Crystals

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Crystals
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Crystal Structures

• Metal kernels are viewed as hard spheres.
• The packing pattern adopted provides the greatest
  energy stability.
• Ions in salts are also hard spheres
• The packing pattern is the same as metals, now
  the larger ion is the sphere.
• The smaller ion is placed in the holes of the
  structure instead of electrons.

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Unit Cells

• Crystals consist of repeating units which may be
  atoms, ions or molecules.
• The space lattice is the pattern formed by the
  points that represent these repeating structural
  units.

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Metallic Crystals
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Packing types

• Body Centered
• 8 nearest neighbors coordination .
• Alkali metals pack this way, most malleable
• Close Packed
• - Hexagonal cubic
• - Face centered cubic
• 12 nearest neighbors or coordination .

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Hexagonal Close Packing

• Alternating planes of hcp spheres
• Those in the 3rd plane pack directly above those
  in the 1st plane ABA pattern

• 74 of the space is filled by metal kernals, the
  rest is filled by electrons.
• used by Be, Co, Mg, Zn, Sc, Ti, Cd Zr
• Least malleable packing

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Face Centered Packing

• Identical to hexagonal packing, EXCEPT the 3rd
  spheres are in the holes that were not used to
  form the 2nd plane
• The 4th plane is above the 1st plane ABCA

• Used for Ag, Al, Au, Ca, Cu, Ni, Pb and Pt
• More malleable than hexagonal packing

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Alloys

• are solid solutions of metals.
• They are usually prepared by mixing molten
  components.
• They may have a regular or irregular array of
  different atoms.

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Substitutional Alloys

• have a structure in which sites of the solvent
  metal are occupied by solute metal atoms.
• An example is brass, an alloy of zinc and copper.

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Interstitial Alloys

• are solid solutions in which the solute atoms
  occupy holes (interstices) within the solvent
  metal structure.
• An example is steel - iron and carbon.

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Ionic Crystals
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• Ionic crystal structures are based on the metal
  lattices.
• The kernel positions are either the anion or
  cation, usually whichever one is larger in size.
• The opposite ions take the place of electrons in
  the metal structure.
• The ions are not mobile so they sit in a fixed
  position, called holes, in the crystal lattice.

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Holes in Close Packed Crystals

• Tetrahedral holes are formed by a planar triangle
  of atoms, with a 4th atom covering the
  indentation in the center.
• Coordination number 4.

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Tetrahedral Holes

• These holes are used when one ion is very small
  relative to the opposite ion.
• n atoms 2n tetrahedral holes.

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Example
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Holes in Close Packed Crystals

• Octahedral holes lie within two staggered
  triangular planes of atoms.

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• The coordination number 6
• n atoms n octahedral holes.

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Example
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Body Centered Ionic crystal

• Occurs when the ion sizes are very similar.
• The smaller ion fits in the center of a cube
  formed by 8 opposite ions.

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Predicting Crystal Structures

• General rules have been developed, to predict
  crystal structures using ionic radii.
• Radius ratios, (radius of the anion)/(radius of
  the cation) are used.
• CN r-/r Hole/packing
• 8 1.0 1.37 body centered packing
• 6 1.37 2.44 octahedral holes
• 4 2.44 4.55 tetrahedral holes

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Common Ionic Crystals
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1. Rock Salt (NaCl)

• Is viewed as a face-centered cubic array of the
  anions, with the cations in all of the octahedral
  holes, or
• The other way around!
• The coordination number is 6 for both ions.

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2. The CsCl structure

• Chloride ions occupy the corners of a cube, with
  a cesium ion in the center (called a cubic hole)
  or vice versa.
• Both ions have a coordination number of 8, with
  the two ions fairly similar in size.

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3. The Zinc-blende or Sphalerite structure-

• Anions (S2-) ions are in a face-centered cubic
  arrangement, with cations (Zn2) in half of the
  tetrahedral holes.

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4. The Fluorite (CaF2) and Antifluorite
structures

• A face-centered cubic arrangement of Ca2 ions
  with F- ions in all of the tetrahedral holes.

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4. The Fluorite (CaF2) and Antifluorite
structures

• The antifluorite structure reverses the
  positions of the cations and anions. An example
  is K2O.