Clathrate Research Articles

Synthesis, Structure, and Transport Properties of Type-I Derived Clathrate Ge46–xPxSe8–y (x = 15.4(1); y = 0–2.65) with Diverse Host–Guest Bonding

A first clathrate compound with selenium guest atoms, [Ge(46-x)P(x)]Se(8-y)□(y) (x = 15.4(1); y = 0-2.65; □ denotes a vacancy), was synthesized as a single-phase and structurally characterized. It crystallizes in the space group Fm3 with the unit cell parameter a varying from 20.310(2) to 20.406(2) Å and corresponding to a 2 × 2 × 2 supercell of a usual clathrate-I structure. The superstructure is formed due to the symmetrical arrangement of the three-bonded framework atoms appearing as a result of the framework transformation of the parent clathrate-I structure. Selenium guest atoms occupy two types of polyhedral cages inside the positively charged framework; all selenium atoms in the larger cages form a single covalent bond with the framework atoms, relating the title compounds to a scanty family of semiclathrates. According to the measurements of electrical resistivity and Seebeck coefficient, [Ge(46-x)P(x)]Se(8-y)□(y) is an n-type semiconductor with E(g) = 0.41 eV for x = 15.4(1) and y = 0; it demonstrates the maximal thermoelectric power factor of 2.3 × 10(-5) W K(-2) m(-1) at 660 K.

Na-Si Clathrates Are High-Pressure Phases: A Melt-Based Route to Control Stoichiometry and Properties

Three different sodium-silicon clathrate compounds–Na8Si46 (sI), Na24Si136 (sII), and a new structure, NaSi6–were obtained for the first time using high-pressure techniques. Experimental and theoretical results unambiguously indicate that Na-intercalated clathrates are only thermodynamically stable under high-pressure conditions. The sI clathrate can be synthesized directly from the elements at pressures from 2 to 6 GPa in the 900–1100 K range. Over the range of conditions studied, sII clathrate only forms as an intermediate compound prior to the crystallization of sI. At higher pressures, we observed the formation of a new intercalated compound, metallic NaSi6, which crystallizes in the orthorhombic Eu4Ga8Ge16 structure. High-pressure crystallization from Na-Si melts provides significant improvements in the electrical properties of bulk clathrate materials (residual resistance ratio RRR = 24 for sI and > 13 for NaSi6), compared to the typical characteristics achieved for single crystals obtained by conventional routes (RRR < 6). Since the Na-Si clathrates are stable only above 2 GPa, previous reports of their synthesis may be viewed as nonequilibrium, precursor-based routes to high-pressure phases at low-pressure conditions.

Crystal structure refinement of Cu6O8YCl by powder neutron diffraction

We report the crystal structure parameters of clathrate-type copper-oxide Cu6O8YCl, which belongs to the Cu6O8MX (M = cation, X = anion) family, investigated by the powder neutron diffraction technique and the Rietveld method. The crystal structure of Cu6O8YCl was identified to be a cubic structure with the space group Fm3̄m. The Cl atom is located at the 4a site, which is the center position of the Cu6O8 cage, and the Y atom occupies the 4b site in the cuboid space. The Y and Cl atoms have anomalously large atomic displacement parameters (Uiso) at room temperature, which suggests that Y and Cl atoms respectively present in a cuboid and in a Cu6O8 cage exhibit intensive motion such as rattling, as recently observed for clathrate compounds.

Hypothetical methane pulse from clathrates: simulation of atmospheric implications and preservation in ice cores

ARCHER, D., 2007. Biogeosciences 4, 521–544. URL www.biogeosciences.net/4/521/2007/ BRASSEUR, G., HITCHMAN, M. H., WALTERS, S., DYMEK, M., FALISE, E., PIRRE, M., 1990. J. Geophys. Res. 95, 5639–5655 BUIZERT, C., MARTINERIE, P., PETRENKO, V., SEVERINGHAUS, J., TRUDINGER, C. et al., 2011. Atmos. Chem. Phys. Discuss. 11, 15975–16021 FISCHER, H., BEHRENS, M., BOCK, M., RICHTER, U., SCHMITT, J., LOULERGUE, L., CHAPPELLAZ, J., SPAHNI, R. et al., 2008. Nature 452, 864–867 MARTINERIE, P., NOURTIER-MAZAURIC, E., BARNOLA, J.-M., STURGES, W. T., WORTON, D. R. et al., 2009. Atmos. Chem. Phys. 9, 3911–3934 MASLIN, M., OWEN, M., DAY, S., LONG, D., 2004. Geology 32, 53–56 SOWERS, T., 2006. Science 311, 838–840 THORPE, R. B., LAW, K. S., BEKKI, S., PYLE, J. A., NISBET, E. G., 1996. J. Geophys. Res. 101, 28627–28635 UCHIDA, M., OHKUSHI, K., KIMOTO, K., INAGAKI, F., ISHIMURA, T., TSUNOGAI, U. et al., 2008. Geochem. Geophys. Geosyst. 9, Q04N14 WITRANT, E., MARTINERIE, P., SEVERINGHAUS, J., HOGAN, C., LAUBE, J. C., KAWAMURA, K. et al., In Prep. Atmos. Chem. Phys. Discuss. North GRIP (31 years) Devon Island (20 years)

ChemInform Abstract: Synthetic Clathrate Systems

AbstractReview: 169 refs.

Enhancement of thermoelectric properties of CoSb3-based skutterudites by double filling of Tl and In

Thermoelectric (TE) generators can directly generate electrical power from waste heat, and thus could be an important part of the solution to future power supply and sustainable energy management. The main obstacle to the widespread use of TE materials in diverse industries, e.g., for exhaust heat recovery in automobiles, is their low efficiency in converting heat to electricity. The conversion efficiency of TE materials is quantified by the dimensionless figure of merit, ZT, and the way to enhance ZT is to decrease the lattice thermal conductivity (κlat) of the material, while maintaining a high electrical conductivity, i.e., to create a situation in which phonons are scattered but electrons are unaffected. Here, we report skutterudites filled by Tl and In, Tl0.1InxCo4Sb12, which allow a dramatic reduction of κlat, yielding a ZT of 1.2 at 700 K. We demonstrate that the reduction of κlat is due to the effective phonon scattering induced both by the rattling of Tl and In and by the naturally formed In2O3 nanoparticles (&amp;lt;50 nm). The combined approach of double filling and self-formed nanostructures might be applicable to various clathrate compounds. Thus, our results point to a new strategy in the improvement of bulk TE materials.

Observation of Rattling Vibrations in Clathrate under High Pressure and Low Temperature

An experimental system for low-frequency Raman measurements at low temperatures and high pressures was constructed in order to research low frequency vibrations of guest atoms in guest-host materials such as semiconductor clathrates. Raman measurements in the range to 10 cm−1 were attained under low temperature and high pressure by arranging a diamond anvil cell fixed on a cryostat in a quasi-back-scattering geometry. Raman spectra of a clathrate compound Eu8Ga16Ge30 were measured using this experimental system. The low frequency Eu vibration so called the rattling vibration located at ∼20 cm−1 was clearly observed under high pressures and low temperatures.

The structural and electronic properties of type-I clathrates Ba8Cu x Ga16−x Sn30 from first-principle calculations

The effects of Cu–doping concentration on the structural and electronic properties of Ba8Cu x Ga16−x Sn30 (0 ≤ x ≤ 3) type-I clathrate compounds have been investigated by first-principle calculations. Starting with the stable structures of the type-I clathrate semiconductors Ba8Ga16Sn30 containing no Ga–Ga bonds, we have constructed unit cells of Ba8Cu x Ga16−x Sn30. The dependence of the structural and electronic properties of these compounds with x is discussed. The results reveal that the interaction between the rattler atom and the cage is affected by Cu doping, which suggests that Cu doping may play a role on the thermal conductivity. It is shown that the Ba8Ga16Sn30 is an intrinsic semiconductor, but Cu doping leads to p-type transport characteristics. Detailed analysis of the electronic structures for Cu-doped clathrates imply that Cu 3d electrons may display both itinerant and localized features and the electronic states near the top of the valence band are modified by the Cu 3d states. The number of states near the Fermi level is increased by the coupling of sp–d, which may lead to reduced stability of the structure.

Equation of State of a Model Methane Clathrate Cage

We investigate the behavior of a model methane clathrate cage under high hydrostatic pressures. The methane clathrate cage consists of 20 water molecules forming 12 pentagonal faces, with a methane molecule positioned at the cage center. The clathrate compound is located inside a fullerene-type arrangement of 180 He atoms to simulate an isotropic pressure. Different pressures are simulated by decreasing the radius of the He array. The minimal energy of the total system for each configuration is calculated by using density functional theory. The variation of the energy with the volume of the imprisoned clathrate cage leads to the proposal of a (cold) equation of state in the pressure range [0,60] GPa. The elastic parameters of the state equation are found in agreement with equivalent quantities measured on clathrates in their sI conformation. Special attention is given to the distribution of the confined atoms and the eventual symmetry lost from the clathrate cage with the pressure, as the clathrate cage constitutes a basic structural unit of the crystal. Finally, the strengths and limitations of the model are discussed.

Quadrupole Ordering and Rattling Motion of Clathrate Compound Pr3Pd20Ge6

We have investigated the quadrupole ordering of 4f-electrons and the rattling motion of guest rare-earth ions in the clathrate compound Pr 3 Pd 20 Ge 6 using ultrasonic measurements. The elastic constants ( C 11 - C 12 )/2 and C 44 exhibit considerable softening at low temperatures. A minimum of ( C 11 - C 12 )/2 at T Q1 = 0.25 K indicates antiferroquadrupole ordering associated with a Γ 3 doublet of a crystal-field ground state at the 8c site, while a minimum of C 44 at T Q2 = 60 mK indicates the ferroquadrupole ordering of a Γ 5 triplet of a crystal-field ground state at the 4a site. The antiferroquadrupole phase depending on magnetic field direction shows a highly anisotropic behavior. The field-induced ordered phase due to the level crossing of the CEF ground state at the 8c site in magnetic fields along the [001] direction has been found. Furthermore, for the elastic constant C 44 and the attenuation coefficient α 44 we found a frequency dependence at two different temperatures of 15 and 25 K, which ...

Optical Conductivity of Rattling Phonons in Type-I Clathrates Ba&lt;sub&gt;8&lt;/sub&gt;Ga&lt;sub&gt;16&lt;/sub&gt;Ge&lt;sub&gt;30&lt;/sub&gt; and Ba&lt;sub&gt;8&lt;/sub&gt;Ga&lt;sub&gt;16&lt;/sub&gt;Sn&lt;sub&gt;30&lt;/sub&gt;

We Review Our Dynamical Property Measurements of Rattling Phonons in Type-I Clathrate Compounds, the Quasi-on-Center Ba8Ga16Ge30 (BGG) and Off-Center Ba8Ga16Sn30 (BGS), Using a Terahertz Time-Domain Spectrometer. The Lowest-Lying Vibrational Modes of Rattling Ba Ions in the Oversized Cage Show Anomalous Temperature Dependence in their Spectra. For BGG, the Temperature Dependence Is Mostly Consistent with a Local Anharmonic Potential Approximation that Predicts Softening towards Low Temperature. On the other Hand, for BGS, a Single Broad Peak of Off-Center Rattling Phonons Splits into Two Subpeaks below 120 K, and, with Further Lowering Temperature, the Spectra Show Anomalous Broadening. While the Splitting Can Be Understood by the Double Well Potential, the Linewidth Broadening Must Be Attributed to some Relevant Interactions with such Excitations as Acoustic Phonons and Doped Carriers.

High temperature thermoelectric properties of the type-I clathrate Ba8AuxSi46−x

The thermoelectric properties of the type-I clathrate Ba8AuxSi46−x (4.10 ≤ x ≤ 6.10) were characterized from 300 to 700 K. Increasing the Au concentration leads to a transition from an n-type (x &amp;lt; 5.43) to a p-type (x ≥ 5.43) electrical conduction. The experimental data are well described by a single-parabolic-band model assuming a single scattering mechanism of the charge carriers in this temperature range. The lattice thermal conductivity, inferred from degeneracy-adjusted Lorenz numbers, is low regardless of the composition. However, the measured values are significantly lower in the p-type samples possibly due to a combination of a higher degree of disorder in the crystal structure at high Au contents and an enhanced phonon-charge carrier coupling. Even though high thermopower values are achieved, the high electrical resistivity remains the main obstacle to push the dimensionless figure of merit ZT (∼0.2 at around 600 K for x = 5.59) beyond the level of the best Si-based clathrate compounds.

Lattice instability and elastic dispersion due to the rattling motion in the type-I clathrate Ba8Ga16Sn30

To investigate the off-center rattling motion and its charge-carrier dependence in type-I clathrate compounds, we carried out ultrasonic measurements on type-I Ba${}_{8}$Ga${}_{16}$Sn${}_{30}$ and a reference compound, K${}_{8}$Ga${}_{8}$Sn${}_{38}$. We found elastic softening of ${C}_{44}$ originating from a lattice instability due to the off-center rattling motion of Ba atom in Ba${}_{8}$Ga${}_{16}$Sn${}_{30}$. Elastic softening below 1 K suggests that the lattice instability remains at very low temperatures. We also found ultrasonic dispersion which has no mode selectivity. No-mode-selective ultrasonic dispersion in Ba${}_{8}$Ga${}_{16}$Sn${}_{30}$ would be caused by a strong electron-phonon coupling. No charge-carrier dependence is observed between $n$-type and $p$-type Ba${}_{8}$Ga${}_{16}$Sn${}_{30}$. The significant softening on the bulk modulus in Ba${}_{8}$Ga${}_{16}$Sn${}_{30}$ contrasts to the continuous hardening in K${}_{8}$Ga${}_{8}$Sn${}_{38}$, indicating the central role of the rattling motion in the softening.

Spectroscopic identification and conversion rate of gaseous guest-loaded hydroquinone clathrates

The formation of hydroquinone clathrates with CO2, CH4, N2, and H2 was investigated using X-ray diffraction, Raman spectroscopy, and solid-state 13C NMR spectroscopy. Of the hydroquinones prepared at 5.0MPa, the CH4- and CO2-loaded hydroquinones show complete conversion to β-form hydroquinone clathrates while the N2-loaded hydroquinone shows only partial conversion; there is no indication of the formation of hydroquinone clathrate with H2. The CO2-loaded hydroquinone is fully converted to the β-form hydroquinone clathrate even at 1.0MPa. Solid-state 13C NMR measurements of the hydroquinone clathrate samples prepared at different pressures reveal that the conversion rate for the formation of clathrate compounds from CH4 and N2 decreases with decreasing pressure.

Electrochemical Lithiation of Silicon Clathrate-II

The clathrate-II compound NaxSi136 (x → 0) was synthesized as a precursor for electrochemical lithiation via sodium extraction from the Zintl-Phase NaSi. The final composition was determined to be Na1.3Si136 via a Rietveld refinement. Electrochemical lithiation of this clathrate-II compound yielded a potential plateau at 300 mV whose capacity is in good agreement with 24 lithium atoms being inserted in the clathrate-II structure. In order to prove this assumption, several clathrate-containing electrodes were prepared and electrochemically lithiated. Afterwards the electrode material was investigated using 7Li magic-angle spinning (MAS)-NMR spectroscopy. Two signals in a 2:1 ratio at extremely high chemical shifts of 412 and 433 ppm, respectively, relative to 1M aqueous lithium chloride solution, were detected and give the indisputable evidence that lithiation of the clathrate-II framework was successful. Hence, this work presents a new method for filling clathrate cages and moreover, the first lithium containing clathrate compound.

High Pressure Synthesis of Silicon Clathrates and Related Compounds

The use of high pressure and high temperature conditions is essential in the preparation of silicon clathrate compounds with electron excess compositions and related Si-rich binary compounds. The preparation conditions and crystal structures of Ba8Si46, Ba24Si100, BaSi6, LaSi5, LaSi10, NaxSi136 (x>24) and related Zintl silicides are discussed. These are the systems rich in superconductors.

Design of Clathrate Compounds that Use Only Weak Intermolecular Attractions

Intermolecular attractive forces that are considerably weaker than hydrogen bonding and coordination complexation may be used in the design of new molecules that function as host molecules in the solid-state. Known literature examples of accidentally discovered hosts (clathrands), which do not involve strong interactions in their crystals, are identified and discussed. Their molecular symmetry and supramolecular interactions are analysed in order to identify structural features that facilitate and promote molecular inclusion. The solid-state properties of a family of designed compounds that embody these principles are then described. Prediction of their inclusion behaviour was 95 % successful and a wide variety of crystal packing arrangements were encountered. This is an inevitable consequence of competition between many different molecular interactions of comparable energy during the crystallisation process. The lowest energy combination of these host–host and host–guest associations generates the observed outcome. One consequence of this behaviour is that detailed prediction of a new clathrate crystal packing arrangement is extremely difficult. However, a second consequence is that crystal structure analysis provides a rich source of information about weak intermolecular forces and new supramolecular synthons that previously had remained hidden.

하이드로퀴논 크러스레이트를 이용한 배가스 분리 특성 연구

배가스로부터 이산화탄소의 분리 특성을 파악하기 위해 유기 화합물인 하이드로퀴논을 이용하여 크러스레이트 화합물을 형성하였다. 형성된 크러스레이트 화합물은 고체 NMR 및 라만 분광법을 이용하여 기체의 포집 거동을 확인하였으며, 기체 분리 효율을 계산하기 위하여 원소 분석기를 통한 정량분석도 함께 수행하였다. 분석 결과 배가스에 포함된 이산화탄소는 질소에 비해 동일한 조건에서 크러스레이트 화합물 내로 더 잘 포집되는 것으로 확인되었다. 또한 다양한 압력에서 형성된 시료들을 분석한 결과, 이러한 이산화탄소의 선택적 포집 특성이 매우 낮은 압력에서도 뚜렷한 것으로 확인되어 추가적인 에너지 소모를 적게 하면서도 배가스로부터 이산화탄소를 대규모로 분리/회수하는 것이 가능할 것이라 판단된다. 본 연구에서 얻어진 결과는 향후 배가스에 대한 분리 응용 기술이나 혼합 가스의 선택적 분리와 같은 분야에서 중요한 정보를 제공할 수 있을 것으로 기대된다. An organic substance, hydroquinone is used to form clathrate compounds in order to identify separation characteristics of carbon dioxide in flue gas. Formed samples were analyzed by means of the solid-state <TEX>$^{13}C$</TEX> nuclear magnetic resonance (NMR) and Raman spectroscopic methods to examine enclthration behaviors of guest species. In addition, elemnetal analysis was also performed in order to evaluate separation efficiency of <TEX>$CO_2$</TEX> in a quantitative way. Based on the experimental results obtained, <TEX>$CO_2$</TEX> molecules are found to be captured into the clathrate compound more readily than <TEX>$N_2$</TEX> molecules. Moreover, because such preferential enclathration is even more significant at low pressure conditions, <TEX>$CO_2$</TEX> separation/recovery from flue gas can be achieved with minimizing additional energy cost for the technique. Experimental results obtained in this study can provide useful information on separation techniques of flue gas or selective separation of gas mixtures in the future.

Applicability of Duncan-Chang Model and its Modified Versions to Methane Hydrate-Bearing Sands

Methane hydrate is ice-like clathrate compound that attracts global attention due to its huge potential as a future energy source. The constitutive law of methane hydrate-bearing sediments remains unknown and becomes a barrier in sustainable exploitation of methane hydrate from marine sediments. The Duncan-Change model is a nonlinear elastic model which was widely accepted by the geotechnical community in approximating the constitutive response of geo-materials. This model and its evolved versions were employed in this study to model the stress-strain response observed in triaxial tests on methane hydrate-bearing sands. Duncan-Chang type models capture well the strain hardening behaviors. However, they fall short of incorporating the dependency of temperature and saturation degree of methane hydrate, which have to be taken into account in future constitutive models of methane hydrate-bearing deposits.

Diversity of gossypol clathrates with pyrazine

Diversity of gossypol clathrates with pyrazine