First Principles Study of the Vibrational and Thermal Properties of Sn-Based Type II Clathrates, CsxSn136 (0 ≤ x ≤ 24) and Rb24Ga24Sn112

After performing first-principles calculations of structural and vibrational properties of the semiconducting clathrates Rb24Ga24Sn112 along with binary CsxSn136 (0 ≤ x ≤ 24), we obtained equilibrium geometries and harmonic phonon modes. For the filled clathrate Rb24Ga24Sn112, the phonon dispersion relation predicts an upshift of the low-lying rattling modes (~25 cm−1) for the Rb (“rattler”) compared to Cs vibration in CsxSn136. It is also found that the large isotropic atomic displacement parameter (Uiso) exists when Rb occupies the “over-sized” cage (28 atom cage) rather than the 20 atom counterpart. These guest modes are expected to contribute significantly to minimizing the lattice’s thermal conductivity (κL). Our calculation of the vibrational contribution to the specific heat and our evaluation on κL are quantitatively presented and discussed. Specifically, the heat capacity diagram regarding CV/T3 vs. T exhibits the Einstein-peak-like hump that is mainly attributable to the guest oscillator in a 28 atom cage, with a characteristic temperature 36.82 K for Rb24Ga24Sn112. Our calculated rattling modes are around 25 cm−1 for the Rb trapped in a 28 atom cage, and 65.4 cm−1 for the Rb encapsulated in a 20 atom cage. These results are utilized to predict the lattice’s thermal conductivity (approximately 0.62 W/m/K) in Rb24Ga24Sn112 within the kinetic theory approximation.

Thermal Shifts and Electron-Phonon Coupling Parameters for the Three Luminescence Lines of 5DJ→7FJ in SrFCl:Sm2+ Crystal

The thermal blue shifts of three spectral lines E1(5D0→7F0), E2(5D0→7F1), and E3(5D1→7F0) in SrFCl:Sm2+ crystal are investigated by a complete expression consisting of both the static contribution due to lattice thermal expansion and the vibrational contribution owing to electron-phonon interaction. The obtained true electron-phonon coupling parameters α′ in both sign and magnitude are considerably different from the apparent electron-phonon coupling parameters α acquired in the previous paper by considering only the vibrational contribution. It is suggested that differing from the simple expression including only the vibrational contribution (many authors thought that it cannot be used to explain the thermal blue shifts), the complete expression containing both the vibrational and static contributions is effective in the studies of thermal shift (whether red shift or blue shift) and true electron-phonon coupling parameter for a spectral line in crystals.

Role of the vibrational contribution in Coulomb explosion of dicationic neon gas clusters: a parallel tempering based study

Distribution plots for interatomic lengths for a stable structure and for one showing Coulomb explosion.

On EXAFS Debye-Waller factor and recent advances

The effects of structural and vibrational disorder on the EXAFS signals are parameterized in terms of the Debye Waller (DW) factor. Here the vibrational contribution is addressed, which for most systems can be singled out by studying the temperature dependence of the EXAFS DW factor, which corresponds to a good accuracy to the parallel mean square relative displacement (MSRD) around the inter-atomic equilibrium distance. By comparing the first-shell EXAFS thermal expansion with the crystallographic thermal expansion one can evaluate the perpendicular MSRD. The results of recent measurements on copper and on several tetrahedral semiconductors are here critically compared and some properties of the MSRDs are discussed, such as the dependence of correlation, force constants and vibrational anisotropy on crystal structure and bond ionicity as well as the relative merits of the correlated Debye and Einstein models. The anharmonic contribution to the parallel MSRD of CdTe has been evaluated and a quasi-harmonic analysis has been attempted, leading to an estimation of the bond Grüneisen parameter.

Investigations of the Thermal Shifts and Electron–Phonon Coupling Parameters of R1 and R2 Lines for Cr3+-doped Forsterite

The thermal shifts of R1 and R2 lines in Cr3+-doped forsterite (Mg2SiO4) are studied by considering both the static contribution due to lattice thermal expansion and the vibrational contribution due to electron-phonon interaction. In the studies, the thermal expansion coefficient of the Cr3+ center is assumed reasonably as that of the corresponding cluster in the host crystal. The results suggest that for R1 and R2 lines the static contributions are opposite in sign and in magnitude about 37% and 45%, respectively, of the corresponding vibrational contributions. The true electron-phonon coupling coefficients α' (obtained by considering both contributions) increase by about 58% and 81%, respectively, for R1 and R2 lines in comparison with the corresponding parameters α obtained by considering only the vibrational contribution. It appears that for the reasonable explanation of thermal shift of spectral lines and the exact estimation of electron-phonon coupling coefficient, both the static and vibrational contributions should be taken into account