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Deriving fractional atomic coordination numbers

  1. Nov 19, 2011 #1
    hi guys

    i was wondering how the coordination numbers for the atomic positions of atoms in crystals such as NaCl are derived

    ...more specifically why is Na at (0,0,0) and Cl at (1/2,1/2,1/2) and not
    (0,0,0) and (1/4,1/4,1/4)

    of course this is in fractional coordinates
  2. jcsd
  3. Nov 19, 2011 #2


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    What are the units?
  4. Nov 19, 2011 #3
    why do the units matter.....NaCl has at halite structure....or the atomic positions of (0,0,0) and(.5,.5,.5) in fractional coordinates for the unit cell

    maybe i didnt explain clearly enough.... let me try again

    So I know that the NaCl unit cell is based on a FCC lattice with a basis of Na Cl atoms at fractional coordinates (0,0,0) and (1/2,1/2,1/2) respectively

    the zincblend (ZnS) unit cell (specifically the sphalerite not wurtzite) i know is also based on a fcc lattice but instead has a basis of Zn at (0,0,0) and S at (1/4,1/4,1/4) respectively which makes it tetrahedraly coordinated instead

    i was wondering fundamentally why the S is at 1/4,1/4,1/4 and not 1/2,1/2,1/2 like Cl

    i.e. the atomic positions...i was wondering if there was a way to derive this based on the lattice parameters.
  5. Nov 20, 2011 #4


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    I am not familiar with the terminology here. You talk about fractions of unit cells. Could you be more specific.
  6. Sep 29, 2012 #5
    hmm, i guess in the end i was wondering how the position of the atoms in a fcc structure are determined. i know the lattice parameters are determined experimentally. But how are the positions of the atoms determined
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