Free evaporation coefficients of solids

For the majority of metals, the evaporation coefficient is found to be unity, but, as mentioned before, the coefficient of many non-metallic elements with a complex vaporization mechanism such as 

Oxides such as MgO and A1203, also have coefficients which are less than unity, between 0.1 and 0.5, depending on the temperature. Data for the evaporation mechanisms of these systems can be obtained from mass spectrometry and, as is the case for the elements with a low coefficient, the vapour does not usually consist of one species only, but has a number of components. The partial pressures of the various species are a function of the oxygen partial pressure, and in the vaporization of alumina and magnesia where the processes 

The number of atoms per unit area, N/A, multiplied by the bond energy, s, and the net number of bonds which are broken yields the surface energy. The relative surface energies of the (111) and (100) planes is derived thus 

O A atoms above and below plane (Hexagonal structure) 

Here a0 is the lattice parameter of the crystal. An approximate value for the bond energy, e, for this structure where the co-ordination number, Z, equals twelve is given by 

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