Bond energy in alpha-Al2O3

  1. Hello,

    I am looking for a value and a reference for the energy of the
    chemical bonds in sapphire (alpha-Al2O3). Apparently, the bond is a
    mix of ionic and covalent, and there is data on bonding things to
    sapphire, but I can't find anything for bonds in the sapphire itself.
    Any ideas ?
     
  2. jcsd
  3. yo_bbd wrote:
    >
    > Hello,
    >
    > I am looking for a value and a reference for the energy of the
    > chemical bonds in sapphire (alpha-Al2O3). Apparently, the bond is a
    > mix of ionic and covalent, and there is data on bonding things to
    > sapphire, but I can't find anything for bonds in the sapphire itself.
    > Any ideas ?


    The "CRC Handbook" gives bond dissociation energy Al-O = 511 (+/-)3
    kJ/mole at 298 K.

    Faraday Discuss. Chem. Soc. 84 75 (1987)

    Check the numbers. I find the standard enthalpy of formation of
    aluminium oxide from the elements is between -1675.7 ± 1.2 kJ/mole
    (first reference) and –1676.4 kJ/mole, –1643.7 kJ/mole (second
    reference).

    Scripta Materialia 50(1) 143 (2004)
    J. Electrochem. Soc. 124(12) 1836 (1977)
    http://link.aip.org/link/?JESOAN/124/1836/1

    Divide by 6.02214x10^23 for enthalpy of formation/formula unit, Al_2
    O_3_. Count the number of bonds to both Al and O, divide by the
    energy/molecule, get the average energy/bond. Note that the
    coordination numbers of Al and O in the solid are not the chemical
    bond numbers. Wang together a CIF file and have ORTEP III for Windows
    display bonding in the crystal lattice based upon separation
    distance. As you can see, the structures for corundum are consistent
    (the temps aren't listed in the database),

    Corundum crystal structure (neutron diffraction),

    Materials Science Forum 378 288-293 (2001)

    a = b = 4.7597(1) A
    c=12.9935(3) A
    alpha = beta = 90 degrees
    gamma = 120 degrees
    sg = R3-CH (167)
    V = 254.93 A^3
    R = 0.0330 (smaller is better)

    oxid.
    Atom state x/a y/b z/c
    Al 3 0.0 0.0 0.3523
    O -2 0.3065 0.0 0.2500

    Corundum crystal structure (x-ray diffraction),

    J. Appl. Crystall. (1999) 32 799 (1999)

    a = b = 4.7540(2) A
    c = 12.9935(1) A
    alpha = beta = 90 degrees
    gamma = 120 degrees
    sg = R3-CH (167)
    V = 254.73 A^3
    R = 0.0410

    oxid.
    Atom state x/a y/b z/c
    Al 3 0.0 0.0 0.35214
    O -2 0.3065 0.0 0.2500

    Corundum crystal structure (synchrotron x-ray diffraction),

    Acta Crystall. B (1993) 49 973 (1993)

    a = b = 4.7597(1) A
    c=12.9935(3) A
    alpha = beta = 90 degrees
    gamma = 120 degrees
    sg = R3-CH (167)
    V = 254.09 A^3
    R = 0.0240 (smaller is better)

    oxid.
    Atom state x/a y/b z/c
    Al 3 0.0 0.0 0.35223
    O -2 0.30622 0.0 0.2500

    --
    Uncle Al
    http://www.mazepath.com/uncleal/
    (Toxic URL! Unsafe for children and most mammals)
    http://www.mazepath.com/uncleal/lajos.htm#a2
     
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