This work presents theoretical calculations of the electrical transport properties of the Ag, Au, and La fractionally filled bulk skutterudites: CoSb3, CoAs3, and CoP3. Density functional theory (DFT), along with projector augmented wave (PAW) potentials, was used to calculate bulk band structures and partial density of states. The Seebeck coefficient (S), electrical conductivity (s), and power factor (S2 s) were calculated as a function of temperature and filling fraction using the momentum matrix method along the entire first Brillouin zone. Calculated trends in the electrical transport properties agree with previously published experimental measurements for p-type unfilled and La filled CoSb3. The calculated S and s values for the Ag and Au filled compounds indicates that the most promising electronic properties are exhibited by p-type Au0.125(CoSb3)4, Au0.25(CoSb3)4, and Au(CoSb3)4. Au is therefore recommended as a promising filler for improved thermoelectric properties in cobalt antimonides. Ag is also a good filler for cobalt phosphides; the creation of a negative indirect band gap is observed in Ag(CoP3)4, which indicates semimetallic behavior, so this compound may possibly exhibit lower thermal conductivity than metallic CoP3. Finally, we recommend future directions for improving the thermoelectric figure of merit of these materials.
On the basis of density functional theory (DFT) calculations thermoelectric properties are derived for Sb-doped skutterudites MPt$_4$Ge$_{12-x}$Sb$_x$ with M=Ba,La,Th. It is predicted that the originally very small absolute values of Seebeck coefficients $|S|$ of the undoped compounds is increased by factors of 10 or more for suitable dopings. The optimal dopings correspond to a "magic" valence electron number for which all electronic states up to a (pseudo)gap are filled. The theoretical findings are corroborated by measurements of $S$ for LaPt$_4$Ge$_{12-x}$Sb$_x$ skutterudites. DFT derived …
Based on the interplay of theory and experiment, a large new family of filled group 9 (Co, Rh and Ir) skutterudites is designed and synthesized. The new materials fill the empty cages in the structures of the known binary CoSb3, RhSb3 and IrSb3 skutterudites with alkaline, alkaline earth, and rare earth atoms to create compounds of the type AyB4X12; A atoms fill the cages to a fraction y, B are the group 9 transition metals, and X is a mixture of electronegative main group elements chosen to achieve chemical stability by adjusting the electron counts to electron-precise values. Forty-three new…
A filled skutterudite, La$_{0.5}$Rh$_4$Sb$_{12}$, with a lattice constant of 9.284(2) Å was synthesized using a high-pressure technique. The electrical resistivity showed semiconducting behavior and the energy gap was estimated to be more than 0.08 eV. Magnetic susceptibility measurements indicated temperature-independent diamagnetism, which originates from Larmor diamagnetism. The electrical properties of this compound are more similar to those of the La$_{0.5}$Rh$_4$As$_{12}$ semiconductor with an energy gap of 0.03 eV than to those of the La$_{0.6}$Rh$_4$P$_{12}$ superconductor.
p-type Ce1.05Fe4Sb12.04 filled skutterudites with much improved thermoelectric properties have been synthesized by rapidly converting nearly amorphous ribbons into crystalline pellets under pressure. It is found that this process greatly suppresses grain growth and second phase formation/segregation, and hence results in the samples consisting of nano-sized grains with strongly-coupled grain boundaries, as observed by transmission electron microscopy. The room temperature carrier mobility in these samples is significantly higher (nearly double) than those in the samples of the same starting co…
Filling the large voids in the crystal structure of the skutterudite CoSb$_3$ with rattler atoms $R$ provides an avenue for both increasing carrier concentration and disrupting lattice heat transport, leading to impressive thermoelectric performance. While the influence of $R$ on the lattice dynamics of skutterudite materials has been well studied, the phase stability of $R$-filled skutterudite materials and the influence of the presence and ordering of $R$ on the electronic structure remain unclear. Here, focusing on the Yb-filled skutterudite Yb$_x$Co$_4$Sb$_{12}$, we employ first-principles…
We propose new topological insulators in cerium filled skutterudite (FS) compounds based on ab initio calculations. We find that two compounds CeOs4As12 and CeOs4Sb12 are zero gap materials with band inversion between Os-d and Ce-f orbitals, which are thus parent compounds of two and three-dimensional topological insulators just like bulk HgTe. At low temperature, both compounds become topological Kondo insulators, which are Kondo insulators in the bulk, but have robust Dirac surface states on the boundary. This new family of topological insulators has two advantages compared to previous ones.…
We studied nanoprecipitates and defects in p-type filled skutterudite CeFe4Sb12 prepared by non-equilibrium melt-spinning plus spark plasma sintering method using transmission electron microscopy. Nanoprecipitates with mostly spherical shapes and different sizes (from several nm to several tens of nm) have been observed. The most typically observed nanoprecipitates are shown to be Sb-rich. Superlattices with a periodicity of about 3.576 nm were induced by the ordering of excessive Sb atoms along the c direction. These nanoprecipitates usually share coherent interfaces with the surrounding matr…
La-filled skutterudites LaxCo4Sb12 (x : 0.25 and 0.5) have been synthesized and sintered in one step under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. The structural properties of the reaction products were characterized by synchrotron X-ray powder diffraction, clearly showing an uneven filling factor of the skutterudite phases, confirmed by transmission electron microscopy. The non-homogeneous distribution of La filling atoms is adequate to produce a significant decrease in lattice thermal conductivity, mainly due to strain field scattering of high-energy phono…
In the last two decades, a nanostructuring paradigm has been successfully applied in a wide range of thermoelectric materials, resulting in significant reduction in thermal conductivity and superior thermoelectric performance. These advances, however, have been accomplished without directly investigating the local thermoelectric properties, even though local electric current can be mapped with high spatial resolution. In fact, there still lacks an effective method that links the macroscopic thermoelectric performance to the local microstructures and properties. Here, we show that local thermal…
The filled-skutterudite compound CePt4Ge12 is situated close to the border between intermediate-valence of Ce and heavy-fermion behavior. Substitution of Ge by Sb drives the system into a strongly correlated and ultimately upon further increasing the Sb concentration into an antiferromagnetically ordered state. Our experiments evidence a delicate interplay of emerging Kondo physics and the formation of a local 4f moment. An extended non-Fermi-liquid region, which can be understood in the framework of a Kondo-disorder model, is observed. Band-structure calculations support the conclusion that t…
The interplay of superconductivity and magnetism is a subject of ongoing interest, stimulated most recently by the discovery of Fe-based superconductivity and the recognition that spin-fluctuations near a magnetic quantum critical point may provide an explanation for the superconductivity and the order parameter. Here we investigate magnetism in the Na filled Fe-based skutterudites using first principles calculations. NaFe4Sb12 is a known ferromagnet near a quantum critical point. We find a ferromagnetic metallic state for this compound driven by a Stoner type instability, consistent with prio…
It is shown that quadrupole susceptibility can be detected in Gd compounds contrary to our textbook knowledge that Gd$^{3+}$ ion induces pure spin moment due to the Hund's rules in an $LS$ coupling scheme. The ground-state multiplet of Gd$^{3+}$ is always characterized by $J$=7/2, where $J$ denotes total angular momentum, but in a $j$-$j$ coupling scheme, one $f$ electron in $j$=7/2 octet carries quadrupole moment, while other six electrons fully occupy $j$=5/2 sextet, where $j$ denotes one-electron total angular momentum. For realistic values of Coulomb interaction and spin-orbit coupling, th…
Measurements of the phonon density of states by inelastic neutron \emph{time-of-flight} scattering and specific heat measurements along with first principles calculations, provide compelling evidence for the existence of an Einstein oscillator ("\emph{rattler}") at $ω_{E1} \approx$ 5.0 meV in the filled skutterudite Yb$_{0.2}$Co$_{4}$Sb$_{12}$. Multiple dispersionless modes in the measured density of states of Yb$_{0.2}$Co$_{4}$Sb$_{12}$ at intermediate transfer energies (14 meV $\leq$ \emph{$ω$} $\leq$ 20 meV) are exhibited in both the experimental and theoretical \emph{density-of-states} of …
In the unconventional f-electron-associated charge order phase of filled skutterudite PrRu4P12, the low-temperature behaviors of the triplet crystalline-electric-field ground state of Pr ions have been studied by specific heat and magnetization measurements using high quality single crystals. Specific heat shows an anomalous Schottky-type peak structure at 0.30 K in zero field in spite of the absence of any symmetry breaking. Magnetization curve at 0.06 K shows a remarkable rounding below 1 T. It has been revealed that these anomalies provide compelling evidence for the formation of a lattice …
Vibrational and elastic properties of the $R$Fe$_{4}$Sb$_{12}$ skutterudites are investigated by, respectively, temperature $(T)$ dependent extended X-ray absorption fine structure (EXAFS) and pressure $(P)$ dependent x-ray diffraction (XRD) experiments. The Fe $K$-edge EXAFS experiments of the $R=$ K, Ca and Ba materials were performed in the $T$-interval $6<T<300$ K and XRD experiments of the $R=$ Na, K, Ca, Sr and Ba materials were performed in the $P$-interval $1\text{ atm }<P<16$ GPa. From EXAFS, we obtained the correlated Debye-Waller parameters that were thus analyzed to extract effecti…
Muon spin rotation ($μ$SR) and inelastic X-ray scattering (IXS) were used to investigate the superconducting properties of the filled-skutterudite compound LaRu$_{4}$As$_{12}$. A two-gap isotropic ($s+s$)-wave model can explain the temperature dependence of the superfluid density. Zero field $μ$SR measurements confirm that the time-reversal symmetry does not break upon entering the superconducting state. The measurements of lattice dynamics at 2, 20 and 300 K revealed temperature dependencies of the phonon modes that do not follow strictly a hardening of phonon frequencies upon cooling as expe…
The best p-type skutterudites so far are didymium filled, Fe/Co substituted, Sb-based skutterudites. Substitution at the Sb-sites influences the electronic structure, deforms the Sb4-rings, enhances the scattering of phonons on electrons and impurities and this way reduces the lattice thermal conductivity. In this paper we study structural and transport properties of p-type skutterudites with the nominal composition DD0.7Fe2.7Co1.3Sb11.7{Ge/Sn}0.3, which were prepared by a rather fast reaction-annealing-melting technique. The Ge-doped sample showed impurities, which did not anneal out complete…
Novel filled skutterudites EpyNi4Sb12-xSnx (Ep = Ba and La) have been prepared by arc melting followed by annealing at 250C, 350C and 450C up to 30 days in sealed quartz vials. A maximum filling level of y = 0.93 and y = 0.65 was achieved for the Ba and La filled skutterudite, respectively. Single-phase samples with the composition Ni4Sb8.2Sn3.8, Ba0.42Ni4Sb8.2Sn3.8 and Ba0.92Ni4Sb6.7Sn5.3 were employed for measurements of the physical properties i.e. temperature dependent electrical resistivity, Seebeck coefficient and thermal conductivity. Resistivity data showed a crossover from metallic to…
Highly filled conductive polymer composites (CPCs) are widely used in applications such as bipolar plate materials for polymer electrolyte membrane fuel cells and redox flow batteries, electromagnetic interference shielding and sensors due to their useful electrical properties. A common method for determining through-plane electrical conductivities $\left(σ_{\rm tp} \right)$ of such highly filled CPCs applies a conductive carbon paper between electrodes and sample with application of external pressure to improve electrical contact. We show the pressure-dependence of the measured $σ_{\rm tp}$ c…
We report $^{59}$Co NMR and transport measurements on $n$-type filled skutterudites Ba$_x$Yb$_y$Co$_4$Sb$_{12}$ and $A$$_x$Co$_4$Sb$_{12}$ ($A$= Ba, Sr), promising thermoelectric materials. The results demonstrate consistently that a shallow defect level near the conduction band minimum dominates the electronic behavior, in contrast to the behavior of unfilled CoSb$_3$. To analyze the results, we modeled the defect as having a single peak in the density of states, occupied at low temperatures due to donated charges from filler atoms. We fitted the NMR shifts and spin-lattice relaxation rates a…
The magnetic, thermodynamic and electrical/thermal transport properties of the caged-structure quasi-skutterudite Gd$_3$Ir$_4$Sn$_{13}$ are re-investigated. The magnetization $M(T)$, specific heat $C_p(T)$ and the resistivity $ρ(T)$ reveal a double-phase transition -- at $T_{N1}\sim$ 10~K and at $T_{N2}\sim$ 8.8~K -- which was not observed in the previous report on this compound. The antiferromagnetic transition is also visible in the thermal transport data, thereby suggesting a close connection between the electronic and lattice degrees of freedom in this Sn-based quasi-skutterudite. The temp…
The electrical conductivity of the polyimide R-BAPB polymer filled with single-wall carbon nanotubes (CNT) with chirality (5,5) is modeled using a multi-scale approach. The modeling starts with molecular dynamics simulations of time-dependent fluctuating atomic configurations of polymer filled CNTs junctions. Then the atomic positions obtained in the first step are used to perform fully first-principles microscopic calculations of the CNTs junctions contact resistances using the Green's function based quantum transport technique. And finally, those contact resistances are supplied as an input …
The inherent network of nanopores and voids in silicon dioxide (SiO2) is generally undesirable for aspects of film quality, electrical insulation and dielectric performance. However, if we view these pores as natural nano-patterns embedded in a dielectric matrix then that opens up new vistas for exploration. The nano-pattern platform can be used to tailor electrical, optical, magnetic and mechanical properties of the carrier film. In this article we report the tunable electrical properties of thermal SiO2 thin-film achieved through utilization of the metal-nanopore network where the pores are …
Polydimethylsiloxane (PDMS) is an insulator and it is commonly used in fabrication of micro-structures but there is a need of electrical conductive PDMS. Therefore, this research is mainly focused on producing PDMS nanocomposites with high electrical conductivity. Thus, we produced highly conductive PDMS nanocomposites which were filled with multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs). Due to the synergistic effect between CNTs and GNPs inside a polymeric matrix, we expected to obtain more conductive nanocomposites than PDMS nanocomposite filled with only CNTs. Addi…
A compound with metallic electrical conductivity usually has a considerable total thermal conductivity because both electrons and photons contribute to thermal transport. Here, we show an exceptional example of iridium oxide, Bi3Ir3O11, that concurrently displays metallic electrical conductivity and ultralow thermal conductivity approaching 0.61 W m-1 K-1 at 300 K. The compound crystallizes into a cubic structural framework with space group Pn-3. The edge- and corner-sharing IrO6 octahedra with a mixed Ir4.33+ charge state favor metallic electrical transport. Bi3Ir3O11 exhibits an extremely lo…
Here we report the structural, electrical and magnetic properties of Fe doped La0.7Ca0.3Mn1-xFexO3 with x = 0.0 to 1.0 prepared by conventional solid state reaction method. Simulated data on XRD shows an increase in volume with an increase in Fe ion concentration. XPS supports that Fe3+ ions directly substitute Mn3+ ions. Shifting towards lower wave-number and symmetric IR band structure confirms increase in volume and homogeneous distribution of Fe ions. Fe ion doesn't contribute in double-exchange (DE) conduction mechanism due to its stable half filled 3d orbital. The presence of Fe3+ ions e…
A realistic transport model showing the interplay of the hopping transport between the outer shells of iron filled entangled multi-walled carbon nanotubes (MWNT) and the diffusive transport through the inner part of the tubes, as a function of the filling percentage, is developed. This model is based on low-temperature electrical resistivity and magneto-resistance (MR) measurements. The conductivity at low temperatures showed a crossover from Efros-Shklovski (E-S) variable range hopping (VRH) to Mott VRH in 3 dimensions (3D) between the neighboring tubes as the iron weight percentage is increa…
