Phonon Scattering in Sodium Chloride Containing Oxygen

The thermal conductivity of supposedly pure NaCl crystals from several sources was found to vary by as much as two orders of magnitude at low temperatures. The conductivity of Harshaw crystals was particularly low. This effect was quantitatively related to the presence of an ultraviolet absorption band at 185 mμ known to be caused by oxygen-containing anionic impurities. Both phenomena were considerably reduced by treatment of the crystals in chlorine vapor at high temperatures; conversely both were enhanced by growing crystals from melts doped with NaOH, NaOD, and ${\mathrm{Na}}_2$ ${\mathrm{O}}_2$. There was little evidence, however, that the dopants appeared in these forms in the crystals. Infrared measurements and $p\mathrm{H}$ titrations suggested that the most likely result of the dopings was to introduce carbonate into the crystals. The active impurity scattered phonons very strongly at low temperatures; at 5°K approximately 3000 times more strongly than is usually observed for point defects. The cross section was proportional to the first power of the phonon wave vector and was found to be independent of the defect concentration. No detailed model was found to explain these results. A likely explanation would be in terms of an interaction between the phonon field and localized modes of the scattering center.