Intermetallic Clathrates Research Articles

Si‐based Clathrates with Partial Substitution by Zn and Ga: K8Zn3.5Si42.5, Rb7.9Zn3.6Si42.4, and Cs8–xGa8–ySi38+y

AbstractThe clathrate Cs8–xGa8–ySi38+y was obtained from a stoichiometric reaction of the elements in an alkali metal halide flux, whereas K8Zn3.5Si42.5 and Rb7.9Zn3.6Si42.4 were synthesized using a combined alkali metal halide/zinc flux. These methods allow to get virtually single‐phase materials with good crystallinity. The compounds were analyzed and characterized by means of powder and single‐crystal X‐ray diffraction methods. Rather unexpectedly, the substitution of Si by Zn in K8Zn3.5Si42.5 and Rb7.9Zn3.6Si42.4 also occurs at the atomic site 24k beside the usually observed 6c site, which has not been found before for intermetallic clathrates with alkali metal guest atoms and substitution of the host atom by group 12 elements. The size effect of the guest atoms Rb and Cs on the alkali metal content of Ga‐Si and Zn‐Si clathrates is established. Furthermore, magnetic and transport measurements (electrical resistivity, Seebeck coefficient and thermal conductivity) are reported.

The Clathrate Ba8–xSi46 Revisited: Preparation Routes, Electrical and Thermal Transport Properties

Abstract The clathrate‐I Ba8–xSi46 was manufactured from BaSi2 and α‐Si by high‐pressure high‐temperature synthesis at 5 GPa and 900 °C and by redox reaction from Ba4Li2Si6. The transport properties were measured on specimens from the high‐pressure high‐temperature synthesis. The composition of the Ba8–xSi46 phase [space group Pm$\bar{3}$n, a = 10.3051(1) Å] corresponds to x = 0.8(2) according to the refined lattice parameter, the value determined from full‐profile refinement of powder X‐ray diffraction data amounts to x = 0.73(1), and the chemical analysis results in x = 1.0(1), yielding the average composition Ba7.2(2)Si46. The electrical transport properties of the phase with x = 0.8 were determined in the temperature range 2–300 K using a polycrystalline bulk specimen. The material is a metallic conductor with ρ(300 K) = 3.3 μΩ m and S(300 K) = –6.8 μV K–1. The thermal conductivity shows a relatively large – for an intermetallic clathrate – value of about 8 W·K–1 m–1.

NMR study of Ba8Cu5Si(x)Ge(41-x) clathrate semiconductors.

We have performed (63)Cu, (65)Cu, and (137)Ba NMR on Ba8Cu5SixGe41-x, a series of intermetallic clathrates known for their potential as thermoelectric materials, in order to investigate the electronic behavior of the samples. The spectra and spin-lattice relaxation times were measured at 77 K and 290 K for the entire composition range 0 ≤ x ≤ 41. Magnetic and quadrupole shifts and relaxation rates of the Cu NMR data were extracted, and thereby carrier-induced metallic contributions identified. The observed shifts change in a nonlinear way with increasing Si substitution: from x = 0 to about 20 the shifts are essentially constant, while approaching x = 41 they increase rapidly. At the same time, Ba NMR data indicate greater Ba-site participation in the conduction band in Ba8Cu5Si41 than in Ba8Cu5Ge41. The results indicate surprisingly little change in electronic features vs. Si content for most of the composition range, while Ba8Cu5Si41 exhibits enhanced hybridization and a more metallic framework than Ba8Cu5Ge41.

Localization of propagative phonons in a perfectly crystalline solid.

Perfectly crystalline solids are excellent heat conductors. Prominent counterexamples are intermetallic clathrates, guest-host systems with a high potential for thermoelectric applications due to their ultralow thermal conductivities. Our combined experimental and theoretical investigation of the lattice dynamics of a particularly simple binary representative, Ba(8)Si(46), identifies the mechanism responsible for the reduction of lattice thermal conductivity intrinsic to the perfect crystal structure. Above a critical wave vector, the purely harmonic guest-host interaction leads to a drastic transfer of spectral weight to the guest atoms, corresponding to a localization of the propagative phonons.

Effective potentials for simulations of the thermal conductivity of type-I semiconductor clathrate systems

Intermetallic clathrates are promising candidates for thermoelectric applications. For theoretical investigations of their lattice thermal conductivity, effective potentials for silicon- and germanium-based structures have been developed. To stabilize the fourfold coordinated cage framework, angular dependent potentials are necessary. The phononic properties obtained from these potentials are in good agreement with ab initio calculated and experimental data. Molecular dynamics simulations using the Green-Kubo method are used to calculate the thermal conductivity. Type-I clathrate structures with different degrees of complexity have been studied. The influence of the rattling modes of the guest atoms inside the cages is identified as one source of the low thermal conductivity. However, the vacancies in the host framework also reduce the lattice thermal conductivity of clathrate systems. The results are compared to experimental data where available.

Crystal growth of intermetallic clathrates: Floating zone process and ultra rapid crystallization

We studied the crystal growth process of type-I transition metal clathrates in two different regimes: a regime of moderate cooling rate, realized with the floating zone technique, and a regime of ultra rapid cooling, realized by the melt spinning technique. In the former regime, bulk Ba8AuxSi46−x and Ba8Cu4.8GaxGe41.2−x single crystals were grown. We investigated segregation effects of the constituting elements by measurements of the composition profiles along the growth direction. The compositional non-uniformity results in a spatial variation of the electrical resistivity which is discussed as well. Structural features of clathrates and their extremely low thermal conductivities imply specifics in growth behavior which manifest themselves most pronouncedly in a rapid crystallization process. Our melt spinning experiments on Ba8Au5Si41 and Ba8Ni3.5Si42.5 (and earlier on some other clathrates) have revealed surprisingly large grains of at least 1μm. Because of the anomalously high growth rate of the clathrate phase the formation of impurity phases is considerably kinetically suppressed. We present our scanning and transmission electron microscopy investigations of melt spun samples and discuss structural, thermodynamic and kinetic aspects of the unusual clathrate nucleation and crystallization.

BaRh2Si9– a new clathrate with a rhodium–silicon framework

The semiconducting compound BaRh2Si9 is a new kind of intermetallic clathrate. It was obtained by reacting BaSi, Rh and Si at 950 °C. The crystal structure (space group C2/c; Pearson symbol mC48, a = 6.2221(5) Å, b = 21.399(2) Å, c = 6.2272(5) Å, β = 90.306(7)°) displays a covalently bonded Rh-Si framework, in which four-connected Si atoms partly show unusually small bond angles. The Ba atoms are encapsulated in large polyhedral cages formed by 18 Si and 4 Rh atoms. The compound is a diamagnetic p-type semiconductor, which is in agreement with band structure calculations resulting in a band gap of 0.12 eV. Quantum chemical calculations reveal positively charged Ba atoms (Ba(+1.3)) and negatively charged Rh atoms (Rh(-1)). Si atoms with neighboring Rh atoms are positively charged, while those connected only with Si atoms are negatively charged.

Thermopower enhancement by encapsulating cerium in clathrate cages

The increasing worldwide energy consumption calls for the design of more efficient energy systems. Thermoelectrics could be used to convert waste heat back to useful electric energy if only more efficient materials were available. The ideal thermoelectric material combines high electrical conductivity and thermopower with low thermal conductivity. In this regard, the intermetallic type-I clathrates show promise with their exceedingly low lattice thermal conductivities [1]. Here we report the successful incorporation of cerium as guest atom into the clathrate crystal structure. In many simpler intermetallic compounds, this rare earth element is known to lead, via the Kondo interaction, to strong correlation phenomena including the ocurrence of giant thermopowers at low temperatures [2]. Indeed, we observe a 50% enhancement of the thermopower compared to a rare earth-free reference material. Importantly, this enhancement occurs at high temperatures and we suggest that a `rattling' enhanced Kondo interaction [3] underlies this effect.

Structural and thermoelectric properties of Ba8Cu5SixGe41−xclathrates

Intermetallic clathrates are known for their high thermoelectric efficiency. However, to realize mass market applications, efficient low-cost representatives have to be found. Here we investigate the candidate material Ba${}_{8}$Cu${}_{5}$Si${}_{x}$Ge${}_{41\ensuremath{-}x}$, $0\ensuremath{\le}x\ensuremath{\le}41$ by x-ray powder diffraction (XPD), energy dispersive x-ray spectroscopy, and electrical and thermal transport property measurements. Polycrystalline samples are prepared by high frequency melting and subsequent ball milling and hot pressing. The type-I clathrate structure is confirmed in all samples by XPD. The structural details are evaluated by the Rietveld method. The results show that when Ge is replaced by Si, linear variations can be observed for the lattice parameter, cages size, atomic displacement parameters of Ba in the large cage, and atomic parameters $x$ of $16i(x,x,x)$ and $z$ of $24k(0,y,z)$. However, the atomic parameters $y$ of $24k(0,y,z)$ and interatomic distance between the atoms at the $24k$ site vary nonlinearly with the Si content ${x}_{\text{Si}}$. Nonlinear behavior also appears for the Si occupation at the $24k$ site. These nonlinear variations result in the electronic band structure changing nonmonotonically with ${x}_{\text{Si}}$, which explains the observed nonmonotonic variations of the transport properties. The lattice thermal conductivity shows ${x}_{\text{Si}}$ independence, which escapes from our anticipation that the larger the tetrakaidecahedra cage, the lower the lattice thermal conductivity in clathrates. The insignificance of resonant scattering for the lattice thermal conductivity might be the reason for the observed independence. The highest dimensionless thermoelectric figure of merit $ZT$ of 0.5 at about $550{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ is achieved for Ba${}_{8}$Cu${}_{5}$Si${}_{3}$Ge${}_{38}$.

Combined X-ray and neutron diffraction study of vacancies and disorder in the dimorphic clathrate Ba8Ga16Sn30 of type I and VIII

We report detailed structural investigations of the dimorphic clathrate Ba8Ga16Sn30 that crystallizes in both type I and VIII clathrate structures. Single crystals of type I and VIII have been examined using single crystal X-ray and Laue neutron diffraction in the temperature range T = 10 K-500 K. The utilization of both X-ray and neutron diffraction gives a unique ability to reveal the occurrence of minute vacancy occupancies in the host structure. The vacancies are shown to be located on the 6c (type I) and 24g (type VIII) framework sites. Largest vacancy densities are observed for type I p-Ba8Ga16Sn30, 1.3(4)%, and type VIII n-Ba8Ga16Sn30, 0.7(2)%. The relation between guest atom disorder and occurrence of glasslike thermal conductivity in intermetallic clathrates was also investigated. In type VIII Ba8Ga16Sn30 neither n-type (crystalline thermal conductivity) nor p-type (glasslike thermal conductivity) showed any significant disorder of the guest atoms; they do however show anharmonic motion. The glasslike thermal conductivity of p-type Ba8Ga16Sn30 is interpretable as a result of higher effective mass of p-type charge-carriers affecting phonon scattering. In type I Ba8Ga16Sn30 guest atoms are highly disordered for both carrier types and samples of both charge carrier types have glasslike thermal conductivity.

Melt Spinning of Clathrates: Electron Microscopy Study and Effect of Composition on Grain Size

The perspective of melt spinning–rapid quenching of a melt to produce nanocrystalline intermetallic clathrates is discussed, using main-group and transition-metal clathrates as examples. While melt spinning was originally developed for production of amorphous materials, our first experiments on melt spinning of clathrates revealed a surprisingly large grain growth rate, resulting in grain sizes of at least 1 μm. However, using the “confusion effect,” i.e., complicating the chemical composition of the material by increasing the number of constituent elements, we have succeeded in reducing the grain size to 200 nm. We present our scanning electron microscopy, transmission electron microscopy, and transport property investigations and discuss the effect of composition on grain size, as well as thermodynamic and kinetic aspects of clathrate crystallization.

Study of α ↔ β transformation in the dimorphic clathrate Ba8Ga16Sn30

The intermetallic clathrate Ba8Ga16Sn30 (BGS) is dimorphic, possessing α-phase (type-VIII) and β-phase (type-I). To determine the stability ranges of the two phases, we performed a study of the phase diagram by combining differential thermal analysis (DTA) and power X-ray diffraction (XRD) on both samples of bulk single crystals (≅1 mm) and powdered crystals (<50 µm). It was found that bulk samples of both α- and β-phases melt congruently at 500°C, but a powder specimen of β-phase has a broad endothermic DTA peak at 466°C, just below the melting point. Intensive studies on this event revealed a partially reversible and strongly kinetically controlled β → α structural phase transition. This transition has been confirmed by in situ synchrotron XRD at SPring-8. After heating the powder specimens to 530°C and cooling to room temperature, the main phase remained intact in both modifications. When the soaking temperature was increased from 530 to 910°C, a weak exothermic peak for nucleation of Ba(Ga/Sn)4 appeared at about 570°C in the cooling process. The main phase of the resulting materials was found to be β-phase, irrespective of the starting phases and grain sizes. Based on the results of DTA and XRD, we constructed a tentative Sn–Ba(Ga/Sn)4 pseudo-binary phase diagram. It suggests that large single crystals of β-BGS can be grown from the Sn flux without being disturbed by the α ↔ β structural phase transition.

Thermoelectric properties of type-VIII clathrate Ba8Ga16Sn30 doped with Cu

The intermetallic clathrate Ba8Ga16Sn30 with type-VIII structure is one of the promising thermoelectric materials. We report on significant effects of Cu doping on both the crystal growth and thermoelectric properties of type-VIII clathrate Ba8Ga16Sn30. By using the Ga flux and Sn flux, respectively, the carrier type could be tuned as p- and n-types. The p-type single crystals were grown to approximately 6mm in diameter, which is twice as large as that grown without Cu. The crystals were characterized by electron-probe microanalysis and powder X-ray diffraction and used for the measurements of the electrical resistivity, Seebeck coefficient, and thermal conductivity. The values of the dimensionless figure of merit ZT of the p- and n-type crystals reached 0.88 and 1.45 at around 500K, respectively, the latter of which is the highest among all substituted systems of type-VIII Ba8Ga16Sn30. This enhancement of ZT is discussed in relation with the charge balance and modification of the band structure caused by Cu doping.

Structural and Thermoelectric Properties of Ba8Cu x Si23-x Ge23 (4.5 ≤ x ≤ 7)

Intermetallic clathrates are promising materials for thermoelectric applications at elevated temperatures. In the search for cost-competitive representatives, we investigate the thermoelectric properties of quaternary type I clathrates with nominal compositions Ba8Cu x Si23-x Ge23 (4.5 ≤ x ≤ 7). The specimens were prepared by melting and annealing with subsequent ball milling and hot pressing. X-ray diffraction and energy-dispersive x-ray spectroscopy were used for an accurate structure determination and for an analysis of the phase purity and microstructure, respectively. The transport properties show a systematic variation with the Cu content in the clathrate phase. The highest dimensionless thermoelectric figure of merit ZT of 0.31 is achieved for Ba8Cu6Si17Ge23 at about 250°C (520 K).

Off-center rattling and thermoelectric properties of type-II clathrate (K, Ba)24(Ga, Sn,□)136single crystals

We report the synthesis and temperature-dependent structural, transport, and thermal properties of type-II clathrate K${}_{8+x}$Ba${}_{16\ensuremath{-}x}$Ga${}_{40\ensuremath{-}y}$Sn${}_{96\ensuremath{-}z}$${\ensuremath{\square}}_{y+z}$ (1.2 $\ensuremath{\le}$ $x$ $\ensuremath{\le}$ 2.8, 2.0 $\ensuremath{\le}$ $y$ $\ensuremath{\le}$ 3.3, 0.8 $\ensuremath{\le}$ $z$ $\ensuremath{\le}$ 6.6, $\ensuremath{\square}$ $=$ framework vacancy). Single-crystal x-ray diffraction analysis reveals that the guest K${}^{+}$ and Ba${}^{2+}$ ions are preferentially incorporated into the hexakaidecahedral cages and dodecahedral cages, respectively. The guest site in the former splits into four sites 0.67 $\AA{}$ away from the center to the 32$e$ site of the cage. The splitting is consistent with the presence of four minima in the electrostatic potential in the hexakaidecahedron. The thermopower is negative and relatively large, $\ensuremath{-}50\ensuremath{\sim}\ensuremath{-}120$ $\ensuremath{\mu}$V$/$K at 300 K, indicating that the dominant charge carriers are electrons. The thermal conductivity displays a glasslike behavior with a plateau at around 20 K. The analysis of the specific heat indicates that the motion of the K${}^{+}$ ion in the hexakaidecahedron can be described by the soft-potential model, including the tunneling term. The characteristic energy of 21 K for the soft mode is as low as that of the off-center rattling of the Ba${}^{2+}$ ion in type-I clathrate Ba${}_{8}$Ga${}_{16}$Sn${}_{30}$. The present result on the type-II clathrate verifies the idea that the low-energy off-center rattling in oversized cages of intermetallic clathrates couples to the acoustic phonons to lead to the glasslike thermal conductivity.

Simple Approach for Selective Crystal Growth of Intermetallic Clathrates

We report on a new method for the synthesis of single-crystal inter-metallic clathrates. Alkali-metal is slowly removed from an alkali-silicide precursor by reaction of the vapor phase with spatial...

A study of low-energy guest phonon modes in clathrate-IINaxSi136 (x = 3, 23, and 24)

Single-crystal x-ray diffraction from clathrate-IINaxSi136 (x = 24) prepared by a new technique reveals the exceptionally largeNa@Si28 atomic displacementparameter (Ueq) is strongly temperature dependent, and can be attributed to low-energy rattling modesassociated with the Na guest. Inelastic neutron scattering (INS) spectra collected fromNaxSi136 powderspecimens (x = 3, 23) confirm the presence of low-energy guest-derived phonon modes forNa@Si28 andNa@Si20. The lowerenergy Na@Si28 rattler mode falls in the frequency range of the silicon host acoustic phonons, indicating thepossibility for interaction with these phonons. The presence of these low-energy modescombined with the ability to controllably vary the guest content presents a uniqueopportunity for exploring the influence of guest-framework interactions on the latticedynamics in intermetallic clathrates.

BaGe5: A New Type of Intermetallic Clathrate

BaGe(5) constitutes a new type of intermetallic clathrate obtained by decomposition of clathrate-I Ba(8)Ge(43)(3) at low temperatures. The crystal structure consists of characteristic layers interconnected by covalent bonds. BaGe(5) is a semiconducting Zintl phase.

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 ${\ensuremath{\kappa}}_{\text{L}}(T)$.

Intrinsic Electrical and Thermal Properties from Single Crystals ofNa24Si136

The observation of intrinsic structural, electrical, and thermal properties from measurements on single-crystal specimens of clathrate-II Na24Si136 is reported, revealing metallic conduction in agreement with electronic structure calculations. Low-temperature heat capacity measurements corroborate a substantial electronic density of states at the Fermi level, and reveal an Einstein-like mode that can be attributed to Na guest "rattling". The large thermal conductivity of Na24Si136, compared to literature data for other intermetallic clathrates, can be understood in terms of the predominant electronic contribution for the fully filled Na24Si136 composition.