Experimental evidence of strong phonon scattering in isotopical disordered systems: The case ofLiHxD

Abstract

We present Raman spectra of LiH x D 1−x mixed crystals at room temperature and in a wide range of isotope compositions (0 < x <1). The line broadening and energy shift of the Raman lines for the optical phonons were investigated for the first time. The observation of the local mode vibration, the two-mode behaviour of the LO (Γ) phonons at large isotope concentration as well as large lines broadening directly evidence the strong additional phonon scattering due to the isotope-induced disorder. The obtained results could not describe in the weak phonon scattering approximation of the CPA model.

Ab initio electronic, dynamic, and thermodynamic properties of isotopic lithium hydrides (6LiH, 6LiD, 6LiT, 7LiH, 7LiD, and 7LiT)

“…From the comparison between the data generated in the current work and experimental data at ! -point, the LO (TO) mode phonon frequencies at 1105 cm À 1 (590 cm À 1 ) are found to be in excellent agreement with experimental values 1100 cm À 1 (587 cm À 1 ) for 7 LiH measured by Plekhanov [45] and Zemlianov et al [46]. The dipole-dipole contribution is found to be responsible for splitting between the longitudinal and transverse optic (LO and TO, respectively) mode T 1u with infrared active at the !…”

Second-order Raman spectroscopic study of lithium hydride and lithium deuteride at high pressure

“…They used a similar force-constant model to the one used by Anderson and Lüty, but because they considered other high-symmetry points in addition to the X and L points they arrived at different assignments. More recently, Plekhanov and co-workers [11][12][13][14][15][16][17][18] have investigated the second-order Raman spectra at room temperature for mixed LiH x D 1Ϫx (0рxр1) crystals. These authors attributed the strong resonant component in the 2LO features to the 2LO ͑⌫͒ phonon.…”

Isotope and disorder effects in the Raman spectra of LiHxD1−x crystals

“…Note that the study of the concentration dependence of the shape of this peak allows one to examine the percolation threshold, which separates the local states from continuous phonon states. 20,21,33 Figure 3 shows the dependence of wavenumber of the localized mode on the concentration of hydrogen ions obtained from Raman spectra presented in Figs 1 and 2 and spectra of other crystals. 21,26 Figure 3 also shows the theoretical dependence of ω loc ¾ f x calculated in the literature.…”

“…20,21,33 Figure 3 shows the dependence of wavenumber of the localized mode on the concentration of hydrogen ions obtained from Raman spectra presented in Figs 1 and 2 and spectra of other crystals. 21,26 Figure 3 also shows the theoretical dependence of ω loc ¾ f x calculated in the literature. 30,31,32 In the paper by Elliott and Taylor, 31 concentration dependences of the wavenumber and intensity of the localized mode in LiH x D 1 x were calculated by means of Green's thermal function.…”

“…19 In addition, because the isotope concentration can be varied over a wide range (0 < x < 100%), one can hope that this will give the rare opportunity to follow experimentally the percolation threshold, which separates localized states from extended states in the phonon spectrum. 20,21 It is shown that in an isotopically mixed crystal the lines of the Raman spectra are not only shifted but also broadened. This phenomenon is discussed by using the coherent approximation potential (CPA) model.…”