The Theory of Defect Concentration in Crystals

The theory of the formation of vacancy and interstitial defects in crystals is re-examined. It is suggested that a Born-von Karman type of lattice provides an adequate model for the calculation of the essential parameters giving the concentration of defects as a function of temperature. It is concluded that the entropy of formation depends only on the lattice frequencies before and after formation of the defect, and no further contribution to the entropy arises even though the enthalpy of formation may appear to remain temperature dependent at constant pressure. Data on vacancy concentration in NaCl and LiF are analyzed roughly in the light of present ideas. The results for NaCl are understandable in a simple way; those for LiF seem to require more complete calculations.