PARALLEL BORN-VON KARMAN STUDY OF DIAMOND AND THE DIAMOND TYPE CRYSTALS

The measured dispersion curves of diamond have been reexamined within the Born-von Karman framework parallel to Ge, Si and α-Sn for which long range Born-von Karman forces have been established. Contrary to earlier reports, it was found that fifth neighbor interactions are absolutely necessary for a correct description of the dispersion curves. The importance of fifth neighbors is a feature common to all four diamond type materials related to their structure. A few systematic trends and interrelationships observed in the numerical values of the force constants are discussed. With the fitted values of the force constants the frequency distributions were calculated accurately and some new information was obtained. Concerning the two-phonon bound state controversy, in diamond, the fifth neighbor fit obtained here clearly showed that the Raman frequency is indeed the absolute maximum frequency for the Δ2′(LO) branch. Thus, a two phonon bound state in diamond cannot be ruled out on the basis of opposite claims. © 1979.