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Clathrates have been reported to form in a variety of different structure types; however, inorganic clathrate-I materials with a low-cation concentration have yet to be investigated. Furthermore, tin-based compositions have been much less investigated as compared to silicon or germanium analogs. We report the temperature-dependent structural and thermal properties of single-crystal Eu2Ga11Sn35 revealing the effect of structure and composition on the thermal properties of this low-cation clathrate-I material. Specifically, low-temperature heat capacity, thermal conductivity, and synchrotron single-crystal x-ray diffraction reveal a departure from Debye-like behavior, a glass-like phonon mean-free path for this crystalline material, and a relatively large Grüneisen parameter due to the dominance of low-frequency Einstein modes. Our analyses indicate thermal properties that are a direct result of the structure and composition of this clathrate-I material. [ABSTRACT FROM AUTHOR]

Temperature-dependent transport properties of the recently discovered layered bismuth-rich tellurobromides BinTeBr (n = 2, 3) are investigated for the first time. Dense homogeneous polycrystalline specimens prepared for different electrical and thermal measurements were synthesized by a ball milling-based process. While the calculated electronic structure classifies Bi2TeBr as a semimetal with a small electron pocket, its transport properties demonstrate a semiconductorlike behavior. Additional bismuth bilayers in the Bi3TeBr crystal structure strengthens the interlayer chemical bonding thus leading to metallic conduction. The thermal conductivity of the semiconducting compositions is low, and the electrical properties are sensitive to doping with a factor of four reduction in resistivity observed at room temperature for only 3% Pb doping. Investigation of the thermoelectric properties suggests that optimization for thermoelectrics may depend on particular elemental substitution. The results presented are intended to expand on the research into tellurohalides in order to further advance the fundamental investigation of these materials, as well as investigate their potential for thermoelectric applications. [ABSTRACT FROM AUTHOR]

Quaternary chalcogenides composed of earth-abundant and primarily nontoxic constituents are currently being explored for thermoelectric applications. The representatives of this class, CuZn2InTe4 and AgZn2InTe4, have been synthesized, and here, we present a comparative study of their structure–property relations using first principles simulations. Our calculations show that the lattice structure for both materials is very similar in terms of characteristic atomic distances and lattice structures, which compare well with experimental data. The electronic structure results indicate that both materials are direct gap semiconductors whose electron localization and charge transfer properties reveal polar covalent bonding in the lattice. The calculated phonon structure shows dynamic stability with unique vibrational properties for each material. [ABSTRACT FROM AUTHOR]

Expanding new avenues for renewable energy and reducing the consumption of fossil fuels in a time of ever-increasing demand for energy is challenging. Thermoelectricity, associated with generating electric power due to an applied temperature gradient or thermal flow due to an applied voltage, offers the potential to utilize some of this "free" energy in all-solid-state conversion devices for power generation. [Extracted from the article]

Low temperature resistivity measurements on dense polycrystalline quaternary chalcogenides Ag2+xZn1-xSnSe4, with x = 0, 0.1, and 0.3, indicate polaronic type transport which we analyze employing a two-component Holstein model based on itinerant and localized polaron contributions. Electronic structure property calculations via density functional theory simulations on Ag2ZnSnSe4 for both energetically similar kesterite and stannite structure types were also performed in order to compare our results to those of the compositionally similar but well known Cu2ZnSnSe4. This theoretical comparison is crucial in understanding the bonding that results in polaronic type transport for Ag2ZnSnSe4, as well as the structural and electronic properties of both crystal structure types. In addition to possessing this unique electronic transport, the thermal conductivity of Ag2ZnSnSe4 is low and decreases with increasing silver content. This work reveals unique structure-property relationships in materials that continue to be of interest for thermoelectric and photovoltaic applications. [ABSTRACT FROM AUTHOR]

Quaternary chalcogenides form in different structure types and compositions and are of scientific interest, while their diversity of physical properties exemplifies why they continue to be investigated for potential technological applications. We investigate the thermal properties of BaCu2SnQ4 with trigonal (Q = S) and orthorhombic (Q = Se) crystal structures. BaCu2SnS2Se2 was also synthesized and characterized in order to investigate the effect of alloying on the thermal properties of these quaternary chalcogenides. The low thermal conductivity these materials possess originates from complex phonon spectra and local dynamics of distorted CuQ4 tetrahedra. Our results and analyses are presented in light of the ongoing fundamental interest in these materials as well as their continued interest for energy-related and opto-electronic applications. [ABSTRACT FROM AUTHOR]

Raman-scattering spectra of polycrystalline Cu2ZnSnSe4, Ag2ZnSnSe4, and Cu2CdSnTe4 were investigated in order to investigate the vibrational modes upon substitution on the metal and chalcogen sites. The non-symmetric modes shift dramatically with metal substitution in the kesterite materials, with Ag2ZnSnSe4 modes shifting by 50 cm−1 in comparison with that of Cu2ZnSnSe4. Tellurium substitution results in a lower frequency of the symmetric modes. Structural data are employed to elucidate our results. These results are intended to provide further insight into these materials as they continue to be of interest for potential thermoelectric and photovoltaic applications. [ABSTRACT FROM AUTHOR]

The low temperature thermal conductivity of YbB44Si2 is investigated. The rare earth boron icosahedra compounds are of interest as potential high temperature thermoelectric materials. The thermal conductivity of YbB44Si2 is relatively large below room temperature, showing crystalline behavior and having a peak exceeding 10 W/m/K at ∼35 K. This departs markedly from the low amorphouslike behavior observed in another rare-earth-containing boron cluster compound: RB66 (R=rare earth). From a comparison and consideration of the various models, we conclude that disorder is the important factor in the generally observed low thermal conductivities of these compounds, with RB66 being exceptionally disordered as compared to YbB44Si2. [ABSTRACT FROM AUTHOR]

The low-density clathrate-II modification of silicon, Si136, contains two distinct cage-like voids large enough to accommodate various types of guest atoms which influence both the host structure and its properties. Although the linear coefficient of thermal expansion of Si136 (293 K < T < 423 K) is only about 20% larger than that of the ground state α-Si (diamond structure), the coefficient of thermal expansion monotonically increases by more than 150% upon filling the framework cages with Na atoms in NaxSi136 (0 < x < 24), ranging from α = 2.6 × 10−6 K−1 (x = 0) to 6.8 × 10−6 K−1 (extrapolated to x = 24) by only varying the Na content, x. Taken together with the available heat capacity and bulk modulus data, the dramatic increase in thermal expansion can be attributed to an increase in the mode-averaged Grüneisen parameter by a factor of nearly 3 from x = 0 to x = 24. These results highlight a potential mechanism for tuning thermal expansion, whereby guest atoms are incorporated into the voids of rigid, covalently bonded inorganic frameworks to influence the lattice dynamics. [ABSTRACT FROM AUTHOR]

In its common, thermodynamically stable state, silicon adopts the same crystal structure as diamond. Although only a few alternative allotropic structures have been discovered and studied over the past six decades, advanced methods for structure prediction have recently suggested a remarkably rich low-density phase space that has only begun to be explored. The electronic properties of these low-density allotropes of silicon, predicted by first-principles calculations, indicate that these materials could offer a pathway to improving performance and reducing cost in a variety of electronic and energy-related applications. In this focus review, we provide an introduction and overview of recent theoretical and experimental results related to low-density allotropes of silicon, highlighting the significant potential these materials may have for technological applications, provided substantial challenges to their experimental preparation can be overcome. [ABSTRACT FROM AUTHOR]

We have developed a custom apparatus for the consecutive measurement of the electrical resistivity, the Seebeck coefficient, and the thermal conductivity of materials between 300 K and 12 K. These three transport properties provide for a basic understanding of the thermal and electrical properties of materials. They are of fundamental importance in identifying and optimizing new materials for thermoelectric applications. Thermoelectric applications include waste heat recovery for automobile engines and industrial power generators, solid-state refrigeration, and remote power generation for sensors and space probes. The electrical resistivity is measured using a four-probe bipolar technique, the Seebeck coefficient is measured using the quasi-steady-state condition of the differential method in a 2-probe arrangement, and the thermal conductivity is measured using a longitudinal, multiple gradient steady-state technique. We describe the instrumentation and the measurement uncertainty associated with each transport property, each of which is presented with representative measurement comparisons using round robin samples and/or certified reference materials. Transport properties data from this apparatus have supported the identification, development, and phenomenological understanding of novel thermoelectric materials. [ABSTRACT FROM AUTHOR]

The thermoelectric properties of PEDOT:PSS/Bi0.5Sb1.5Te3 polymer/inorganic bulk composites with different Bi0.5Sb1.5Te3 content were investigated. The composites were prepared at various concentrations of Bi0.5Sb1.5Te3 by a solution-phase process before grinding to fine powders in liquid N2 for hot pressing into bulk polymer composite materials. The measured transport properties are well described within a theoretical model for effective media involving a tunneling mechanism induced by thermal voltage fluctuations. Our results present a strategy for the preparation of bulk polymer composites and demonstrate an avenue for optimization of the thermoelectric properties of PEDOT:PSS/Bi0.5Sb1.5Te3 bulk composites. [ABSTRACT FROM AUTHOR]

A new stannite phase was synthesized and its temperature dependent transport properties were investigated. Cu2CdSnTe4 possesses strong p-type conduction, while the temperature dependence of the thermal conductivity exhibits typical dielectric behavior. Electronic structure calculations allowed for a description of the transport characteristics in terms the energy band structure, density of states, and Fermi surface. The potential for thermoelectric applications is also discussed. [ABSTRACT FROM AUTHOR]

We have designed and modeled new techniques, based on the 3ω method, to measure thermal conductivity of liquids (κl) and solids (κs) under hydrostatic pressure (P). The system involves a solid sample immersed in a liquid pressure medium, both of which have unknown thermal properties. The temperature (T) and P dependance of κl are first determined through the use of a modified 3ω technique. This method uses a conducting wire (Pt, in this work), which is immersed in the pressure medium, as the heater/sensor. In addition to κl, this allows for the accurate determination of the specific heat per volume of the liquid and Pt, (ρC)l and (ρC)Pt, respectively. The information of κl and (ρC)l can then be used to make corrections to measurements of κs, in which the sample is immersed in the pressure medium, and a metal strip acts as the heater/sensor. We present the T and P dependence of κl and (ρC)l for the widely used pressure medium 3M Fluorinert FC77 up to 0.8 GPa. The measurement of κs for a thermoelectric clathrate material, Sr8Ga16Ge30, in FC77 is analyzed in detail, and the refined data achieves an accuracy of 1%. The setup can be modified to measure κ and ρC up to 3.5 GPa. [ABSTRACT FROM AUTHOR]

A large reversible magnetocaloric effect (MCE) and enhanced refrigerant capacity (RC) were observed in multiphase composite materials composed of type-I clathrate Eu8Ga16Ge30 and EuO. Eu8Ga16Ge30 undergoes two successive ferromagnetic transitions at 10 K and 35 K, and EuO exhibits a ferromagnetic transition at 75 K. A large RC of 794 J/kg for a field change of 5 T over a temperature interval of 70 K was achieved in the Eu8Ga16Ge30-EuO composite with a 40%-60% weight ratio. This is the largest value ever achieved among existing magnetocaloric materials for magnetic refrigeration in the temperature range 10 K-100 K. Adjusting the Eu8Ga16Ge30 to EuO ratio is shown to produce composites with table-like MCE, desirable for ideal Ericsson-cycle magnetic refrigeration. The excellent magnetocaloric properties of these Eu8Ga16Ge30-EuO composites make them attractive for active magnetic refrigeration in the liquid nitrogen temperature range. [ABSTRACT FROM AUTHOR]

The distribution of gallium/germanium in the type-I clathrates Sr[sub 8]Ga[sub 16]Ge[sub 30] and Sr[sub 4]Eu[sub 4]Ga[sub 16]Ge[sub 30] over the three crystallographically distinct framework sites has been determined by resonant diffraction. The analyses indicate a strong preference for the occupation of the 6c site by gallium. This is consistent with theoretical predictions, but contrary to the results of a previous neutron diffraction study. The gallium distribution, and, hence, the thermoelectric properties of these materials, may be amenable to manipulation by heat treatment. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Clathrates have been reported to form in a variety of different structure types; however, inorganic clathrate-I materials with a low-cation concentration have yet to be investigated. Furthermore, tin-based compositions have been much less investigated as compared to silicon or germanium analogs. We report the temperature-dependent structural and thermal properties of single-crystal Eu2Ga11Sn35 revealing the effect of structure and composition on the thermal properties of this low-cation clathrate-I material. Specifically, low-temperature heat capacity, thermal conductivity, and synchrotron single-crystal x-ray diffraction reveal a departure from Debye-like behavior, a glass-like phonon mean-free path for this crystalline material, and a relatively large Grüneisen parameter due to the dominance of low-frequency Einstein modes. Our analyses indicate thermal properties that are a direct result of the structure and composition of this clathrate-I material. [ABSTRACT FROM AUTHOR]

The low temperature thermal conductivity of YbB44Si2 is investigated. The rare earth boron icosahedra compounds are of interest as potential high temperature thermoelectric materials. The thermal conductivity of YbB44Si2 is relatively large below room temperature, showing crystalline behavior and having a peak exceeding 10 W/m/K at ∼35 K. This departs markedly from the low amorphouslike behavior observed in another rare-earth-containing boron cluster compound: RB66 (R=rare earth). From a comparison and consideration of the various models, we conclude that disorder is the important factor in the generally observed low thermal conductivities of these compounds, with RB66 being exceptionally disordered as compared to YbB44Si2. [ABSTRACT FROM AUTHOR]

A new stannite phase was synthesized and its temperature dependent transport properties were investigated. Cu2CdSnTe4 possesses strong p-type conduction, while the temperature dependence of the thermal conductivity exhibits typical dielectric behavior. Electronic structure calculations allowed for a description of the transport characteristics in terms the energy band structure, density of states, and Fermi surface. The potential for thermoelectric applications is also discussed. [ABSTRACT FROM AUTHOR]

Temperature-dependent transport properties of the recently discovered layered bismuth-rich tellurobromides Bi n TeBr (n = 2, 3) are investigated for the first time. Dense homogeneous polycrystalline specimens prepared for different electrical and thermal measurements were synthesized by a ball milling-based process. While the calculated electronic structure classifies Bi 2 TeBr as a semimetal with a small electron pocket, its transport properties demonstrate a semiconductorlike behavior. Additional bismuth bilayers in the Bi 3 TeBr crystal structure strengthens the interlayer chemical bonding thus leading to metallic conduction. The thermal conductivity of the semiconducting compositions is low, and the electrical properties are sensitive to doping with a factor of four reduction in resistivity observed at room temperature for only 3% Pb doping. Investigation of the thermoelectric properties suggests that optimization for thermoelectrics may depend on particular elemental substitution. The results presented are intended to expand on the research into tellurohalides in order to further advance the fundamental investigation of these materials, as well as investigate their potential for thermoelectric applications.