Rattling Motion Research Articles

Large-cage occupation and quantum dynamics of hydrogen molecules in sII clathrate hydrates.

Hydrogen clathrate hydrates are ice-like crystalline substances in which hydrogen molecules are trapped inside polyhedral cages formed by the water molecules. Small cages can host only a single H2 molecule, while each large cage can be occupied by up to four H2 molecules. Here, we present a neutron scattering study on the structure of the sII hydrogen clathrate hydrate and on the low-temperature dynamics of the hydrogen molecules trapped in its large cages, as a function of the gas content in the samples. We observe spectral features at low energy transfer (between 1 and 3meV), and we show that they can be successfully assigned to the rattling motion of a single hydrogen molecule occupying a large water cage. These inelastic bands remarkably lose their intensity with increasing the hydrogen filling, consistently with the fact that the probability of single occupation (as opposed to multiple occupation) increases as the hydrogen content in the sample gets lower. The spectral intensity of the H2 rattling bands is studied as a function of the momentum transfer for partially emptied samples and compared with three distinct quantum models for a single H2 molecule in a large cage: (i) the exact solution of the Schrödinger equation for a well-assessed semiempirical force field, (ii) a particle trapped in a rigid sphere, and (iii) an isotropic three-dimensional harmonic oscillator. The first model provides good agreement between calculations and experimental data, while the last two only reproduce their qualitative trend. Finally, the radial wavefunctions of the three aforementioned models, as well as their potential surfaces, are presented and discussed.

Direct Imaging of Atomic Rattling Motion in a Clathrate Compound

Controlling nanoscale heat generation, dissipation, and transport is crucial for miniaturizing electronic devices and for designing highly efficient thermoelectric materials. However, it has been challenging to directly measure thermal properties at individual atom level. Herein, direct atomic‐resolution column‐by‐column imaging of the rattling motion of Ba atoms in a clathrate compound Ba8Ga16Ge30 using atomic‐resolution scanning transmission electron microscopy with a segmented detector is shown. The directional anisotropy of the rattling motion is clearly visualized in real space and its amplitude and anisotropy are quantitatively evaluated by Bayesian analysis of the thermal diffuse scattering distribution. These results open a new possibility for directly characterizing nanoscale thermal properties in materials and devices, even those containing heavy elements such as thermoelectric materials.

Effects of Si substitution on ferromagnetic order and off-center rattling of Eu ions in the type-I clathrate Eu8Ga16Ge30

In the ferromagnetic type-I clathrate Eu8Ga16Ge30, the divalent Eu ions are known to rattle among off-center positions in the tetrakaidecahedral cage. We have studied effects of Si substitution on the ferromagnetic state and the rattling motion of the Eu ions in Eu8Ga16Ge30 by measuring the magnetization M, electrical resistivity ρ, thermopower, specific heat C, and Eu partial phonon density of states (Eu pDOS) of Eu8Ga16−xGe30+x−y−zSiy□z (□ = vacancy, 0 ≤y≤ 7.3). Upon increasing y from 0 to 7.3, the lattice parameter decreases by 0.7%, which has no effect on the divalent state of Eu. The Curie temperature TC decreases from 36.2 K to 32.6 K, whereas the specific heat jump at TC, ΔC, increases twofold. For y= 0, both ρ (T) and dM(T)/dT exhibit anomalies below TC at T∗∼ 24 K, which are suppressed by applying magnetic field of 1 T. The anomalies at T∗ disappear by Si substitution at a low level, y = 3.7. The measured Eu pDOS indicates that the low-lying phonon energy of the Eu ions decreases with Si substitution. This softening means that the off-center rattling is changed to on-center one with large amplitude in an anharmonic potential. Based on these findings, we discuss relationship between the off-center Eu rattling and their collective magnetic behavior.

Three mechanical relaxation modes associated with nanoscale structural heterogeneity in Pd42.5Ni7.5Cu30P20 fragile glass

The dynamic relaxation behavior of Pd42.5Ni7.5Cu30P20 metallic glass was studied at 173–563 K (∼0.98Tg) and at angular frequency 10−2 to 102 rad s−1. Master curves of 16-digit dynamic moduli at angular frequency 10−4 – 1012 rad s−1 were constructed and characterized by the stretched exponent relaxation function, which exhibits three discrete relaxation modes with different activation energies, i.e., α-relaxation (7.98 eV) associated with the cooperative motion of ∼5 atoms, slow β-relaxation (1.39 eV) associated with single atomic diffusion, and fast β-relaxation (0.25 eV) associated with rattling motion. These three modes were also detected as slope changes on the quasi-static creep compliance under an isochronal heating process, which was successfully reproduced by dynamic complex compliance using the dynamic moduli with fitting parameters related to the considerable inhomogeneous structural characteristics, elastic modulus and volume fraction, generated in the glass.

Dynamics and local ordering of pentachloronitrobenzene: a molecular-dynamics investigation.

Plastic phases are constituted by molecules whose centers of mass form a long range ordered crystalline lattice, but rotate in a more or less constrained way. Pentachloronitrobenzene (PCNB) is a quasi-planar hexa-substituted benzene formed by a benzene ring decorated with a -NO2 group and five chlorine atoms that displays below the melting point a layered structure of rhombohedral (R3̄) planes in which the molecules can rotate around a six-fold-like axis. Dielectric spectroscopy [Romanini et al., The Journal of Physical Chemistry C, 2016, 120, 10614] of this highly anisotropic phase revealed a complex relaxation dynamics with two coupled primary α processes, initially ascribed to the in-plane and out-of-plane components of the molecular dipole. In this work, we perform a series of molecular dynamics simulations together with single crystal X-ray synchrotron diffraction experiments to investigate the puzzling dynamics of PCNB. We conclude that the molecule undergoes very fast movements due to the high flexibility of the -NO2 group, and two slower movements in which only the in-plane rotation of the whole ring is involved. These two movements are related to fast attempts to perform a 60° in-plane rotation, and a diffusive motion that involves the rotation of the molecule completely decorrelating the dipole orientation. We have also investigated whether a homogeneous or a heterogeneous scenario is better suited to describe the restricted orientational disorder of this anisotropic phase both from a structural and dynamical point of view.

Ion Correlation in Choline Chloride-Urea Deep Eutectic Solvent (Reline) from Polarizable Molecular Dynamics Simulations.

In recent years, deep eutectic solvents (DESs) emerged as highly tunable and environmentally friendly alternatives to common ionic liquids and organic solvents. In this work, a polarizable model based on the CHARMM Drude polarizable force field is developed for a 1:2 ratio mixture of choline chloride/urea (reline) DES. To successfully reproduce the structure of the liquid as compared to first-principles molecular dynamics simulations, a damping factor was introduced to correct the observed over-binding between the chloride and the hydrogen bonding site of choline. Investigated radial distributions reveal the formation of hydrogen bonds between all the constituents of reline and similar interactions for chloride and urea's oxygen atoms, which could contribute to the melting point depression of the mixture. Predicted dynamic properties from our polarizable force field are in good agreement with experiments, showing significant improvements over nonpolarizable models. Similar to some ionic liquids, an oscillatory behavior in the velocity autocorrelation function of the anion is visible, which can be interpreted as a rattling motion of the lighter anion surrounded by the heavier cations. The obtained results for ionic conductivity of reline show some degree of correlated ion motion in this DES. However, a joint diffusion of ion pairs cannot be observed during the simulations.

Ultra-low lattice thermal conductivity and high thermoelectric efficiency in Cs2SnX6 (X=Br, I): A DFT study

High-performance thermoelectric materials provide an environmentally friendly route for future large-scale electricity production from waste heat recovery. In this work, we present an in-depth analysis of electronic, phononic, and thermoelectric properties of vacancy-ordered double perovskite Cs2SnX6 (X = Br, I) compounds based on generalized gradient approximations (GGA) and post-density functional theory treatments. We find that the optical phonon scattering mechanism plays a more critical role than the acoustical phonon and impurity scattering in both materials' transport properties. These materials also exhibit a relatively large Seebeck coefficient and ultra-low lattice thermal conductivity about 0.123/0.062 W/mK for Cs2SnBr6/Cs2SnI6 at room temperature calculated by single relaxation time model (SMRT). Such ultra-low lattice thermal conductivity arises from significant phonon-phonon scattering due to highly overlapped acoustic and optical phonon branches around ~1 THz. This occurrence introduces the Cs atom's rattling motion inside [SnX6]2- (X = Br, I) cages. The most considerable ZT values with the SMRT method at 300/500 K are about 0.32/0.82 for Cs2SnBr6 and 0.9/1.96 for Cs2SnI6. Our theoretical results suggest that both compounds have a great potential to be good candidates for thermoelectric materials and can be used to guide experimental investigation to obtain the optimal ZT performance.

Ultrafast rattling motion of a single atom in a fullerene cage sensed by terahertz spectroscopy

We have investigated the dynamical motion of a single Ce atom encapsulated in a C82 fullerene cage (Ce@C82) by using terahertz (THz) spectroscopy. A Ce@C82 molecule is placed in a sub-nm gap of metal electrodes and excited by THz radiation. Two broad peaks are observed in the THz photocurrent spectra around 5 meV and 15 meV and are attributed to the lateral and longitudinal vibrational modes of the encapsulated single Ce atom, respectively. The present results demonstrate that the THz spectroscopy using nanogap electrodes has an extremely high sensitivity that can detect the dynamical motion of a single atom.

Carrier mobilities, thermoelectric properties, and band structures of type-I clathrates Ba8M16Ge30 (M = Al, Ga, In)

Analysis of the carrier mobilities and band structure calculations for the titled clathrates indicate that the primary carrier scattering mechanism around room-temperature is not acoustic phonon scattering but alloy disorder scattering due to substituting M atoms on the frameworks. Also, it is studied whether the rattling motion of the guest atoms in the cages affects carrier mobility due to optical phonon scattering. Electron densities of conduction-band bottoms exist not around the frameworks but between the guest and framework atoms and are anisotropic because of disordered placements of substituting atoms. Band-gap energies depend strongly on the presence of the substituting atoms. However, these features do not vary with the kind of substituting group-III elements very much. The maximum dimensionless figure-of-merits ZT were 0.65 at 940 K, 0.74 at 890 K, and 0.51 at 800 K for M = Al, Ga, and In, respectively, which depended on the experimentally achieved carrier concentrations.

Ferrimagnetism in EuFe4As12 revealed by Eu153 NMR and As75 NQR measurements

Filled skutterudite compound EuFe$_4$As$_{12}$ shows the highest magnetic ordering temperature of $T_{\rm C}$ = 154 K among Eu-based skutterudite compounds, but its magnetic ground state has not been determined yet. Here, we performed $^{153}$Eu nuclear magnetic resonance (NMR) and $^{75}$As nuclear quadrupole resonance (NQR) measurements on EuFe$_4$As$_{12}$ to reveal its magnetic ground state as well as the physical properties from a microscopic point of view. From the temperature and magnetic field dependence of $^{153}$Eu NMR spectrum in the magnetically ordered state, we found that the Eu ions are in Eu$^{2+}$ state with a nearly 7 $\mu_{\rm B}$ corresponding to $S$ = 7/2 spins. Combined with the magnetization measurements which show the reduced saturation moments of 4.5 $\mu_{\rm B}$/f.u., we determined the ground magnetic structure in EuFe$_4$As$_{12}$ to be ferrimagnetic where the Eu$^{2+}$ 4$f$ and the Fe 3$d$ ordered moments are ferromagnetically aligned in each sublattice but the moments between the sublattices are antiferromagnetically aligned. We also found the local distortion at the Eu site from the cubic symmetry in the magnetically ordered state. The relationship between the rattling motion of Eu atoms and the local symmetry of the Eu ions is discussed. From the $^{75}$As NQR nuclear spin-lattice relaxation time measurements as well as $^{153}$Eu NMR measurements, we found that the 4$f$ electrons of the Eu ions are well described by the local moment picture in both the magnetic and paramagnetic metallic states.

Quasielastic neutron scattering studies on couplings of protein and water dynamics in hydrated elastin.

Performing quasielastic neutron scattering measurements and analyzing both elastic and quasielasic contributions, we study protein and water dynamics of hydrated elastin. At low temperatures, hydration-independent methyl group rotation dominates the findings. It is characterized by a Gaussian distribution of activation energies centered at about Em = 0.17 eV. At ∼195 K, coupled protein-water motion sets in. The hydration water shows diffusive motion, which is described by a Gaussian distribution of activation energies with Em = 0.57 eV. This Arrhenius behavior of water diffusion is consistent with previous results for water reorientation, but at variance with a fragile-to-strong crossover at ∼225 K. The hydration-related elastin backbone motion is localized and can be attributed to the cage rattling motion. We speculate that its onset at ∼195 K is related to a secondary glass transition, which occurs when a β relaxation of the protein has a correlation time of τβ ∼ 100 s. Moreover, we show that its temperature-dependent amplitude has a crossover at the regular glass transition Tg = 320 K of hydrated elastin, where the α relaxation of the protein obeys τα ∼ 100 s. By contrast, we do not observe a protein dynamical transition when water dynamics enters the experimental time window at ∼240 K.

Strong Chemical Bond Hierarchy Leading to Exceptionally High Thermoelectric Figure of Merit in Oxychalcogenide AgBiTeO

The chemical bond hierarchy (CBH) in prototype cage structures has been considered important for achieving high thermoelectric performance. By performing first principles calculations and lattice dynamics, we demonstrate CBH hosted distinct rattlers in a non-caged oxychalcogenide AgBiTeO, cause ultralow κl of 0.9 W/m-K at room temperature. The CBH in this compound leads to a unique structural bonding, where Ag and Te are loosely bonded to the rigid framework of the lattice and form distorted four-centered Ag-Te tetrahedra. These clusters exhibit large atomic vibrational motions in a very shallow potential energy surface resulting into rattling motion. The presence of multiple avoided crossing points of low lying optical mode with longitudinal acoustic mode in phonon dispersion further confirms the rattling induced thermal damping. Additionally, unique in-plane off phase collective vibrations of Ag- Te tetrahedra introduce localized flat phonon dispersions that lower the group velocity and significantly reduce the lattice thermal conductivity. Most importantly, it prevents carrier-phonon scattering leading to a high electrical conductivity in AgBiTeO. Combination of intrinsic low lattice thermal conductivity and excellent electronic transport properties give an unprecedented range of ZT from 1.00 to 1.99 in the large temperature range of 700-1200K, for n-type charge carriers.

Solvation Structure and Dynamics of Alkali Metal Halides in an Ionic Liquid from Classical Molecular Dynamics Simulations.

The structure and dynamics of the solvation of several alkali metal halides in an ionic liquid (IL), 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIm][OTf]), were investigated by classical molecular dynamics simulations. Various properties such as density, radial distribution functions, coordination numbers, spatial distribution functions, mean-square displacements, self-diffusion coefficients, and velocity–velocity autocorrelation functions were calculated to understand the solvation environment of alkali metal halide salts in IL at various salt concentrations. We observe that the halide anions are coordinated in two different ways with [BMIm]+ in all of the mixtures. However, the alkali metal cations interact more with the anion of the ionic liquid as we go from fluorides to iodides. When a common anion was used for the salt and the ionic liquid, we observe significant coordination of Na+ with the anion of the ionic liquid in two different ways, which was not observed in the case of lithium salt. We also find that the Li+ and Na+ ions are involved in the formation of a aggregate-like, stable kinetic entity with anions in their first solvation shells. These aggregate-like entities were seen to be relatively stable, and we noted a rattling motion of salt ions inside them.

Na3Pt10Si5: A Non-Centrosymmetric Superconductor Having Rattling Na Atoms in the Tunnel Framework Structure.

Single crystals of a novel Na-Pt-Si ternary compound, Na3Pt10Si5, were synthesized by heating the constituent elements at 1423 K. It crystallizes in a non-centrosymmetric trigonal structure of space group R32 (Z = 3) with lattice constants of a = 10.1536(3) Å and c = 10.1539(3) Å at 300 K. The structure consists of a three-dimensional framework made of Pt and Si atoms, and the Na atoms are contained in the tunnels of the framework. The large magnitude and the temperature dependence of the atomic displacement parameter of the Na site reveal a large thermal vibration indicative of a "rattling" motion of Na atoms in the oversized tunnel. The electronic structure calculations explain the observed metallic properties on the basis of the covalent bonds between the Pt and Si atoms in the framework and the ionic bonding of the Na atoms to the framework. A type II superconductivity with a transition temperature of 2.9 K and an upper critical field of 2.5 kOe are observed for a polycrystalline sintered bulk sample of Na3Pt10Si5 prepared by heating at 1353 K in Na vapor. Heat capacity measurements reveal a strong coupling superconductivity that is probably caused by an electron-phonon interaction enhanced by the rattling motion of the Na atoms.

Electronic properties across metal-insulator transition in β -pyrochlore-type CsW2O6 epitaxial films

In CsW2O6, which undergoes a metal-insulator transition (MIT) at 213 K, the emergence of exotic properties associated with rattling motion of Cs is expected owing to its characteristic \beta-pyrochlore-type structure. However, a hurdle for crystal growth hampers elucidation of detailed properties and mechanisms of the MIT. Here we report on the epitaxial growth of \beta-pyrochlore-type CsW2O6 films and their electronic properties across the MIT. Using pulsed-laser deposi-tion technique, we grew single-crystalline CsW2O6 films exhibiting remarkably lower resistivity compared with a poly-crystalline bulk and sharp MIT around 200 K. Negative magnetoresistance and positive Hall coefficient were found, which became pronounced below 200 K. The valence-band and core-levels photoemission spectra indicated the drastic changes across the MIT. In the valence band photoemission spectrum, the finite density of states was observed at the Fermi level in the metallic phase. In contrast, an energy gap appeared in the insulating phase. The split of W 4f core-level spectrum suggested the charge disproportionation of W5+ and W6+ in the insulating phase. The change of spectral shape in the Cs 4d core levels reflected the rattling motion of Cs+ cations. These results strongly suggest that CsW2O6 is a novel material, in which MIT is driven by the charge disproportionation associated with the rattling motion.

Static and dynamic structures of liquid Ba8Ga16Sn30: a melt of the thermoelectric clathrate compounds

X-ray diffraction and inelastic x-ray scattering measurements of liquid Ba8Ga16Sn30 have been carried out to investigate local structure and atomic dynamics in the liquid. The pair distribution function shows shorter and longer interatomic distances in the first coordination shell. The dynamic structure factor exhibits the inelastic excitations on both sides of the quasielastic central peak. The inelastic excitations disperse with increasing the momentum transfer, suggesting the longitudinal acoustic mode. We found a low energy excitation in addition to the longitudinal acoustic excitation in the dynamic structure factor and it reminds us a strong relationship with a rattling motion of a guest (Ba) atom in the solid state. The temperature dependence of the pair distribution function and the longitudinal acoustic excitation energy is very weak in a range from 600 to 900 °C. The result suggests that Ba and other atoms in the melt are located around minimum positions of the effective pair potential approximated as a harmonic one.

Rattling of Oxygen Ions in a Sub-Nanometer-Sized Cage Converts Terahertz Radiation to Visible Light.

A simple and robust approach to visualization of continuous wave terahertz (CW-THz) light would open up opportunities to couple physical phenomena that occur at fundamentally different energy scales. Here we demonstrate how nanoscale cages of Ca12Al14O33 crystal enable conversion of CW-THz radiation to visible light. These crystallographic cages are partially occupied with weakly bonded oxygen ions and give rise to a narrow conduction band that can be populated with localized, yet mobile electrons. CW-THz light excites a nearly stand-alone rattling motion of the encaged oxygen species, which promotes electron transfer from them to the neighboring vacant cages. When the power of CW-THz light reaches tens of watts, the coupling between forced rattling in the confined space, electronic excitation and ionization of oxygen species, and corresponding recombination processes result in emission of bright visible light.

Thermodynamic scaling of vibrational dynamics and relaxation.

We investigate by thorough molecular dynamics simulations the thermodynamic scaling (TS) of a polymer melt. Two distinct models, with strong and weak virial-energy correlations, are considered. Both evidence the joint TS with the same characteristic exponent γts of the fast mobility-the mean square amplitude of the picosecond rattling motion inside the cage-and the much slower structural relaxation and chain reorientation. If the cage effect is appreciable, the TS master curves of the fast mobility are nearly linear, grouping in a bundle of approximately concurrent lines for different fragilities. An expression of the TS master curve of the structural relaxation with one adjustable parameter less than the available three-parameter alternatives is derived. The novel expression fits well with the experimental TS master curves of thirty-four glassformers and, in particular, their slope at the glass transition, i.e., the isochoric fragility. For the glassformer OTP, the isochoric fragility allows to satisfactorily predict the TS master curve of the fast mobility with no adjustments.

An Overview on Short and Long Time Relaxations in Glass-forming Supercooled Liquids

Density fluctuations in supercooled liquids near the glass transition relax in multiple steps. The short time relaxation is known as β-relaxation and the final long time relaxation is called α-relaxation. It is believed that the long time α-relaxation is a cooperative phenomena associated with a growing length scales, whereas the short-time β-relaxation is often attributed to spatially local processes involving the rattling motion of a particle in the transient cage formed by its neighbors. Using molecular dynamics simulations of few model glass-forming liquids, we show that the β-relaxation is also cooperative in nature and the length scale extracted from the detailed finite-size scaling analysis of β-relaxation is found to be the same as that of the length scale that describes the spatial heterogeneity of local dynamics in the long-time α-relaxation regime. These results provide a clear connection between short-time dynamics and long-time structural relaxation in glass-forming liquids.

Initial Atomic Motion Immediately Following Femtosecond-Laser Excitation in Phase-Change Materials.

Despite the fact that phase-change materials are widely used for data storage, no consensus exists on the unique mechanism of their ultrafast phase change and its accompanied large and rapid optical change. By using the pump-probe observation method combining a femtosecond optical laser and an x-ray free-electron laser, we substantiate experimentally that, in both GeTe and Ge_{2}Sb_{2}Te_{5} crystals, rattling motion of mainly Ge atoms takes place with keeping the off-center position just after femtosecond-optical-laser irradiation, which eventually leads to a higher symmetry or disordered state. This very initial rattling motion in the undistorted lattice can be related to instantaneous optical change due to the loss of resonant bonding that characterizes GeTe-based phase change materials. Based on the amorphous structure derived by first-principles molecular dynamics simulation, we infer a plausible ultrafast amorphization mechanism via nonmelting.