Electronic and Thermal Properties of the Cation Substitution-Derived Quaternary Chalcogenide CuInSnSe4.

Quaternary chalcogenides continue to be of interest for a variety of technological applications, with physical properties stemming from their structural complexity and stoichiometric variation. In certain structure types, partial vacancies on specific lattice positions present an opportunity to investigate electrical and thermal properties in light of these lattice defects. In this work, we investigated the structural, thermal, and electronic properties of CuInSnSe4, a material that belongs to a relatively unexplored class of quaternary chalcogenides with a defect adamantine crystal structure. First-principles calculations together with experimental measurements revealed a chalcopyrite-like structure with inherent vacancies and characteristic s-p and p-d orbital hybridizations in the electronic structure of the material. Cation disorder and lattice anharmonicity result in very low thermal conductivity with values significantly lower than those for related compositions. This work reveals the fundamental physical properties of a previously uninvestigated quaternary chalcogenide and may aid investigations of similar as well as other quaternary chalcogenide compositions.

Thermal properties of cubic NaSbS2: diffusion dominant thermal transport above the Debye temperature.

We elucidate the thermal properties of superionic conductors, which are of intense current interest for solid-state battery applications. The temperature-dependent thermal properties of superionic NaSbS2 were investigated by analyses of appropriate models revealing that a predominant contribution to thermal transport above the Debye temperature is from thermal diffusion.

Structure, electrical and thermal properties of single-crystal BaCuGdTe3.

Single crystals of the quaternary chalcogenide BaCuGdTe3 were obtained by direct reaction of elements allowing for a complete investigation of the intrinsic electrical and thermal properties of this previously uninvestigated material. The structure was investigated by high-resolution single-crystal synchrotron X-ray diffraction, revealing an orthorhombic crystal structure with the space group Cmcm. Although recently identified as a semiconductor suitable for thermoelectric applications from theoretical analyses, our electrical resistivity and Seebeck coefficient measurements show metallic conduction, the latter revealing strong phonon-drag. Temperature dependent hole mobility reveals dominant acoustic phonon scattering. Heat capacity data reveal a Debye temperature of 183 K and a very high density of states at the Fermi level, the latter confirming the metallic nature of this composition. Thermal conductivity is relatively high with Umklapp processes dominating thermal transport above the Debye temperature. The findings in this work lay the foundation for a more detailed understanding of the physical properties of this and similar multinary chalcogenide materials, and is part of the continuing effort in investigating quaternary chalcogenide materials and their suitability for use in technological applications.

Thermal and Electronic Properties of Ba2MnSe3.

The structural, thermal, and electronic properties of Ba2MnSe3 were investigated. Analysis of the low-temperature heat capacity revealed a low Debye temperature and a low average speed of sound that, together with the bonding in this material, result in a low thermal conductivity over a relatively large temperature range. Density functional theory and calculated electron localization were employed to investigate the electronic structure and bonding. Absorption and photoluminescence spectroscopy measurements corroborated our calculations and revealed a direct band gap of 1.75 eV. This study expands on our understanding of the physical properties of this material and reveals previously unascertained properties, the knowledge of which is imperative for any potential application of interest.

Europium Clustering and Glassy Magnetic Behavior in Inorganic Clathrate-VIII Eu8Ga16Ge30.

The temperature- and field-dependent, electrical and thermal properties of inorganic clathrate-VIII Eu8Ga16Ge30 were investigated. The type VIII clathrates were obtained from the melt of elements as reported previously. Specifically, the electrical resistivity data show hysteretic magnetoresistance at low temperatures, and the Seebeck coefficient and Hall data indicate magnetic interactions that affect the electronic structure in this material. Heat capacity and thermal conductivity data corroborate these findings and reveal the complex behavior due to Eu2+ magnetic ordering and clustering from approximately 13 to 4 K. Moreover, the low-frequency dynamic response indicates Eu8Ga16Ge30 to be a glassy magnetic system. In addition to advancing our fundamental understanding of the physical properties of this material, our results can be used to further the research for potential applications of interest in the fields of magnetocalorics or thermoelectrics.

Structural and thermal properties of ultralow thermal conductivity Ba3Cu2Sn3Se10.

The thermal properties of Ba3Cu2Sn3Se10 were investigated by measurement of the thermal conductivity and heat capacity. The chemical bonding in this diamagnetic material was investigated using structural data from Rietveld refinement and calculated electron localization. This quaternary chalcogenide is monoclinic (P21/c), has a large unit cell with 72 atoms in the primitive cell, and a high local coordination environment. The Debye temperature (162 K) and average speed of sound (1666 m s-1) are relatively low with a very small electronic contribution to the heat capacity. Ultralow thermal conductivity (0.46 W m-1 K-1 at room temperature) is attributed to the relatively weak chemical bonding and intrinsic anharmonicity, in addition to a large unit cell. This work is part of the continuing effort to explore quaternary chalcogenides with intrinsically low thermal conductivity and identify the features that result in a low thermal conductivity.

Synthesis, structure, electronic and thermal properties of sphalerite CuZn2InS4.

Quaternary chalcogenides continue to be of interest due to the variety of physical properties they possess, as well as their potential for different applications of interest. Investigations on materials with the sphalerite crystal structure have only recently begun. In this study we have synthesized sulfur-based sphalerite quaternary chalcogenides, including off-stoichiometric compositions, and investigated the temperature-dependent electronic, thermal and structural properties of these materials. Insulating to semiconducting transport is observed with stoichiometric variation, and analyses of heat capacity and thermal expansion revealed lattice anharmonicity that contributes to the low thermal conductivity these materials possess. We include similar analyses for CuZn2InSe4 and compare these sphalerite quaternary chalcogenides to that of zinc blende binaries in order to fully understand the origin of the low thermal conductivity these quaternary chalcogenides possess.

Synthesis, Bottom up Assembly and Thermoelectric Properties of Sb-Doped PbS Nanocrystal Building Blocks.

The precise engineering of thermoelectric materials using nanocrystals as their building blocks has proven to be an excellent strategy to increase energy conversion efficiency. Here we present a synthetic route to produce Sb-doped PbS colloidal nanoparticles. These nanoparticles are then consolidated into nanocrystalline PbS:Sb using spark plasma sintering. We demonstrate that the introduction of Sb significantly influences the size, geometry, crystal lattice and especially the carrier concentration of PbS. The increase of charge carrier concentration achieved with the introduction of Sb translates into an increase of the electrical and thermal conductivities and a decrease of the Seebeck coefficient. Overall, PbS:Sb nanomaterial were characterized by two-fold higher thermoelectric figures of merit than undoped PbS.

Structural, Electronic, and Thermal Properties of CdSnAs2.

Structural, electrical, and thermal properties of CdSnAs2, with analyses from temperature-dependent transport properties over a large temperature range, are reported. Phase-pure microcrystalline powders were synthesized that were subsequently densified to a high-density homogeneous polycrystalline specimen for this study. Temperature-dependent transport indicates n-type semiconducting behavior with a very high and nearly temperature independent mobility over the entire measured temperature range, attributed to the very small electron effective mass of this material. The Debye model was successfully applied to model the thermal conductivity and specific heat. This work contributes to the fundamental understanding of this material, providing further insight and allowing for investigations into altering this and related physical properties of these materials for technological applications.

Synthesis, transport properties and electronic structure of p-type Cu1+xMn2-xInTe4 (x = 0, 0.2, 0.3).

The synthesis, electronic structure and temperature dependent transport properties of polycrystalline Cu1+xMn2-xInTe4 (x = 0, 0.2, 0.3) are reported for the first time. These quaternary chalcogenides were synthesized by direct reaction of the elements, followed by solid state annealing and hot press densification. The thermal conductivity is low for all specimens and intrinsic to the material system. Furthermore, the off-stoichiometry specimens illustrate the sensitivity of the transport properties to stoichiometry, with a greater than two-orders-of magnitude increase in carrier concentration with increased Cu content. First principles calculations of the electronic structure are also reported, and are in agreement with the experimental data. This fundamental investigation shows the potential towards further optimization of the electrical properties that, in addition to the intrinsically low thermal conductivity, provides a basis for further research into the viability of this material system for potential energy-related applications.

Enhanced thermoelectric performance of heavy-fermion compounds YbTM 2Zn20 (TM = Co, Rh, Ir) at low temperatures.

Thermoelectricity allows direct conversion between heat and electricity, providing alternatives for green energy technologies. Despite these advantages, for most materials the energy conversion efficiency is limited by the tendency for the electrical and thermal conductivity to be proportional to each other and the Seebeck coefficient to be small. Here we report counter examples, where the heavy fermion compounds YbTM 2Zn20 (TM = Co, Rh, Ir) exhibit enhanced thermoelectric performance including a large power factor (PF = 74 µW/cm-K2; TM = Ir) and a high figure of merit (ZT = 0.07; TM = Ir) at 35 K. The combination of the strongly hybridized electronic state originating from the Yb f-electrons and the novel structural features (large unit cell and possible soft phonon modes) leads to high power factors and small thermal conductivity values. This demonstrates that with further optimization these systems could provide a platform for the next generation of low temperature thermoelectric materials.

Zintl Phases as Reactive Precursors for Synthesis of Novel Silicon and Germanium-Based Materials.

Recent experimental and theoretical work has demonstrated significant potential to tune the properties of silicon and germanium by adjusting the mesostructure, nanostructure, and/or crystalline structure of these group 14 elements. Despite the promise to achieve enhanced functionality with these already technologically important elements, a significant challenge lies in the identification of effective synthetic approaches that can access metastable silicon and germanium-based extended solids with a particular crystal structure or specific nano/meso-structured features. In this context, the class of intermetallic compounds known as Zintl phases has provided a platform for discovery of novel silicon and germanium-based materials. This review highlights some of the ways in which silicon and germanium-based Zintl phases have been utilized as precursors in innovative approaches to synthesize new crystalline modifications, nanoparticles, nanosheets, and mesostructured and nanoporous extended solids with properties that can be very different from the ground states of the elements.

Structural, Chemical, Electrical, and Thermal Properties of n-Type NbFeSb.

We report on the structural, chemical, electrical, and thermal properties of n-type polycrystalline NbFeSb synthesized by induction melting of the elements. Although several studies on p-type conduction of this half-Heusler composition have recently been reported, including reports of relatively high thermoelectric properties, very little has been reported on the transport properties of  n-type compositions. We combine transport property investigations together with short- and long-range structural data obtained by Mössbauer spectroscopy of iron-57 and antimony-121 and by neutron total scattering, as well as first-principles calculations. In our investigation, we show that n-type conduction can occur from antiphase boundaries in this material. This work is intended to provide a greater understanding of the fundamental properties of NbFeSb as this material continues to be of interest for potential thermoelectric applications.

Transport properties of topologically non-trivial bismuth tellurobromides BinTeBr.

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.

Compositional Effects and Electron Lone-pair Distortions in Doped Bournonites.

Bornite materials are naturally occurring systems composed of earth-abundant constituents. Bournonite, a representative of this class of materials, is of interest for thermoelectric applications due to its inherently low thermal conductivity, which has been attributed to the lattice distortions due to stereochemically active electron lone pair distributions. In this computational and experimental study, we present analyses of the lattice structure, electron and phonon dynamics, and charge localization and transfer properties for undoped and Ni and Zn doped bournonites. The results from our simulations reveal complex relations between bond length and bond angle characteristics, chemical bonding, and charge transfer upon doping. Analysis of the experimental results indicate that a microscopic description for bournonite and its doped compositions is necessary for a complete understanding of these materials, as well as for effective control of the transport properties for targeted applications.

Synthesis, Structure, and Electrical Properties of the Single Crystal Ba8Cu16As30.

Single crystals of clathrate-I Ba8Cu16As30 have been synthesized and their structure and electronic properties determined using synchrotron-based X-ray diffraction and first-principles calculations. The structure is confirmed to be Pm3̅ n (No. 223), with lattice parameter a = 10.4563(3) Å, and defined by a tetrahedrally bonded network of As and Cu that forms two distinct coordination polyhedra, with Ba residing inside these polyhedra. All crystallographic positions are fully occupied with no vacancies or superstructure with the Cu atoms, while occupying all framework sites in the network, exhibiting a preference for the 6c site. Agreement between the experimental and theoretically predicted structures was achieved after accounting for spin-orbit coupling. Our calculated Fermi surface, electron localization, and charge transfer, as well as a comparison with the results for elemental As46, provide insight into the fundamental properties of this clathrate-I material.

Zintl Ions within Framework Channels: The Complex Structure and Low-Temperature Transport Properties of Na4Ge13.

Single crystals of a complex Zintl compound with the composition Na4Ge13 were synthesized for the first time using a high-pressure/high-temperature approach. Single-crystal diffraction of synchrotron radiation revealed a hexagonal crystal structure with P6/m space group symmetry that is composed of a three-dimensional sp3 Ge framework punctuated by small and large channels along the crystallographic c axis. Na atoms are inside hexagonal prism-based Ge cages along the small channels, while the larger channels are occupied by layers of disordered sixfold Na rings, which are in turn filled by disordered [Ge4]4- tetrahedra. This compound is the same as "Na1-xGe3+z" reported previously, but the availability of single crystals allowed for more complete structural determination with a formula unit best described as Na4Ge12(Ge4)0.25. The compound is the first known example of a guest-host structure where discrete Zintl polyanions are confined inside the channels of a three-dimensional covalent framework. These features give rise to temperature-dependent disorder, as confirmed by first-principles calculations and physical properties measurements. The availability of single-crystal specimens allowed for measurement of the intrinsic low-temperature transport properties of this material and revealed its semiconductor behavior, which was corroborated by theoretical calculations.

Synthesis, Structure, Te Alloying, and Physical Properties of CuSbS2.

Materials with very low thermal conductivities continue to be of interest for a variety of applications. We synthesized CuSbS2 employing a mechanical alloying technique in order to investigate its physical properties. The trigonal pyramid arrangement of the S atoms around the Sb atoms allows for lone-pair electron formation that results in very low thermal conductivity. In addition to thermal properties, the structural, electrical, and optical properties, as well as compositional stability measurements, are also discussed. CuSbS1.8Te0.2 was similarly synthesized and characterized in order to compare its structural and transport properties with that of CuSbS2, in addition to investigating the effect of Te alloying on these properties.

BC8 Silicon (Si-III) is a Narrow-Gap Semiconductor.

Large-volume, phase-pure synthesis of BC8 silicon (Ia3[over ¯], cI16) has enabled bulk measurements of optical, electronic, and thermal properties. Unlike previous reports that conclude BC8-Si is semimetallic, we demonstrate that this phase is a direct band gap semiconductor with a very small energy gap and moderate carrier concentration and mobility at room temperature, based on far- and midinfrared optical spectroscopy, temperature-dependent electrical conductivity, Seebeck and heat capacity measurements. Samples exhibit a plasma wavelength near 11 µm, indicating potential for infrared plasmonic applications. Thermal conductivity is reduced by 1-2 orders of magnitude depending on temperature as compared with the diamond cubic (DC-Si) phase. The electronic structure and dielectric properties can be reproduced by first-principles calculations with hybrid functionals after adjusting the level of exact Hartree-Fock (HF) exchange mixing. These results clarify existing limited and controversial experimental data sets and ab initio calculations.

Apparatus for the measurement of electrical resistivity, Seebeck coefficient, and thermal conductivity of thermoelectric materials between 300 K and 12 K.

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.