Guest Modes Research Articles

Heat Transport in Clathrate Hydrates Controlled by Guest Frequency and Host-Guest Interaction.

The underlying mechanism of common limited lattice thermal conductivity (κ) in energy-related host-guest crystalline compounds has been an ongoing topic in recent decades. Here, the guest-triggered intrinsic ultralow κ of the representative xenon clathrate hydrate was investigated using the time domain thermoreflectance technique and theoretical calculations. The localized guest modes were observed to hybridize with acoustic branches and severely limit the acoustic κ contribution. Besides, the strong mode coupling enables the reshaping of the overall lattice dynamics, especially for optical branches. More importantly, we identified that guest fillers prompt great phonon scattering in wide frequencies, which originates from both the guest-frequency-controlled enhancement of phase space and the host-guest-interaction-governed lattice anharmonicity. The extremely low guest frequency and strong host-guest interaction and coupling were thereby underlined to play vital but distinct roles in κ minimization. Our results unveil the dominant factors of guest reduction effects and facilitate the design of efficient thermoelectric or other thermal-related materials.

Anharmonic phonon renormalization and thermal transport in the type-I Ba8Ga16Sn30 clathrate from first principles

Effects of strong phonon anharmonicity of a type-I clathrate ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Sn}}_{30}$ induced by the quadruple-well potential of guest atoms were investigated. Phonon transport including a coherent interbranch component was analyzed using a first-principles-based self-consistent phonon (SCP) theory that gives temperature-dependent harmonic interatomic force constants and by solving off-diagonal components of group velocity operator. Experimentally observed thermal conductivities have been reasonably reproduced by considering both lattice and electron contributions. Through the analysis with the SCP theory, we found that hardening of guest modes leads to an increase in lattice thermal conductivity at frequencies below those of framework-dominant flat modes ($<40\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$), which finally results in the slow decay of the total lattice thermal conductivity with increasing temperature. Detailed analyses revealed that the increase in lattice thermal conductivity at low frequency is attributed to (a) the increase in both group velocities and lifetimes of phonon modes located at frequencies below that of the flat guest modes and (b) abnormal increase in lifetimes of phonon modes located between frequencies of the flat guest and framework modes with increasing temperature.

Anharmonic lattice dynamics and thermal transport in type-I inorganic clathrates

The anharmonic phonon properties of type-I filled inorganic clathrates Ba8Ga16Ge30 and Sr8Ga16Ge30 are obtained from the first-principles calculations by considering the temperature-dependent sampling of the potential energy surface and quartic phonon renormalization. Owing to the weak binding of guest atoms with the host lattice, the obtained guest modes undergo strong renormalization with temperature and become stiffer by up to 50% at room temperature in Sr8Ga16Ge30. The calculated phonon frequencies and associated thermal mean squared displacements are comparable with experiments despite the on-centering of guest atoms at cage centers in both clathrates. Lattice thermal conductivities are obtained in the temperature range of 50–300 K accounting for three-phonon scattering processes and multi-channel thermal transport. The contribution of coherent transport channel is significant at room temperature (13% and 22% in Ba8Ga16Ge30 and Sr8Ga16Ge30) but is insufficient to explain the experimentally observed glass-like thermal transport in Sr8Ga16Ge30.

VCD spectroscopy reveals conformational changes of chiral crown ethers upon complexation of potassium and ammonium cations.

Two chiral derivatives of 18-crown-6, namely the host molecules 2,3-diphenyl- and 2-phenyl-18c6, serve as model systems to investigate whether VCD spectroscopy can be used to monitor conformational changes occurring upon complexation of guests. Host-guest complexes of both crown ethers were prepared by addition of KNO3. The more bulky 2,3-diphenyl-18c6 is found to undergo major conformational changes upon encapsulation of K+, which are revealed as characteristic changes of the VCD spectral signatures. In contrast, while 2-phenyl-18c6 also incorporates K+ into the macrocycle, strong conformational changes are not occurring and thus spectral changes are negligible. With an octyl ammonium cation as guest molecule, 2,3-diphenyl-18c6 shows the same conformational and spectral changes that were observed for K+-complexes. In addition, the asymmetric NH3-deformation modes are found to gain VCD intensity through an induced VCD process. An analysis of the vibrational spectra enables a differentiation of VCD active and inactive guest modes: There appears to be a correlation between the symmetry of the vibrational mode and the induced VCD intensity. While this finding makes the host-guest complexes interesting systems for future theoretical studies on the origin of induced VCD signatures, the observations described in this study demonstrate that VCD spectroscopy is indeed a suitable technique for the characterization of supramolecular host-guest complexes.

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.

Power-efficient and high-performance block I/O framework for mobile virtualization systems

A virtualized system generally suffers from low I/O performance, mainly caused by its inherent abstraction overhead and frequent CPU transitions between the guest and hypervisor modes. The recent research of polling-based I/O virtualization partly solved the problem, but excessive polling trades intensive CPU usage for higher performance. This article presents a power-efficient and high-performance block I/O framework for a virtual machine, which allows us to use it even with a limited number of CPU cores in mobile or embedded systems. Our framework monitors system status, and dynamically switches the I/O process mode between the exit and polling modes, depending on the amounts of current I/O requests and CPU utilization. It also dynamically controls the polling interval to reduce redundant polling. The highly dynamic nature of our framework leads to improvements in I/O performance with lower CPU usage as well. Our experiments showed that our framework outperformed the existing exit-based mechanisms by 10.8 % higher I/O throughput, maintaining similar CPU usage by only 3.1 % increment. In comparison to the systems solely based on the polling mechanism, ours reduced the CPU usage roughly down to 10.0 % with no or negligible performance loss.

Inelastic X-ray Scattering of Heavy Fermion Compounds ROs4Sb12 (R: Light Rare-Earth)

Inelastic X-ray scattering experiments were carried out in the series of filled skutterudites ROs4Sb12 (R: light rare-earth). Phonon energy of the rare-earth guest modes in the longitudinal (1 0 0) direction decreases with increase of atomic number of rare-earth elements from La to Sm inserted into a Sb cage. This fact demonstrates that the guest mode energy is affected by the ionic radius of the rare-earth atom. In addition, comparison between inelastic X-ray scattering and X-ray diffraction experiments reveals the correlation of the guest mode energy in the longitudinal (1 0 0) direction and the Einstein temperature obtained by the temperature dependence of the atomic displacement factor.

Atomic dynamics of low-lying rare-earth guest modes in heavy fermion filled skutteruditesROs4Sb12(R=light rare-earth)

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Observations of Host Guest Interactions Specific to Molecular Matrices: Water Monomers and Dimers in Hydrogen Matrices

Water monomers and dimers have been studied at low temperatures in matrices of solid p-H(2), o-D(2), n-H(2), and n-D(2) using infrared spectroscopy. Our data demonstrate interaction mechanisms between host matrix and guest molecules that are different from the ones observed in atomic noble gas matrices. Notably both guest/host rotational--rotational interaction and matrix induced modifications of the guest libration modes are observed. We also show that different types of interaction influence the relaxation times of some of guest modes. Water rotates freely in p-H(2) and o-D(2) but librates in n-H(2) and n-D(2). Rotational relaxation is faster in o-D(2) than in p-H(2) and faster in p-H(2) than in Ne. This is attributed to interactions between water rotation and matrix molecule rotation in p-H(2) and o-D(2). In n-H(2) and n-D(2), a strong water libration band is observed in the far-infrared, and strong water monomer vibration bands have libration satellites. Water dimer bands, close to matrix rotation bands, are perturbed by the matrix motions. The H-bonded isomer H(2)O--HOD rapidly converts to the D-bonded form H(2)O-DOH in p-H(2) and in o-D(2) but slowly in n-H(2) and n-D(2).

Raman Scattering Study of Lattice Dynamics in Caged Compounds

Raman scattering spectra of caged compounds PrB 6 , PrT 4 Sb 12 , and PrRu 2 Zn 20 have been measured at room temperature. In there spectra, cage modes due to B, Sb, and Zn vibrations and Pr guest modes are observed. In the excitation energy dependence, the guest modes and some of cage modes show the resonance behavior at low energy excitation.

Off-center rattling modes and glasslike thermal conductivity in the type-I clathrateBa8Ga16Sn30

Type-I clathrate ${\text{Ba}}_{8}{\text{Ga}}_{16}{\text{Sn}}_{30}$ is unique for showing glasslike behavior in the lattice thermal conductivity ${\ensuremath{\kappa}}_{\text{L}}$ irrespective of the charge carrier type. For better understanding the relation between this behavior and guest rattling, polarized Raman-scattering measurements have been performed on carrier-tuned single crystals. The appearance of a symmetry-forbidden mode in the ${E}_{g}$ symmetry spectrum indicates that the Ba atoms are rotating among the off-center positions in the tetrakaidecahedron. On cooling from 300 to 4 K, the energies of ${E}_{g}$ and ${T}_{2g}$ guest modes decrease by 15% and 25%, respectively. This drastic decrease originates from the strongly anharmonic potential with a quartic term of the atomic displacement. The energy of $14\text{ }{\text{cm}}^{\ensuremath{-}1}$ for the ${T}_{2g}$ mode at 4 K is the lowest among intermetallic clathrates. These results for ${\text{Ba}}_{8}{\text{Ga}}_{16}{\text{Sn}}_{30}$ are compared with those for type-I ${\text{Eu}}_{8}{\text{Ga}}_{16}{\text{Ge}}_{30}$ and ${\text{Sr}}_{8}{\text{Ga}}_{16}{\text{Ge}}_{30}$ whose ${\ensuremath{\kappa}}_{\text{L}}(T)$ show similar glasslike behavior with a plateau. The comparison between three systems indicates that the guest rattling energy is proportional to the temperature range of the plateau in ${\ensuremath{\kappa}}_{\text{L}}(T)$.

Simultaneous structure and carrier tuning of dimorphic clathrateBa8Ga16Sn30

We report structural, transport, and thermal properties of carrier-tuned ${\text{Ba}}_{8}{\text{Ga}}_{16}{\text{Sn}}_{30}$ single crystals with the type-1 clathrate structure ($\ensuremath{\beta}$ phase), demonstrating that ${\text{Ba}}_{8}{\text{Ga}}_{16}{\text{Sn}}_{30}$ is a unique thermoelectric clathrate material wherein both the structure type and the carrier type are tunable. The results are compared with the properties of the better known type-8 structure ($\ensuremath{\alpha}$ phase) and of ${A}_{8}{\text{Ga}}_{16}{\text{Ge}}_{30}$ $(A=\text{Sr},\text{Eu})$. Differential thermal analysis and powder x-ray diffraction show that both phases are stable up to their virtually identical melting point of $520\ifmmode\pm\else\textpm\fi{}3\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. Refinements of single-crystal x-ray diffraction data indicate that the Ba(2) guest ion in the tetrakaidecahedron occupies the off-center $24k$ sites which are $0.43--0.44\text{ }\text{\AA{}}$ away from the centered $6d$ site. The temperature-linear coefficient of the specific heat is $29\text{ }\text{mJ}/\text{mol}\text{ }{\text{K}}^{2}$ for both $n$- and $p$-type carriers in the $\ensuremath{\beta}$ phase, four times larger than that for the $\ensuremath{\alpha}$ phase, suggesting contributions from tunneling of the Ba(2) guest ions between off-center minima. Analysis of specific heat with a soft-potential model (SPM) gives a characteristic energy of 20 K for the Ba(2) vibration, significantly lower than 50 K for the $\ensuremath{\alpha}$ phase and in fact the lowest among type-1 clathrates. The lattice thermal conductivities ${\ensuremath{\kappa}}_{L}$ for the $\ensuremath{\beta}$ phase with both charge carrier types are very similar and show a glasslike temperature dependence. This behavior in ${\ensuremath{\kappa}}_{L}(T)$ is also described using SPM analysis, where it is found that the coupling strength between guest modes and acoustic phonons for the $\ensuremath{\beta}$ phase is significantly larger than that for ${\text{Sr}}_{8}{\text{Ga}}_{16}{\text{Ge}}_{30}$.

Investigation of the Anharmonic Guest Modes in Filled Skutterudites Using Inelastic X-ray Scattering Techniques

We have carried out high-resolution inelastic X-ray scattering and 149 Sm nuclear resonant inelastic scattering to investigate the anharmonicity of the mode of the atoms enclosed in the cages of filled skutterudites. The temperature dependence of the phonon energy obtained by 149 Sm nuclear resonant inelastic scattering from SmRu 4 P 12 suggests an anharmonic vibration of the Sm atoms. A similar decrease of the vibration energy of the La atoms in LsOs 4 Sb 12 with decreasing temperature, which suggests that this motion too is anharmonic, was observed by high resolution inelastic X-ray scattering.

Lattice dynamics study of low energy guest–host coupling in clathrate hydrate

Our lattice dynamics simulation of Xe-hydrate with four-site TIP4P oxygen-shell model can accurately reproduce each peak position in the inelastic incoherent neutron scattering spectrum at the acoustic band (below 15meV) and yield correct relative intensity. Based on the results, the uncertain profile at ∼6 meV is assigned to anharmonic guest modes coupled strongly to small cages. Blue shift is proposed in phonon dispersion sheet in the case of anticrossing and found to be an evident signal for guest-host coupling that explains the anomalous thermal conductivity of clathrate hydrate.

Raman scattering study of type-I clathrate compounds: [formula omitted] ( [formula omitted], Sr, Ba)

Lattice dynamical properties, especially dynamical motion of a guest ion, of type-I clathrate compounds A 8 Ga 16 Ge 30 ( A = Eu , Sr, Ba) have been studied by Raman scattering. The Raman active phonon modes are fully assigned by polarized Raman scattering on single crystalline A 8 Ga 16 Ge 30 with referring to the results of the first principles calculations. Additional phonon modes due to an off-center rattling of the guest ion, whose energy decreases with decreasing temperature, have been observed. This anomalous temperature dependence is a common property for A 8 Ga 6 Ge 30 . In addition, a soft mode is also observed in the guest modes. This softening suggests a new type transition named an incoherent transition.

Infrared vibrational spectra of tert-butyl halides in low-aluminum H-Y Faujasite: Vibrational excitation exchange and other effects of guest–host interactions

Fourier transform infrared (FTIR) studies at 295 K of low-aluminum H-Y (LAHY, Si/Al = 40) faujasite zeolite supercage-included tert-butyl halides, (CH 3) 3C–X (X = Cl, Br, I), are presented in comparison with the adsorbate molecular gas-phase and unloaded host solid-state spectra. The FTIR results, aided by computer modeling studies, reveal a propensity toward the exchange of quanta of vibrational excitation between guest modes, and between guest and host modes. The exchange phenomenon and some pseudo-hydrogen bonding effects are related to the siting of the guest molecules in the host supercage, and to the guest–host interactions at the guest site.

Lattice dynamics of methane and xenon hydrate: Observation of symmetry-avoided crossing by experiment and theory

The collective dynamics of methane and xenon hydrate in the energy range of $\ifmmode\pm\else\textpm\fi{}20\mathrm{meV}$ and the Q range of $1.5--11{\mathrm{nm}}^{\ensuremath{-}1}$ has been investigated by inelastic x-ray scattering and compared to results from inelastic neutron scattering experiments and lattice dynamical calculations. The experiment focused on the low--frequency phonon dispersion curves that were found to point towards the existence of an avoided crossing between the acoustic lattice phonons and the localized guest modes. The calculations reproduce the experimental spectra and show that the localized vibrations of the guest molecules or atoms are mixed with the collective host lattice vibrations, leading to a damping of the intensity of the acoustic host lattice phonons. This observation supports the idea of a resonant scattering mechanism for the strong guest-host phonon interactions in clathrate hydrates.

Vibrational Dynamics of Isotopically Dilute Nitrogen to 104 GPa

Raman studies to 104 GPa of a ${}^{14}{\mathrm{N}}_{2}$ host with dilute ${}^{15}{\mathrm{N}}_{2}$ and ${}^{14}{\mathrm{N}}^{15}\mathrm{N}$ isotopic species indicate an increase in vibrational coupling with pressure. The lowest frequency branch of the isotopic species ${}^{15}{\mathrm{N}}_{2}$ reaches a plateau around 100 GPa suggesting the onset of bond weakening for molecules located on particular sites. An unusual intensity increase is observed for the isotopic ${\ensuremath{\nu}}_{1}$ guest modes. The phase transitions above 20 GPa appear to involve an increase in the number of inequivalent sites, with possibly subtle changes in the orientational behavior of particular molecules in the pressure range 25 to 50 GPa not readily revealed in recent x-ray diffraction studies.

Intramolecular vibrational relaxation of a polyatomic in the solid state. I. DCN in Xe

Vibrational relaxation of DCN isolated in Xe matrices has been studied by laser-excited vibrational fluorescence. The C–D stretch ν3 was excited and fluorescence was observed from overtones and combinations of the ν1 and ν2 modes. The decay rates of the vibrational levels range over several orders of magnitude from 3×108 s−1 for ν3 to 2.1×105 s−1 for ν2. A kinetic model for the relaxation was solved to give a complete set of rate constants for the vibrational transitions. The rates, together with the relative fluorescence intensities, show that there is a dominant relaxation channel. The relative importance of competing channels depends on changes in energy and in vibrational quantum number of the transfers. Within the ν2 manifold, the rate constants for Δv2=1 transitions follow kν,ν−1=vnk1,0 with n&amp;lt;1.3. The rate constants increase only modestly with temperature indicating that the excess vibrational energy is transferred to guest modes such as rotation or translation rather than to bulk phonons.

’’Effective’’ site symmetry in molecular crystals. Orientational energies of dilute guest probes in isotopically mixed, partially deuterated ethylene crystals

The midinfrared and Raman spectra of dilute mixed crystals of the partially deuterated ethylenes (C2H3D; cis-, trans- and 1,1-C2H2D2; C2HD3) in C2H4 and in one another have been obtained. The guest modes often exhibit multiplet structure arising from different relative orientations of the neighboring molecules. The number of energetically inequivalent orientations is diagnostic of the ’’effective’’ site symmetry of the host lattice (i.e., symmetry in the spectroscopic rather than the mathematical sense), which was found to be Ci for each of the partially deuterated ethylenes. Consistent with the predictions of the virtual crystal approximation, the number and energies of the orientational components of the guest molecules are independent of the isotopic nature of the host crystal, which indicates that the intermolecular force field is insensitive to the isotopic composition of the interacting ethylene molecules. The orientational effect can be used to distinguish among several possible effective site symmetries for any crystalline lattice, provided that the number of predicted orientational components differ.