Crystal Structures

  • Chemical looping cycle: Perovskite -> Brownmilleride

Close packing of spheres

  • Many ionic and metallic structures can be seen as a packing of large ions or atoms with smaller ones placed in the voids in-between.
  • Closest packing of spheres can be hexagonal symmetry, ABAB or ABCABC
  • Voids:
    • hcp and fcc structures
    • Tetrahedral voids
      • In-between 4 large spheres
      • is small
      • 2 per large sphere, T and T’
    • Octahedral voids
      • between 6 large spheres
      • larger
      • 1 per large sphere
  • ABAB (hcp) (a)
  • ABCABC - turned 45 degrees, (fcc) (b)
  • Subsitutent solubility increases with increasing temperature because more flexibility and space available.
  • Examples:
    • Rocksalt AX (cations in tetrahedral holes)
    • Fluorite AX\(_2\) ()
    • Perovskite ABX\(_3\)

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