We start with | ||
Next we must do the differentiation, i.e. form ∂F/∂T: | ||
One can go straight ahead, of course. But here comes a little helpful trick: Multiply skillfully by T/T and re-sort; you get | ||
Now we need to resort to approximations | ||
First we realize that whenever h ·ω/2π << kT,
then |
||
This takes care of the first term. | ||
The second term needs a somewhat more sophisticated approach. Substituting x for h · ω/2p · kT, we can use a simple expansion formula, stop after the second term and re-insert the result. This gives | ||
That's as far as one can go. Now use ω' for the circle frequencies of the crystal with a vacancy and form SF = S' S | ||
q.e.d. | ||
Do the Math for the Formation Entropy
© H. Föll