Isotope effects in lattice dynamics

The large number of available stable isotopes and well developed isotope separation technology have enabled growing crystals of C, LiH, ZnO, CuCl, CuBr, Cu₂O, CdS, α-Sn, Ge, Si, etc. with a controlled isotope composition. Experimental and theoretical studies provide evidence that the isotope effect has an influence on the thermal, elastic, and vibrational properties of crystals. In this paper it is shown that in Ge and C crystals isotope effect causes only weak phonon scattering whereas in LiH the scattering potential changes are so strong that they lead to experimentally observable phonon localization. It is emphasized that a systematic description of isotope effects requires that anharmonicity be taken into account.