Intermetallic Clathrates Research Articles

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.

Investigation of Yb substitution in the clathrate phase Eu8Ga16Ge30

Attempts to introduce rare earth elements other than Eu as guest atoms into intermetallic clathrates have remained elusive to date. Besides fundamental interest, their motivation is to create “strongly correlated” clathrates with correlation-enhanced “giant” thermopower values in addition to the much discussed low thermal conductivities, and thus with enhanced thermoelectric figures-of-merit. In Eu clathrates no sizeable hybridization of the 4f electrons with the conduction electrons is observed. Here we present our attempts to incorporate Yb in Eu8Ga16Ge30. Yb leads to strong correlation effects in many intermetallic compounds. Along with the standard melting and annealing procedure an alternative route, namely quick quenching of the melt by melt spinning, was tested. Samples obtained in both ways were investigated by XRD, SEM/EDX, and TEM/EDX techniques. No sizable substitution of Eu by Yb in the clathrate phase was found.

K7B7Si39, a Borosilicide with the Clathrate I Structure

Including boron: K7B7Si39 (see structure; large spheres: K, small spheres: Si, B) is the first clathrate I compound with boron atoms. With 9.952(1) Å, the borosilicide exhibits the smallest lattice parameter among all intermetallic clathrate I compounds. The composition is in agreement with the Zintl rule.

Synthesis of the intermetallic clathrate Na2Ba6Si46 by oxidation of Na2BaSi4 with HCl

A new preparation route to the intermetallic clathrate-I compound Na2Ba6Si46 is introduced, which allows one to make large amounts of product with standard laboratory equipment. The precursor Na2BaSi4 is oxidized with gaseous HCl at 673 K to Na2Ba6Si46, NaCl and BaCl2. Full-profile refinement of the crystal structure from the X-ray powder diffraction data revealed a composition close to Na2Ba6Si46 (Na1.94(1)Ba6.06(1)Si46, space group , a = 10.281(1) Å). Differential scanning calorimetry showed an exothermic effect at 874 K, indicating that Na2Ba6Si46 is metastable. The product was additionally characterized by scanning electron microscopy. The electronic structure of Na2Ba6Si46 was investigated by a first-principles, all-electron full-potential method, predicting metallic conductivity. Na2Ba6Si46 obtained by oxidation with HCl shows Pauli paramagnetism; no bulk superconductivity was found down to 1.8 K in a magnetic field of 20 Oe.

High Thermoelectric Figure of Merit and Nanostructuring in Bulk p‐type Na1−xPbmSbyTem+2

Thermoelectric materials are special types of semiconductors that function as “heat pumps” and as heat-to-electricity converters. Thermoelectric power generation allows for small size, high reliability, and quiet operation. Efficient thermoelectric-based heat-to-electricity converters require higher performance materials than are currently available. Direct conversion of heat to electricity could be achieved with solidstate devices based on thermoelectric materials. These devices could play an important role in future energy production, conversion, management, and utilization. When a temperature gradient is created across a thermoelectric module, a voltage is generated, owing to the Seebeck effect. This voltage can be used to drive an external load. Currently, there is a strong scientific and technological drive to identify new materials with enhanced thermoelectric figures of merit ZT= (sS/k)T (where s is the electrical conductivity, S the thermopower or Seebeck coefficient, k the thermal conductivity, and T the temperature). The numerator sS is called the power factor PF. Several classes of materials are currently under investigation, including complex chalcogenides, doped PbTe and its solid solutions, such as Pb1 xSnxTe, [5,6] superlattice thin films, and quantum-dot superlattices. Also of interest are skutterudites, metal oxides, and intermetallic clathrates. The superlattice thin-film structures of Bi2Te3/Sb2Te3 grown from chemical vapor deposition, and of PbSe0.98Te0.02/PbTe formed by molecular beam epitaxy (MBE) have figures of merit greater than ZT= 2 (at approximately 300 and 550 K, respectively). The MBE-grown thin films PbSe0.98Te0.02/PbTe are n-type materials and contain pyramid-shaped “nanodots” of PbSe of uniform size (approximately 20 nm), which form spontaneously inside a matrix of PbTe. Because energyconversion applications require materials in large quantities, we seek bulk analogues of these systems with similar figures of merit. A recent contribution to these efforts was the discovery of the n-type Ag-based tellurides AgSbTe2/PbTe, which can exhibit high figures of merit (ZT 1.7 at 700 K) when properly doped. To construct a fully functioning optimal thermoelectric device, both nand p-type materials are needed. To date, there is no p-type counterpart to AgSbTe2/ PbTe with similar performance. The highest figure of merit reported for p-type bulk materials (ZT 1.2 at 700 K) is exhibited by the so-called TAGS system (based on Te, Ag, Ge, and Sb: (GeTe)1 x((Ag2Te)1 y(Sb2Te3)y)x). [23] These Ge-containing materials, though more efficient than PbTe, have found limited use, owing to their high cost and to a lowtemperature phase transition. Recently, we described the ptype materials Ag(Pb1 ySny)mSbTe2+m, which show outstanding thermoelectric properties, reaching a maximum figure of merit of ZT 1.45 at 630 K. Herein, we report that the Ag-free system Na1 xPbmSbyTem+2, with appropriate combinations of m, y, and x, achieves record-high ZT values for a p-type bulk thermoelectric material. The effect of the composition on the thermoelectric properties is profound. We show that the high performance of these materials derives mainly from a low thermal conductivity. High-resolution transmission electron microscopy (HRTEM) demonstrates pervasive nanostructuring in Na1 xPbmSbyTem+2, which may be the root cause for the remarkably low thermal conductivity. The Na1 xPbmSbyTem+2 system was selected for study because it should be naturally prone to create Na,Sb-rich clusters in the lattice. The distribution of Na and Sb ions in the Pb sublattice cannot be random, as would be demanded by a solid solution, because Coulombic forces alone tend to drive the system to clustering at the nanoscale, thereby lowering the overall energy. The results described herein are in agreement with long-standing theoretical predictions that nanostructuring in semiconductors would lead to enhanced thermoelectric figures of merit. The Na1 xPbmSbyTem+2 materials could find applications in devices for power generation from a wide variety of hot sources, for example, vehicle exhausts, coal-burning installations, or electric power utilities. Na1 xPbmSbyTem+2 (y 1) samples (see Supporting Information for synthesis details) exhibit p-type conduction from 300 to 700 K. Ingots with the composition Na0.95Pb19SbTe21 (m= 19, x= 0.05, y= 1) exhibit an electrical conductivity of s= 1422 Scm 1 with a positive thermopower of S= 105 mVK 1 at room temperature. This leads to the relatively high power factor of PF= 15.6 mWcm K . The temperature dependence of the electrical conductivity and the thermopower of Na0.95Pb19SbTe21 are shown in Figure 1A. The conductivity decreases with increasing temperature, which is consistent with degenerate semiconductors, and reaches s= 150 Scm 1 at 700 K. However, the thermopower increases rapidly to S= 357.6 mVK 1 at 700 K, yielding a much higher power factor of PF= 19 mWcm K . For samples of composition Na0.95Pb20SbTe22 (m= 20, x= 0.05, y= 1), an electrical conductivity of s= 1541 Scm 1 and a [*] Dr. P. F. P. Poudeu, Prof. M. G. Kanatzidis Department of Chemistry Michigan State University East Lansing, MI 48824 (USA) Fax: (+1)517-353-1793 E-mail: kanatzid@cem.msu.edu

High-pressure crystal chemistry of binary intermetallic compounds

Abstract Effects of high pressure on intermetallic compounds are reviewed with regards to structural stability and phase transitions. Changes of bonding properties and electronic structure are examplified by means of the elemental metals caesium and titanium, the latter forming an internal intermetallic compound at high pressures. After a short systematic overview regarding pressure effects, structural transformations in selected classes of intermetallic compounds like Zintl phases and AlB2-type arrangements precedes sections concerning high-pressure synthesis of Laves phases and intermetallic clathrates.

Relationship between composition and charge carrier concentration inEu8Ga16−xGe30+xclathrates

The charge carrier concentratioin is an important parameter in the search for intermetallic clathrates adequate for thermoelectric applications. This work reveals the influence of the exact composition of ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16\ensuremath{-}x}{\mathrm{Ge}}_{30+x}$ on the charge carrier concentration. The samples with initial composition ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ were processed according to two different heat treatments. Slow cooling of the melt and subsequent annealing produced the $\ensuremath{\beta}$ phase with clathrate I structure (space group $Pm\overline{3}n$). In contrast, samples quenched from 1040 \ifmmode^\circ\else\textdegree\fi{}C and then annealed showed the clathrate-VIII structure ($\ensuremath{\alpha}$ phase, space group $I\overline{4}3m$). Lattice parameter investigations and energy dispersive x-ray spectroscopy analysis show that the $\ensuremath{\beta}$ phase possesses a narrow homogeneity range ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16\ensuremath{-}x}{\mathrm{Ge}}_{30+x}\phantom{\rule{0.3em}{0ex}}(0.49\ensuremath{\leqslant}x\ensuremath{\leqslant}1.01)$. The charge carrier concentration $(n)$ at 2 K, as determined from Hall-effect measurements, correlates with $x$ ($0.15\phantom{\rule{0.3em}{0ex}}{e}^{\ensuremath{-}}$/unit cell $\ensuremath{\leqslant}n\ensuremath{\leqslant}0.50\phantom{\rule{0.3em}{0ex}}{e}^{\ensuremath{-}}$/unit cell). The $\ensuremath{\alpha}$ phase has a homogeneity range with $0.28\ensuremath{\leqslant}x\ensuremath{\leqslant}0.48$ at 650 \ifmmode^\circ\else\textdegree\fi{}C. The ideal Zintl composition $(x=0)$ was not achieved for either modification using the applied preparation conditions. From x-ray absorption spectroscopy and the temperature and magnetic field dependence of the magnetization we find that the electronic configuration of the cations in both phases is $4{f}^{7}({\mathrm{Eu}}^{2+})$. It is believed that the Rudermann-Kittel-Kasuya-Yosida interaction is responsible for the observed ferromagnetic ordering. This agrees well with the observation made in this and another paper [A. Bentien, V. Pacheco, S. Paschen, Yu. Grin, and F. Steglich, Phys. Rev. B 71, 165206 (2005)] on this work that the large difference in ordering temperature between the two phases scales with the effective masses of the charge carriers.