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\sect{Defects in Lattice}
A perfect lattice is composed of ${N}$ atoms on ${N}$ sites. If
${M}$ of these atoms are shifted to interstitial sites (i.e. between
regular positions) we have an imperfect lattice with ${M}$ defects.
The number of available interstitial sites is ${N'}$ and is of order
${N}$. The energy needed to create a defect is ${w}$. Assume that
${T<