Abstract
We develop a model for simple diatomic crystals in which two effective-charge parameters are associated with the infrared-active optical-phonon mode. One of these is , the macroscopic effective charge. The definition of the second charge, here designated as , the localized effective charge, is model dependent and derived from a model which includes two contributions to the TO-phonon frequency. One of these is a mechanical or spring-constant contribution and the second a frequency term which provides a measure of dipole-dipole interactions. is computed from the experimental values of the TO-phonon frequencies and values of calculated from simple force-constant models. For the zinc-blende (ZB) and wurtzite () crystals, , where is the fractional ionicity and is the effective chemical valence. For the rocksalt (NaCl) and CsCl crystals, is not proportional to , but instead shows systematic variations with ion size parameters. For the ten-electron NaCl and CsCl crystals, , where is the optical-frequency dielectric constant. The model calculation for is more reliable for the ZB and crystals than the NaCl and CsCl crystals.
- Received 30 April 1971
DOI:https://doi.org/10.1103/PhysRevB.4.1367
©1971 American Physical Society