Raman scattering, defect luminescence, and phonon spectra ofLi7H,Li6H, andLi7D crystals

Abstract

Two types of phonon spectra, obtained by ${\mathrm{Ar}}^{+}$-ion-laser irradiation of lithium hydride crystals, have been observed and analyzed for the three isotopic components ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$H, ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$H, and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$D. Two-phonon Raman spectra of pure crystals were measured at 300 and 10 K. One-phonon spectra were obtained from measurements of the resolved sideband structure of a strong zero-phonon emission line around 600 nm. This emission is produced by defects of very weak electron-phonon coupling, which are present in crystals either irradiated or containing a slight nonstoichiometric excess of Li. Both Raman and luminescence spectra show in their detailed resolved structure pronounced isotope shifts allowing a classification of the phonon modes involved. Comparison with data from neutron scattering and lattice-dynamics calculations gives very close agreement and identifies the modes as originating essentially from $X$ and $L$ points at the Brillouin-zone edges. Isotope effects on the zero-phonon lines of the defect luminescence are also observed and discussed.