Inelastic Neutron Spectra and the Vibrational Modes of the Hydrogen Layer in Alkali and Alkaline‐Earth Hydroxides

Activated carbons are materials with relevance in different industrial applications. Due to the inherent complexity and heterogeneity of their structures, an easy assignment of the species present on their surface has a challenging result. Only recently, with the possibility to collect well-resolved inelastic neutron spectra and to simulate by DFT methods more or less extended graphitic clusters, this task is starting to become feasible. Here we report our investigation on a steam activated carbon and we show that different vibrations in the region of out-of-plane C-H bending modes are specifically connected to hydrogen terminations belonging to extended and regular borders or to short and defective ones. Furthermore, simulations including heteroatoms such as oxygen allowed us to point out spectral regions with a contribution from carboxyl species.

Download Full-text

The Dynamics of the Guests in Filled Germanium Clathrates

MRS Proceedings ◽

10.1557/proc-0886-f10-01 ◽

2005 ◽

Vol 886 ◽

Cited By ~ 10

Author(s):

Raphael P. Hermann ◽

Fernande Grandjean ◽

Veerle Keppens ◽

Werner Schweika ◽

George S. Nolas ◽

...

Keyword(s):

Thermal Conductivity ◽

Inelastic Neutron Scattering ◽

Inelastic Neutron ◽

Nuclear Inelastic Scattering ◽

Vibrational Modes ◽

Weakly Bound ◽

Typical Time ◽

Tunneling Dynamics ◽

Time And Energy ◽

Localized Vibrational Modes

ABSTRACTIn the filled gallium-germanium clathrates, R8Ga16Ge30, where R is Ba, Sr, or Eu, the guests are located in two large cages and are weakly bound to the crystalline clathrate framework. The caged guests exhibit a localized “rattling” vibrational mode that provides an efficient mechanism for reducing the thermal conductivity. Inelastic neutron scattering and nuclear inelastic scattering measurements have yielded the phonon density of states in R8Ga16Ge30; the line width of the localized vibrational modes is found to be an important parameter in determining the lattice thermal conductivity. Neutron diffraction studies on R8Ga16Ge30 have shown that the guests in the larger cage are located off-center, and it was proposed that their jumping about the four off-center locations is responsible for the observed glass-like thermal conductivity at temperatures below 10 K. The detection of such slow guest motion is challenging because the typical time and energy scales involved are ca. 4 ns and 1 µeV, respectively. We have studied the slow europium tunneling dynamics in Eu4Sr4Ga16Ge30 by both Mössbauer and microwave absorption spectroscopy.

Download Full-text

Filled Skutterudite Antimonides: Validation of the Electron-Crystal Phonon-Glass Approach to New Thermoelectric Materials

MRS Proceedings ◽

10.1557/proc-478-199 ◽

1997 ◽

Vol 478 ◽

Cited By ~ 18

Author(s):

D. Mandrus ◽

B. C. Sales ◽

V. Keppens ◽

B. C. Chakoumakos ◽

P. Dai ◽

...

Keyword(s):

Specific Heat ◽

Thermoelectric Materials ◽

Inelastic Neutron Scattering ◽

Inelastic Neutron ◽

Low Energy ◽

Vibrational Modes ◽

Weakly Bound ◽

Filled Skutterudite ◽

Resonant Ultrasound ◽

Electron Crystal

AbstractAfter a brief review of the transport and thermoelectric properties of filled skutterudite antimonides, we present resonant ultrasound, specific heat, and inelastic neutron scattering results that establish the existence of two low-energy vibrational modes in the filled skutterudite LaFe3CoSb12. It is likely that at least one of these modes represents the localized, incoherent vibrations of the La ion in an oversized atomic “cage.” These results support the usefulness of weakly bound, “rattling” ions for the improvement of thermoelectric performance.

Download Full-text