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Isotope and disorder effects in the Raman spectra of LiH x D 1− x crystals
Author(s) -
Plekhanov V. G.
Publication year - 2001
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.684
Subject(s) - raman spectroscopy , kinetic isotope effect , raman scattering , deuterium , crystal (programming language) , chemistry , analytical chemistry (journal) , isotope , germanium , scattering , brillouin zone , isotopic shift , phonon , spectral line , crystal structure , crystallography , materials science , atomic physics , condensed matter physics , silicon , optics , physics , organic chemistry , chromatography , quantum mechanics , computer science , programming language , astronomy
Most of the physical properties of a solid depend on its isotopic composition in some way or another. Scientific interest, technological promise and increased availability of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled crystals. A great number of stable isotopes and well‐developed methods for their separation have made it possible to grow crystals of C, LiH, ZnO, ZnSe, CuCl, GaN, GaAs, CdS, Cu 2 O, Si, Ge and α‐Sn with a controllable isotopic composition. Among these compounds, LiH possesses the largest value of the isotope effect. The great number of theoretical and experimental data suggest that the isotopic composition of a crystal lattice exerts some influence on the vibrational properties of crystals. These effects are fairly large and can be readily measured by modern experimental techniques (ultrasound, Brillouin and Raman scattering and neutron scattering). In addition, crystals of different isotopic compositions possess different Debye temperatures. This difference between an LiH crystal and its deuterated analogue exceeds a 100 K. Very pronounced and general effects of isotopic substitution are observed in phonon spectra. The scattering lines in isotopically mixed crystals are not only shifted (the shift of LO lines exceeds 100 cm −1 ) but are also broadened. This broadening is related to the isotopic disorder of the crystal lattice. It is shown that the degree of a change in the scattering potential is different for different isotopic mixed crystals. In the case of germanium and diamond crystals, phonon scattering is weak, which allows one to apply successfully the coherent potential approximation (CPA) for describing the shift and broadening of scattering lines. In the case of lithium hydride, the change in the scattering potential is so strong that it results in phonon localization, which is directly observed in experiments. The common nature of the isotopic and disorder effects in a wide range of crystals is emphasized. Copyright © 2001 John Wiley & Sons, Ltd.