packed arrays • CsCl unrelated to close-packed structures • Simple cubic array of anions • Cs+ in cuboctahedral sites • RA /rc> 1  chlorine ideally also has large CN • Ca:Cl = 1:1  all sites filled

  9. Cesium Chloride Cl- 1 Cs+/unit cell 1 Cl-/unit cell CN(Cs) = 8 Cs+

  10. Why do ionic solids stay bonded? • Pair: attraction only • Solid: repulsion between like charges • Net effect? Compute sum for overall all possible pairs Madelung Energy Sum over a cluster beyond which energy is unchanged For simple structures Single rij |Z1| = |Z2| a = Madelung constant Can show

  11. Multiple cations Perovskite Capacitors Related to high Tc superconductors Spinel Magnetic properties Covalency Zinc blende Semiconductors Diamond Semiconductors Silicates Minerals Structures of Complex Oxides

  12. Perovskite • Perovskite: ABO3 [B  boron] • A2+B4+O3 A3+B3+O3 A1+B5+O3 • CaTiO3 LaAlO3 KNbO3 • Occurs when RA ~ RO and RA > RB • Coordination numbers • CN(B) = 6; CN(A) = • CN(O) = 2B + 4A • CN’s make sense? e.g. SrTiO3 • RTi = 0.61 Å • RSr = 1.44 Å • RO = 1.36 Å above/below A 12 O B RTi/RO = 0.45 RSr/RO = 1.06 http://abulafia.mt.ic.ac.uk/shannon/ptable.php

  13. Tolerance factor close-packed directions A B

  14. Covalent Compounds sp3 s2p2 s2p4 s2p3 s2p1 s2 semi-conductors

  15. Covalent Structures  both species tetrahedral Recall: zinc blende ZnS: +2 -2 GaAs: +3 -3 single element: C or Si or Sn or sp3 diamond

  16. Structural Characteristics • Metals • Close-packed structures (CN = 12) • Slightly less close-packed (CN = 8) • Ionic structures • Close-packed with constraints • CN = 4 to 8, sometimes 12 • Covalent structures • Not close-packed, bonding is directional • Any can be strongly or weakly bonded (Tm)

  17. Diamond vs. CCP 8 atoms/cell, CN = 4 4 atoms/cell, CN = 12 ½ tetrahedral sites filled

  18. Cl Na Avogardo’s # Computing density • Establish unit cell contents • Compute unit cell mass • Compute unit cell volume • Unit cell constant, a, given (or a and c, etc.) • Or estimate based on atomic/ionic radii • Compute mass/volume, g/cc • Example: NaCl • Contents = 4 Na + 4 Cl • Mass = 4(atom mass Na + atomic mass Cl)/No • Vol = a3 • Units =

  19. Quartz (SiO2) Single Crystal vs. Polycrystalline Rb3H(SO4)2 Diamond Ba(Zr,Y)O3-d Regions of uninterrupted periodicity amalgamated into a larger compact Periodicity extends uninterrupted throughout entirety of the sample External habit often reflects internal symmetry = grains delineated by grain boundaries

  20. Isotropic vs. Anisotropic graphite* diamond polycrystalline averaging * http://www.electronics-cooling.com/assets/images/2001_August_techbrief_f1.jpg

More Related