Anomalous thermal expansion with infrared spectroscopy.

Anomalous thermal expansion is treated through an analytical approach, based on the anharmonic behavior of lattice vibrations of the solids CuCl and CuBr of which complete ir spectroscopic data were available for the low temperature region. In the two cases examined here, anomalous thermal expansion as well as change of anharmonic factor, as a function of temperature, show a mirrorlike proportionality. In addition, drastic changes of ir energy absorption take place within the temperature region of reexpansion, suggesting a substantial increase of the ionic fraction of binding, coupled with a corresponding decrease of the covalent fraction, within the re-expansion period. These striking events appear to be the basic reason for the re-expansion phenomenon, since the sum value of the ionic radii of the compounds in question is greater than the sum value of the covalent radii, thus enlarging the interatomic distance, instead of contracting it.