Conclusion

Abstract

In this book, we have presented briefly the results of experimental and theoretical studies of the objects of research that earlier were simply inaccessible (except for LiH x D1-x crystals). The use of such objects allows the investigation of isotopic effects in lattice dynamics (elastic, thermal and vibrational properties) and also the influence of such effects on electronic states via electron-phonon coupling (renormalization of the band-to-band transition energy E g , exciton binding energy E B , and the size of longitudinal-transverse splitting ΔLT). The first three chapters of the book briefly describe the influence of isotopic effects on lattice dynamics as well as the energy spectrum renormalization of electronic excitation in isotopically-mixed semiconducting and insulating crystals. The thermal conductivity enhancement in isotopically enriched materials amounts (C; Ge; Si) to almost 60% at room temperature and is close to a factor of 6 at the thermal conductivity maximum around 20K (Si-case). The change in the lattice constant is Δa/a ∼ 10−3-10−4, and the change δc ik in elastic constants amounts to several percent. In addition, crystals of different isotopic compositions possess different Debye temperatures. This difference between a LiH crystal and its deuteride exceeds 100 K. The difference between Debye temperatures for diamond crystals is of the same order of magnitude. Very pronounced and general effects of isotopic substitution are observed in phonon spectra. The Raman lines in isotopically mixed crystals are not only shifted (the shift of LO phonon lines exceeds 100 cm−1) but are also broadened. This broadening is related to the isotopic disorder of a crystal lattice. It is shown in this book that the degree of change in the scattering potential is different for different isotopically mixed crystals. In semiconducting crystals (C, Ge, Si, α-Sn, etc.), phonon scattering is weak, which allows one to successfully apply the coherent potential approximation (CPA) for describing the shift and broadening of scattering lines in Raman spectra. In LiH, the change in the scattering potential is so strong that it results in phonon localization, which is directly observed in experiments.KeywordsDebye TemperatureLattice DynamicMixed CrystalExciton Binding EnergyThermal Conductivity EnhancementThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.