Mean-Square Displacement and Mean-Square Velocity of a Point Mass Defect in NaI

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Abstract
Following the treatment of Dawber and Elliott for a monatomic system we have derived the expressions for the mean‐square displacement (or Debye—Waller factor) and the mean‐square velocity (or second‐order Doppler shift) of a point mass defect in a diatomic cubic crystal in the harmonic approximation. These quantities are computed as functions of temperature for isotopic impurities substituting for positive and negative ions separately in NaI. The lattice dynamics of the perfect crystal required in these calculations is obtained on the basis of the deformation‐dipole model. Finally a brief discussion of the results is given.