A Contribution to the Theory of Heteropolar Crystals

Abstract

It is shown that the failure of theories of the solid state to account for the stability of ideal crystals cannot be due to inadequacy of the force postulate. A number of existing crystals, if ideal, would be dynamically unstable, with respect to certain variations, for any force postulate compatible with the compressibility data. These crystals cannot be ideal. A possible type of departure from the ideal state is suggested which involves a change of energy comparable with the heat of fusion (in agreement with Zwicky's theory). The change in volume at the melting point may be accounted for. Evidence from the photo-electric properties of these crystals indicates that this proposed type of departure may have physical reality although undoubtedly accompanied by asymmetries. A consideration of the changes necessary in the energy function to permit the existence of secondary structure indicate that such secondary may exist even in crystals apparently dynamically stable. It is conjectural whether any crystal is thermodynamically stable in the ideal state. A working hypothesis that all crystals have a secondary structure leads to a rational explanation of the doublet nature of residual rays in cubic crystals and to an improved form of Lindemann's law.