Raman scattering of light in cubic 7LiH and 6LiH crystals at excitation in the range from 514 to 250 nm is investigated experimentally. Raman spectra of pure crystals are measured at 300 K. For the first time, Raman scattering is found to consist of two components. These components are (1) resonant Raman scattering and (2) nonresonant Raman scattering. Resonant Raman spectrum is obtained and identified as the density of states function of V c− ‐type defect, whereas the nonresonant Raman spectrum is obtained and identified as the two‐phonon spectrum of LiH. Interesting behaviour of this two‐phonon spectrum is found and used as a basis of the original assignment method. Comparison with data from lattice‐dynamics calculations gives very close agreement and identifies the modes as originating essentially from X, W, and K points at the Brillouin‐zone edges.
An experimental study is made of resonant secondary emission, reflection, and excitation spectra which indicate the existence of bound excitons in LiH-Mg crystals. The spectroscopic properties of the bound exciton are studied by selectively excited luminescence and resonant Raman scattering. The resonance energy of the bound exciton is estimated to be 4.842 eV. For the first time, the transformation of resonant Raman scattering into luminescence is found when the incident photon energy is above the resonance. pacceman B nmM&iHecueHuEIm, KorAa 3~e p r m nanaiouero @oToHa npeBbImaeT 3~e p r~m p e 3 0~a~c~o r o nepexona.
The isotope effect on the lattice parameter and the coefficient of thermal expansion of the LiH crystal is investigated by means of rigid-ion model. It is demonstrated that the differences in lattice parameters between LiH isotopes can be related directly to changes in the thermal-expansion coefficient which varies inversely as the cation-anion reduced mass. These differences should disappear at a temperature of 0 K. For the first time, the thermal-expansion data are calculated for the six isotopic components 6LiH, 7LiH, 6LiD, 7LiD, 6LiT, and 7LiT. The lattice parameters of LiH isotopes are also calculated and agree with those observed earlier within the experimental error.
IntrGductionLithium hydride crystallizing in NaCl structure and consisting of Li' and H-ions with the Isz electron configuration is the simplest ionic crystal. The large relative mass difference between the isotopes makes the LiH crystals very attractive for studies of the difference in physical properties resulting from isotopic substitution. These substitutions cause changes in the lattice parameter a, and in the coefficient of thermal expansion CI of the solid [l].There have been investigations of lattice parameters for the five isotopic components 7LiH, 7LiD [2 to 91, 6LiD [6,8], 6LiH, and 7LiT [8]. These investigations have shown that the lighter isotopes produce larger lattice parameters than the heavier isotopes. In addition, it was found [8] that the lattice parameters for the isotopic lithium hydrides can, at any given temperature, be related by the expressionwhere B and A are constants. Due to the low reduced mass p of the LiH system, the lattice parameter experiences an observable change on substitution of either hydride or lithium ions. Several attempts [5, 8, 101 have been made to calculate these changes theoretically using the Einstein and Debye models, but poor agreement has been obtained.') 644000 Omsk, Russia.
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