Nearest-neighbor isotopic fine structure of theAsPgap mode in GaP

High-resolution (0.02 ${\mathrm{cm}}^{\mathrm{-}1}$) infrared-absorption measurements have been made in the gap region between the acoustic and optic modes of GaP. A mode at 269 ${\mathrm{cm}}^{\mathrm{-}1}$ showed resolved isotopic fine structure due to the random distribution of ${}_{}{}^{69}{}_{}{}^{}\mathrm{Ga}$ and ${}_{}{}^{71}{}_{}{}^{}\mathrm{Ga}$ nearest neighbors to the impurity that is identified as arsenic on a phosphorus lattice site. The measurements are in excellent agreement with the predictions of a theoretical analysis involving calculations of the properties of perfect lattice modes by ab initio density-functional methods and the use of these properties in the determination of defect mode frequencies by Green’s-function techniques.