Crystal Defects and Semiconductors

Note the slight inconsistency In the second part of the equation, we still use r in units of angstroms. This is because the parameter p is given in emits of A, and to keep units consistent we keep the A unit for r. As a ratio, it is required that the units cancel we could just as easily convert p to units of meters. You should satisfy yourself that the units in the first part of the expression cancel appropriately to yield units of J/mol. Solving numerically  

Compared to an experimental value of 769 kj/mol, we find that equation 21.13 does a much better job of predicting lattice energies than Coulomb s law did. 

Equation 21.13 is defined to give positive values for the lattice energy. It should be understood that when oppositely charged ions come together, total energy is always decreased. Therefore, the values of these processes are always negative, indicating an exothermic process. 

So far in this chapter, we have been working under the assumption that crystalline solids are perfect crystals. Every atom, ion, or molecule is assumed to be in exactly the right place in every unit cell throughout the entire solid crystal. 

In reality, this is not the case. Most real crystals are full of imperfections. Even crystals that are considered very well ordered have an occasional lapse in crystal structure at the atomic and molecular level. These lapses are called defects. 

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