Effect of isotopic composition on phonon modes. Static atomic displacements in crystals

Theoretical and experimental work relevant to isotopic (mass) effects in solids is reviewed. The effect of isotopic composition on the phonon spectrum is discussed with particular emphasis on chemically pure and nearly perfect semiconductor single crystals. The fundamental role of polarization vectors in polyatomic compounds is pointed out. Static atomic displacements around isotopic impurities — an effect potentially leading to considerable phonon (and electron) scattering in strongly anharmonic crystals — are also discussed.