type-I Clathrate Structure Research Articles

High-temperature superconductivity in La4H23 below 100 GPa

High-temperature superconductivity has been observed in binary hydrides such as LaH10 at pressures close to 180GPa, whereby hydrogen cagelike structures have been identified as a common motif beneficial for high critical temperatures Tc. Efforts are now focused on finding hydride high-temperature superconductors at lower pressures. We present evidence for high-temperature superconductivity in binary La4H23 at a pressure of P=95GPa, with A15-type arrangement of the lanthanum atoms in the type-I clathrate structure. We synthesize La4H23 from a lanthanum film capped with a palladium catalyst promoting the dissociation of hydrogen. In resistance measurements, we observe superconductivity with a transition temperature Tc∼90K. The A15-type lanthanum sublattice is identified in x-ray diffraction on the same sample, in agreement with previous x-ray diffraction and structure prediction studies. Our study reinforces the favorability of cagelike networks of the hydrogen lattice for high-temperature superconductivity. Published by the American Physical Society 2024

Lithium metal atoms fill vacancies in the germanium network of a type-I clathrate: synthesis and structural characterization of Ba8Li5Ge41.

Clathrate phases with crystal structures exhibiting complex disorder have been the subject of many prior studies. Here we report syntheses, crystal and electronic structure, and chemical bonding analysis of a Li-substituted Ge-based clathrate phase with the refined chemical formula Ba8Li5.0(1)Ge41.0, which is a rare example of ternary clathrate-I where alkali metal atoms substitute framework Ge atoms. Two different synthesis methods to grow single crystals of the new clathrate phase are presented, in addition to the classical approach towards polycrystalline materials by combining pure elements in desired stoichiometric ratios. Structure elucidations for samples from different batches were carried out by single-crystal and powder X-ray diffraction methods. The ternary Ba8Li5.0(1)Ge41.0 phase crystallizes in the cubic type-I clathrate structure (space group Pm3̄n no. 223, a ≈ 10.80 Å), with the unit cell being substantially larger compared to the binary phase Ba8Ge43 (Ba8□3Ge43, a ≈ 10.63 Å). The expansion of the unit cell is the result of the Li atoms filling vacancies and substituting atoms in the Ge framework, with Li and Ge co-occupying one crystallographic (6c) site. As such, the Li atoms are situated in four-fold coordination environment surrounded by equidistant Ge atoms. Analysis of chemical bonding applying the electron density/electron localizability approach reveals ionic interaction of barium with the Li-Ge framework, while the lithium-germanium bonds are strongly polar covalent.

Impact of temperature and mode polarization on the acoustic phonon range in complex crystalline phases: A case study on intermetallic clathrates

The authors use inelastic neutron scattering to show that the lattice anharmonicity in a single crystal of a complex type-I clathrate structure depends on transverse versus longitudinal phonon polarizations.

Anisotropic low-energy vibrational modes as an effect of cage geometry in the binary barium silicon clathrate Ba24Si100

The low lattice thermal conductivity in inorganic clathrates has been shown recently to be related to the low-energy range of optical phonons dominated by motions of guest atoms trapped in a network of host covalent cages. A promising route to further reduce the heat conduction, and increase the material efficiency for thermoelectric heat waste conversion, is then to lower the energy of these guest-weighted optical phonons. In the present work, the effect of the host cage geometry is explored. The lattice dynamics of the binary type-IX clathrate, Ba24Si100, has been investigated experimentally by means of inelastic neutron scattering as a function of temperature between 5 K and 280 K, and computed by ab-initio density functional techniques. It is compared with the lattice dynamics of Ba8Si46, the simplest representative of the well-known type-I clathrate structure. The binary Ba8Si46 and Ba24Si100 materials have both a cubic unit cell made of different Si cages. The energies, the degree of anharmonicity as well as the anisotropy of the optical phonon modes weighted by Ba motions are found to depend strongly on 2 the size and shape of the cages. The lowest optical phonon energies in Ba24Si100 are found around 2.5-4 meV, while those in Ba8Si46 have higher energies around 7-9 meV. The low-lying optical phonons in Ba24Si100 are mainly weighted by the motion of Ba in the opened Si20 cage, which doesn't exist in Ba8Si46. Moreover, the Ba vibrations within the opened Si20 cages are found intrinsically anisotropic, strongly dispersionless in some directions and exhibit a significant anharmonicity, which is not observed for any optical phonon modes in Ba8Si46.

High-Temperature Thermoelectric Properties of Polycrystalline Silicon Clathrate Ba8TM x Si46−x (TM = Ni, Pt)

The n-/p-type stability of a silicon clathrate in which silicon was substituted with nickel or platinum was evaluated by density functional theory calculations. Then, Ba8Pt5Si41 and Ba8Pt1.5Ni3.5Si41 were synthesized, and their thermoelectric properties were investigated. The polycrystalline compounds, which have a type-I clathrate structure, were prepared through arc melting and spark-plasma-sintering. The crystal structures and elemental compositions of the synthesized samples were characterized via powder x-ray diffraction and electron microprobe analyses, respectively. The temperature dependence of both the electrical resistivity and the Seebeck coefficient was measured.

Simultaneous Pressure-Induced Magnetic and Valence Transitions in Type-I Clathrate Eu8Ga16Ge30

We have performed X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectroscopy (XAS) measurements at pressures up to 17 GPa for the clathrate Eu8Ga16Ge30 (Curie temperature TC = 36 K). The temperature dependence of the XMCD spectra agrees well with that of the DC magnetization at ambient pressure. The TC is gradually enhanced with increasing pressures up to 13.3 GPa, and the divalent state of the Eu ions with J = 7/2 remains stable, but at 17 GPa the XMCD intensity is strongly suppressed and a spectral weight corresponding to the trivalent state of Eu ions (with no magnetic moment) appears in the XAS spectrum. The concurrent change from the type-I clathrate structure to an amorphous phase has been observed by X-ray diffraction experiment. We conclude that the amorphization of this compound induces the mixed valence state, which collapses the ferromagnetism.

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${}_{38}$.

Influence of Sn on the structural and thermoelectric properties of the type-I clathrates Ba8Cu5Si6Ge35-xSnx (0 ≤ x ≤ 0.6)

ABSTRACTOn the search for cost-competitive thermoelectric clathrates we have investigated the influence of Sn substitutions for Ge on the structural and thermoelectric properties of the type-I clathrate Ba8Cu5Si6Ge35. The solid solubility of Sn was found to be limited to 0.6 atoms per unit cell. A series of compounds with the nominal compositions Ba8Cu5Si6Ge35-xSnx (x = 0.2, 0.4, 0.6) was synthesized in a high-frequency furnace. The samples were annealed, and subsequently ball milled and hot pressed. The hot pressed samples were characterized by X-ray powder diffraction, energy-dispersive X-ray spectroscopy and transport property measurements. Our results show that the substitution of Ge by Sn introduces vacancies at the 6d site of the type-I clathrate structure and shifts the highest dimensionless thermoelectric figure of merit ZT from 570 °C for the Sn free sample to lower temperatures. The highest figure of merit ZT = 0.42 is reached at about 320 °C for the Sn-substituted sample Ba8Cu5Si6Ge35Sn0.6.

Crystallographic, thermoelectric, and mechanical properties of polycrystalline type-I Ba8Al16Si30-based clathrates

Crystallographic, thermoelectric, and mechanical properties of polycrystalline Ba8Al16Si30-based samples with type-I clathrate structure prepared by combining arc melting and spark plasma sintering methods were investigated. The major phase of the samples was a type-I clathrate with an actual Al/Si ratio of ~15/31, strongly suggesting that framework deficiency was absent or was present in very low concentration in the samples. The Hall carrier concentration n of the samples was approximately 1 × 1021 cm−3, which is lower than the values reported so far for the Ba8Al16Si30 system. Other important material parameters of the samples were as follows: the density-of-states effective mass m* = 2.3m0, Hall mobility μ = 7.4 cm2 V−1 s−1, and the lattice thermal conductivity κL = 1.2 W m−1 K−1. The thermoelectric figure of merit ZT reached approximately 0.4 (900 K) for a sample with n = 9.7 × 1020 cm−3. Simulation using the experimentally determined values of material parameters showed that ZT reached values >0.5 if the carrier concentration is optimized at about 3 × 1020 cm−3. Young’s, shear, and bulk moduli were estimated to be approximately 98, 39, and 117 GPa, respectively, and Poisson’s ratio was found to be 0.25 from the longitudinal and transverse velocities of sound, vL = 6038 m/s and vT = 3503 m/s, respectively, for a sample with ZT = 0.4. The coefficient of thermal expansion (CTE) ranged from approximately 8 × 10−6 K−1 to 10 × 10−6 K−1 (330–690 K), which is smaller than the values reported for Ba8Ga16Ge30 and Sr8Ga16Ge30 clathrates.

Preparation and thermoelectric properties of sintered n-type K8M8Sn38 (M = Al, Ga and In) with the type-I clathrate structure

The maximum dimensionless figures-of-merit ZTmax were 0.13 at 410 K for K8Al8Sn38, 0.25 at 440 K for K8Ga8Sn38 and 0.21 at 420 K for K8In8Sn38, where their room temperature (RT) mobilities were 4.1 cm2 V−1 s−1, 31.0 cm2 V−1 s−1 and 9.8 cm2 V−1 s−1, respectively, their RT effective masses were 4.3, 1.8 and 3.2 me, respectively, and their RT lattice thermal conductivities were 12 mW cm−1 K−1, 14 mW cm−1 K−1 and 13 mW cm−1 K−1, respectively. The analysis of the measured transport properties suggested that these samples suffered from grain boundary scattering at lower temperatures and from alloy disorder scattering and acoustic phonon scattering at higher temperatures. In particular, such a low mobility of the K8Al8Sn38 sample was considered to be caused by strong alloy disorder scattering. Band structure calculations demonstrated that, in order of K8Ga8Sn38, K8In8Sn38 and K8Al8Sn38, their conduction band edges were sharper and their band structures near the gaps were more similar to that of K8Sn46. These features were well consistent with their measured effective masses and mobilities.

Crystal structure and thermoelectric properties of KxBa8−xZnyGe46−y clathrates

Polycrystalline samples of degenerate n-type KxBa8−xZnyGe46−y (y∼8-x/2) with the type-I clathrate structure (No. 223, Pm3¯n) were prepared by powder metallurgy to obtain a high-efficiency Ge-based clathrate. Their Zn atoms preferred to exist at the 6c site in the framework, and consequently, the samples with x around 4, such as K4Ba4Zn6Ge40, possessed highly ordered Zn/Ge atom frameworks whose 6c, 16i, and 24k sites were occupied almost solely by Zn, Ge, and Ge atoms, respectively. In spite of such ordered structures and small numbers of substituting Zn atoms, these samples exhibited carrier mobilities lower than those of Ba8Zn8Ge38 and Ba8Ga16Ge30. Band structure calculations implied that the combination of the rattler K and Ba atoms in the cages considerably modified the conduction band edge of the corresponding clathrates; such a modification is considered to strengthen alloy disorder scattering, which reduces carrier mobility. The maximum dimensionless figure-of-merit ZT was 0.51 at 1000 K for the K2Ba6Zn7Ge39 sample, which is similar to that of 0.50 at 900 K for the Ba8Zn8Ge38 sample.

Gallium composition dependence of crystallographic and thermoelectric properties in polycrystalline type-I Ba8GaxSi46−x (nominal x=14–18) clathrates prepared by combining arc melting and spark plasma sintering methods

The gallium composition dependence of crystallographic and thermoelectric properties in polycrystalline n-type Ba8GaxSi46−x (nominal x=14–18) compounds with the type-I clathrate structure is presented. Samples were prepared by combining arc melting and spark plasma sintering methods. Powder x-ray diffraction, Rietveld analysis, scanning electron microscopy, and energy-dispersive x-ray spectroscopy show that the solubility limit of gallium in the type-I clathrate phase is close to x=15, which is slightly higher than that for a single crystal. The carrier concentration at room temperature decreases from 2×1021cm−3 to 4×1020cm−3 as the Ga content x increases. The Seebeck coefficient, the electrical conductivity, and the thermal conductivity vary systematically with the carrier concentration when the Ga content x varies. The effective mass (2.0m0), the carrier mobility (10cm2V−1s−1), and the lattice thermal conductivity (1.1Wm−1K−1) are determined for the Ga content x=14.51. The dimensionless thermoelectric figure of merit ZT is about 0.55 at 900K for the Ga content x=14.51. The calculation of ZT using the experimentally determined material parameters predicts ZT=0.8 (900K) at the optimum carrier concentration of about 2×1020cm−3.

Phase Range of the Type-I Clathrate Sr8AlxSi46–x and Crystal Structure of Sr8Al10Si36

Samples of the type-I clathrate Sr(8)Al(x)Si(46-x) have been prepared by direct reaction of the elements. The type-I clathrate structure (cubic space group Pm3n) which has an Al-Si framework with Sr(2+) guest atoms forms with a narrow composition range of 9.54(6) ≤ x ≤ 10.30(8). Single crystals with composition A(8)Al(10)Si(36) (A = Sr, Ba) have been synthesized. Differential scanning calorimetry (DSC) measurements provide evidence for a peritectic reaction and melting point at ∼1268 and ∼1421 K for Sr(8)Al(10)Si(36) and Ba(8)Al(10)Si(36), respectively. Comparison of the structures reveals a strong correlation between the 24k-24k framework sites distances and the size of the guest cation. Electronic structure calculation and bonding analysis were carried out for the ordered models with the compositions A(8)Al(6)Si(40) (6c site occupied completely by Al) and A(8)Al(16)Si(30) (16i site occupied completely with Al). Analysis of the distribution of the electron localizability indicator (ELI) confirms that the Si-Si bonds are covalent, the Al-Si bonds are polar covalent, and the guest and the framework bonds are ionic in nature. The Sr(8)Al(6)Si(40) phase has a very small band gap that is closed upon additional Al, as observed in Sr(8)Al(16)Si(30). An explanation for the absence of a semiconducting "Sr(8)Al(16)Si(30)" phase is suggested in light of these findings.

Crystal structure and thermoelectric properties of clathrate, Ba8Ni3.5Si42.0: Small cage volume and large disorder of the guest atom

Samples with the type-I clathrate composition Ba8NixSi46−x have been synthesized and their structure and thermoelectric properties characterized. Microprobe analysis indicates the Ni incorporation to be 2.62≤x≤3.53. The x=3.5 phase crystallizes in the type-I clathrate structure (space group: Pm-3n) with a lattice parameter of 10.2813(3)Å. The refined composition was Ba8Ni3.5Si42.0, with small vacancies, 0.4 and 0.5 atoms per formula unit, at the 2a and 6c sites, respectively. The position of the Ba2 atom in the large cage was modeled using a 4-fold split position (24j site), displaced 0.18Å from the cage center (6d site). The volume of the large cage is calculated to be 146Å3, smaller than other clathrates with similar cation displacement. The sample shows n-type behavior with a maximum of −50μV/K at 823K above which the Seebeck coefficient decreases, suggesting mixed carriers. Lattice thermal conductivity, κl, is 55mW/K above 600K.

Low-temperature structure and lattice dynamics of the thermoelectric clathrate Sn24P19.3I8

The crystal structure of the thermoelectric clathrate Sn24P19.3I8 was determined down to 10K showing no variations with the temperature. Even at 10K Sn24P19.3I8 crystallizes in the type-I clathrate structure, space group Pm3-n, with the cubic unit cell parameter ranging from 10.9173Å at 10K to 10.9554Å at room temperature. In its crystal structure, tin and phosphorus atoms form the framework that traps the guest iodine atoms in the polyhedral cavities of two different shapes. The temperature-dependent crystal structure data and the results of the heat capacity measurements revealed the localized vibrations of the guest atoms inside the oversized cavities with the characteristic Einstein temperatures of θE1=60K and θE2=78K, whereas the characteristic Debye temperature for Sn24P19.3I8 is 265K. The room temperature lattice thermal conductivity was calculated using the Debye model to be κL=0.85Wm−1K−1, which is in excellent agreement with the experimentally measured data and demonstrates that the thermal conductivity is almost entirely phononic.

Synthesis and clathrate-type crystal structure of a solid solution in the Sn-In-P-Br system

The solid solution of the general formula Sn24−x In x P19.2+0.2x Br8 (x ≤ 10) was synthesized in the Sn-In-P-Br system, and its crystal structure was determined for the composition with x = 7.8. This solid solution crystallizes in the cubic system, the space group Pm \(\bar 3\) n, and has a type-I clathrate structure, in which the clathrate framework is composed of tin, indium, and phosphorus atoms, whereas bromine atoms occupy two types of guest positions. The unit cell parameter a increases with increasing indium content, two linear regions with different increments being observed in the a(x) plot. The crystal structure of the solid solution is characterized by a complicated disorder manifested in the partial occupancy of three closely spaced positions by tin and indium atoms. The application of the Zintl formalism to the rationalization of the composition of the solid solution is discussed, and its structure is compared with the iodide analogs of similar composition but showing differences in structural details.

Preparation and thermoelectric properties of sintered type-I clathrates K8GaxSn46−x

Polycrystalline clathrate samples of nominal K(8)Ga(x)Sn(46-x) were prepared by the spark plasma sintering method to investigate their crystal structures, mobilities and thermoelectric properties. The samples almost had a single-phase type-I clathrate structure, and their relative densities reached as high as 98%. The room-temperature mobility of the K(8)Ga(8)Sn(38) sample was 25 cm(2) V(-1) s(-1), which substantially exceeded the reported mobilities of Rb- and Cs-containing Sn clathrates. Moreover, the mobility was larger than those of the type-I Ba(8)Ga(16)Sn(30), which had more Ga substituting atoms in unit cell. A higher mobility might accordingly be achieved in thermoelectric clathrates with a smaller number of substituting atoms. Their electrical conductivities and Seebeck coefficients in the temperature range 300-450 K were typical of n-type doped semiconductors in the extrinsic region while their room-temperature lattice thermal conductivities were as low as approximately 11 mW cm(-1) K(-1). The maximum dimensionless figure of merit ZT was estimated to be 0.27 at 490 K from the Seebeck coefficient of -262 microV K(-1) and the electrical conductivity of 96 S cm(-1) for the K(8)Ga(8)Sn(38) sample with a carrier concentration of 2.9 x 10(19) cm(-3).

Electron spin resonance (ESR) of Eu2+ in type-I clathrate Eu8Ga16Ge30

Abstract We report temperature dependent X-band ( ν ~ 9.5 GHz ) electron spin resonance (ESR) experiments in type-I clathrate Eu 8 Ga 16 Ge 30 . This system presents interesting thermoelectric properties in its paramagnetic state and orders ferromagnetically below T C = 35 K . Although the Eu 2 + ions sits on two inequivalent sites in the type-I clathrate structure, we found in the paramagnetic state a single Dysonian Eu 2 + ESR line with a nearly temperature independent g ~ 2 is observed, and its linewidth follows a Korringa-like behavior as a function of temperature. The microscopic details of the exchange coupling between the Eu 2 + local moments and the conduction-electrons (c-e) in this compound are investigated through the obtained Korringa rate ( Δ H / Δ T ~ 1 ( 1 ) Oe / K ) and g-shift ( Δ g ~ 0.01 ( 1 ) ) from ESR experiments combined with the magnetic susceptibility and specific heat data.

Synthesis and thermoelectric properties of type-VIII germanium clathrates Sr8AlxGayGe46−x−y

Nominal Sr8AlxGa16−xGe30 samples with x=6, 8, and 10 crystallized in the type-VIII clathrate structure (I4¯3m, No. 217), while the sample with x=4 crystallized in the type-I clathrate structure (Pm3¯n, No. 223). While a large number of the type-I thermoelectric clathrates exist, only three type-VIII clathrates of Ba8Ga16Sn30, Eu8Ga16Ge30, and Sr8AlxGa16−xSi30 had been synthesized before. The type-VIII Sr8AlxGayGe46−x−y samples (6≤x≤7 and 10≤y≤11) with various carrier concentrations were prepared to investigate their thermoelectric properties. They exhibited the temperature dependences of electrical conductivities and the Seebeck coefficients typical of n type degenerate semiconductors, which almost depended on their carrier concentrations systematically. A relatively large dimensionless figure-of-merit ZT of 0.56 at 800 K was obtained for the type-VIII Sr8Al6.3Ga10.3Ge29.4 sample with a carrier concentration of 3.0×1020 cm−3. This ZT value is comparable to that of 0.62 at 800 K for the type-I Sr8Ga16.5Ge29.5 clathrate. The type-VIII clathrate had a smaller effective mass, a higher mobility, and a higher lattice thermal conductivity than those of the type-I clathrate. The difference in transport properties between the type-I and type-VIII clathrates is also discussed.

Ba 8 Ga 16 Sn 30 with type-I clathrate structure: Drastic suppression of heat conduction

For the past decade, intermetallic clathrates have been rattling their way into mainstream research in thermoelectrics. The unusual vibrations of their guest ions inside oversized cages interfere with the cage phonons while leaving the electronic flow intact, providing an exotic way to achieve a best-of-both-worlds scenario in terms of electrical and thermal transport in the same material. Here, we present the structural and thermoelectric properties of Ba8Ga16Sn30 single crystals grown in the type-I clathrate structure (β-BGS), showing one of the lowest recorded thermal conductivities κ(T) for any bulk compound, while still behaving electronically as a heavily doped n-type semiconducting crystal.