Bond energy in alpha-Al2O3

[yo_bbd]

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 ?

[Uncle Al]

Post by yo_bbd
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