Skutterudite Compounds Research Articles

Vibrational Dynamics of Filled Skutterudites LaT4X12 (T = Fe, Ru, Os, X = As, Sb)

We report on the vibrational properties of the lanthanum filled skutterudite compounds LaRu4Sb12, LaOs4Sb12, and LaOs4As12 studied by inelastic neutron scattering. The experimental data are backed up by ab initio lattice dynamics calculations of the complete series of lanthanum filled skutterudites La\(T_{4}X_{12}\) with \(T\) = Fe, Ru, Os and \(X\) = As, Sb. We present details of their structural properties, phonon dispersion relations, vibrational densities of states Z(ω) and specific heats \(C_{V}(T)\) calculated from Z(ω) within the harmonic approximation. Powder averaged lattice dynamics (PALD) technique is applied to approximate the experimental signal presented as the generalized density of states G(ω) with high accuracy. Furthermore, the PALD approach is exploited to compute average transverse \(v_{\text{t}}^{\text{PALD}}\) and longitudinal \(v_{\text{l}}^{\text{PALD}}\) sound velocities, elastic constants \(c_{11}^{\text{PALD}}\), \(c_{44}^{\text{PALD}}\), bulk moduli \(B^{\text{PALD}}\), and Deb...

Effects of Synthesis Temperature on the Microstructure and Thermoelectric Properties of Te-Se Codoped Skutterudites

Skutterudite compounds Co4Sb11.3Te0.5Se0.2 were synthesized by solid-state reaction at different temperatures (853 K, 903 K, 953 K, and 1003 K) with subsequent spark plasma sintering. x-Ray diffraction, field-emission scanning electron microscopy, and electron probe microanalysis were utilized to analyze the phase structure, microstructure, and actual compositions of the samples. The results showed that the actual composition and the grain size vary with the synthesis temperature. The thermoelectric properties of all samples were measured in the temperature range of 300 K to 800 K. As the synthesis temperature increases, the electrical conductivity increases rapidly, the absolute Seebeck coefficient falls, and the thermal conductivity first decreases and then increases. The highest dimensionless figure of merit ZT was achieved for the sample synthesized at 953 K, exceeding 1.0 at high temperature.

THERMOELECTRIC PROPERTIES OF p-TYPE SKUTTERUDITES (Pr0.25Nd0.75)xFe3CoSb12 BY LEVITATION MELTING AND SPARK PLASMA SINTERING

The p-type skutterudite compounds of ( Pr 0.25 Nd 0.75)x Fe 3 CoSb 12 (x = 0.67–0.78) have been successfully synthesized by levitation melting followed by annealing and spark plasma sintering. The thermoelectric properties have been characterized by the measurements of Seebeck coefficient, electrical conductivity and thermal conductivity in the temperature range from 300 K to 850 K. The improvement in the thermoelectric properties was realized due to the reduction in the lattice thermal conductivity when the voids were partially filled by Pr 0.25 Nd 0.75. The maximum ZT value of ~ 0.83 for ( Pr 0.25 Nd 0.75)0.76 Fe 3 CoSb 12 was obtained at 700 K.

Enhanced thermoelectric performance of optimized Ba, Yb filled and Fe substituted skutterudite compounds

Abstract It is well known that Ba and Yb are effective pair-filling species in skutterudites. They offer a large mass and size contrast therefore yielding widely different phonon resonant scattering frequencies. These features of Ba and Yb fillers inspired our investigation of the high temperature thermoelectric properties of Fe substituted and Ba/Yb double-filled skutterudite compounds of compositions BaxYbyFe0.4Co3.6Sb12 (0.25 ⩽ x, y ⩽ 0.4 nominal). We measured the Seebeck coefficient, electrical conductivity, thermal conductivity, and the Hall coefficient from 300 K to 800 K. All samples were prepared using the traditional method consisting of melting-annealing followed by spark plasma sintering. The Hall coefficient and the Seebeck coefficient signs indicate that all samples are n-type conductors. The electrical conductivity increases and the Seebeck coefficient decreases with the increasing Ba content, likely due to the increasing carrier density as the amount of Ba increases. It is seen that Ba is more effective than Yb in tuning electronic transport properties while Yb has a greater effect on the lattice thermal conductivity. EMPA and PXRD analyses indicate that increasing the Ba content reduces the filling fraction of Yb, therefore the solubility limit of combined Ba + Yb filling remains about 0.4 (actual) for BaxYbyFe0.4Co3.6Sb12 (0.25 ⩽ x, y ⩽ 0.4 nominal) compositions. The combination of a high power factor (47 μW/K2 cm) and a reasonably low thermal conductivity (2.79 W/m K) of Ba0.3Yb0.3Fe0.4Co3.6Sb12 (nominal) leads to a value of the thermoelectric figure of merit ZT∼1.35 at 800 K, one of the highest values of ZT for double-filled n-type skutterudite compounds.

Preparation and thermoelectric properties of p-type filled skutterudites CeyFe4−xNixSb12

Abstract In this study, a series of p-type filled skutterudite compounds CeyFe4−xNixSb12 (x = 0–1.5; y = 1–0.3) have been successfully prepared by a conventional melting, quenching, annealing followed by spark plasma sintering technique and their high temperature thermoelectric properties are systematically examined. The results indicate that the introduction of Ni dopants exerts little influence on the electronic structure of CeyFe4−xNixSb12 compounds near top valence band but will cause severe bipolar diffusion effect. The carrier concentration can be facilely adjusted over a broad region through the synergetic regulation of Ce filling fraction and Ni doping amount, thus providing much space for power factor optimization. We also demonstrate that partial filling is an effective way of blocking the phonon propagation. Consequently, the thermoelectric performance is significantly enhanced over a wide temperature range via a moderate Ni doping because of the simultaneous optimization of electron and phonon transport.

Ultrasound measurements on the skutterudite compound SmOs4P12

We have investigated the elastic properties of the Sm-based filled skutterudite compound SmOs4P12 for the first time. The measurements were performed on polycrystalline samples prepared at high temperatures and high pressures by using a wedge-type, cubic anvil, high-pressure apparatus. A clear and steep decrease was observed at an antiferromagnetic transition temperature of 4.5 K in the temperature dependence of both the longitudinal and the transverse elastic constants. Absence of a precursor behavior reminds us of the multipolar ordering, possibly octupolar ordering, observed in the isostructural system SmRu4P12. The transition is robust against an external magnetic field. We argue that the low-lying degenerate levels derived from the 4f-multiplet ground state of the Sm ion is split by the crystalline electric field in SmOs4P12. Furthermore, we discuss the phononic properties, which include the ionic degrees of freedom for the rattling motion, and compare them with the values for other isostructural systems.

Thermoelectric transport properties of nickel-doped Co4−xNixSb11.6Te0.2Se0.2 skutterudites

In this paper, skutterudite compounds Co4−xNixSb11.6Te0.2Se0.2 (x=0, 0.05, 0.1 and 0.2) were successfully prepared by the solid state reaction and spark plasma sintering methods. The component phases and the chemical composition of all samples were characterized by the X-ray diffraction and the electron microprobe analysis, respectively. The Seebeck coefficient, electrical conductivity and thermal conductivity were measured in the 300–800K temperature range. It is found that suitable amount of Ni dopant in the Co4−xNixSb11.6Te0.2Se0.2 system not only resulted in an improvement of the electrical conductivity but also a drop of the lattice thermal conductivity. The maximum dimensionless figure of merit of 0.83 was obtained at 800K for sample Co3.8Ni0.2Sb11.6Te0.2Se0.2, representing a 23% improvement compared with that of the Ni-free sample.

Upper critical field peculiarities in the cubic superconductor LaRu4As12

Electrical resistivity of the filled skutterudite compound LaRu4As12 was investigated both as a function of temperature and magnetic field. Upon lowering temperature below around 0.85Tc ≈ 9K, the upper critical field increases quasilinearly with the slope (−dHc2/dT)=1.11 T/K, i.e., significantly faster than in the vicinity of Tc where (−dHc2/dT)Tc = 0.66T/K was found. As a consequence, a positive curvature emerges whose size is very similar to those observed for anisotropic superconductors. Anticipating a weakly anisotropic Fermi surface in the cubic compound LaRu4As12, we related the anomaly in Hc2(T) to multiband effects rather than to a strong anisotropy of a single energy gap.

The thermoelectric properties of InxM0.2Co4Sb12 (M=Ba and Pb) double-filled skutterudites

Skutterudite compounds InxM0.2Co4Sb12 (M=Ba and Pb) with bcc crystal structure have been prepared by high pressure and high temperature (HPHT) method. Through this method, the processing time can be reduced from a few days to half an hour. Remarkably the combination of In and Ba was much more effective in reducing lattice thermal conductivity κL than In and Pb. The structure of InxM0.2Co4Sb12 skutterudites was evaluated by means of x-ray diffraction, and Seebeck coefficient, electrical resistivity, power factor and thermal conductivity of InxM0.2Co4Sb12 were all presented in the temperature range of 314.9–673K. The transport properties measured on the synthesized samples have been compared with each other. The figures of merit, ZT, of samples all increased with the increasing temperature. Among all the samples, In0.5Ba0.2Co4Sb12 showed the highest power factor of 30.3μWcm−1K−2 at 643.5K, the lowest lattice thermal conductivity 1.128Wm−1K−1 at 643.5K and the maximum ZT value 0.80 at 673K.

High-Field Phase Diagram of SmRu4P12Determined by Ultrasonic Measurements in Pulsed Magnetic Field up to 55 T

Elastic constant measurements were performed for the filled skutterudite compound SmRu4P12 in a pulsed magnetic field up to 55 T of \(H\parallel\langle 111\rangle\) to create a high-field phase diagram. The field dependence of the transverse elastic constant \(C_{\text{T}} = (C_{11} - C_{12} + C_{44})/3\) shows two marked anomalies, one of which was assigned as a magnetic transition \(T^{*}\). The other anomaly was newly discovered. The newly discovered phase boundary is immersed in the known phase III, and joins to the known II–III boundary. This particular behavior is compared with the recently proposed theory, and discussed on the basis of the octupole model for this system.

Thermoelectric Properties of RuSb2Te Ternary Skutterudites

Polycrystalline samples of the RuSb2Te ternary skutterudite compound were prepared by the powder metallurgy method, and the influence of various types of doping on its thermoelectric properties was studied. The phase purity of the prepared samples was checked by means of powder x-ray diffraction, and their compositions were checked by electron probe x-ray microanalysis. Hot-pressed p-type samples were characterized by measurements of electrical conductivity, Hall coefficient, Seebeck coefficient, and thermal conductivity. Various doping strategies, i.e., cation substitution (Ru0.95Fe0.05Sb2Te), anion substitution (RuSb2Sn0.1Te0.9) or partial filling of voids of the ternary skutterudite structure (Yb0.05RuSb2Te), were investigated, and the influence of the dopants on the changes of the resulting transport, thermoelectric, and thermal properties is described.

Indications of a magnetic‐field‐driven anisotropy in the hybridization gap semiconductor CeOs4As12

AbstractThe filled skutterudite compound CeOs4As12 displays phenomena that are associated with hybridization between the 4f‐electron and conduction‐electron states. This holds especially true for a semiconducting ground state with a main energy gap as small as Δ/kB = 45 K. When subjected to magnetic field, the hybridization gap semiconductor CeOs4As12 becomes metallic below around 2.5 K. Preliminary electrical‐resistivity experiments provide evidence for an anisotropy of magnetic‐field‐induced insulator–metal transition in this cubic material. Our findings suggest a contribution of the quartet $4{\rm f \hbox{-} }\Gamma _{67}^{- } $ states to transport properties of CeOs4As12.

Rapid preparation of CeFe4Sb12 skutterudite by melt spinning: rich nanostructures and high thermoelectric performance

In this work, we adopt a non-equilibrium melt spinning technique combined with a subsequent spark plasma sintering technique to successfully synthesize a p-type nanostructured CeFe4Sb12 skutterudite compound with high homogeneity in less than 24 hours. Microstructures of the melt-spun ribbons and the sintered bulk material are systematically investigated. The evolution of multiple-phase melt-spun ribbons into a single-phase skutterudite compound during the heating process is also carefully examined. Greatly refined matrix grains (300–500 nm) and numerous FeSb2 nanodots with sizes below 50 nm are evenly distributed inside the grains, and together contribute to the experimentally observed low lattice thermal conductivity of the sintered bulk material. Both absolute and average ZT values of this melt-spun skutterudite are about 10% higher than in the material of the same composition prepared by traditional melting and long-term annealing. The markedly shortened preparation time coupled with the enhanced thermoelectric performance should make this synthesis process of interest for commercial applications.

Inelastic neutron scattering study of Ni-substituted Ce0.5Fe4Sb12 skutterudite compounds

An inelastic neutron scattering study of the filled and partially-filled skutterudite compounds RFe4Sb12 and R0.5Fe2.75Ni1.25Sb12 (where R = Ce and La) was carried out to understand the nature of the spin dynamics. Strong magnetic scattering was observed in Ce0.5Fe2.75Ni1.25Sb12 at ~ 5 meV. The integrated intensity of this peak does not follow the Ce3+ form factor, but exhibits a maximum at a momentum transfer (|Q|) of 2 Å−1. We attribute this feature to a Ce3+ crystal field excitation in the presence of magnetic exchange interactions. This picture is supported by thermodynamic and magnetic properties. Finally, we confirm the presence of a spin gap in CeFe4Sb12 suggested by our previous work.

Magnetic and electrical properties of filled skutterudite compound (Sm1−xRx)Ru4P12 (R=La, Y)

Sm-site substitution effect on filled skutterudite compound SmRu4P12, which shows an intriguing metal-insulator (MI) transition, has been studied by electrical resistivity, magnetic susceptibility and specific heat measurements. Single-phase polycrystalline (Sm1−xLax)Ru4P12 (x=0.1, 0.2, 0.3) and (Sm1−xYx)Ru4P12 (x=0.1, 0.2, 0.3) have been prepared at high temperature and high pressure. Both La and Y substitutions cause a large suppression of the semiconductor-like behavior at low temperature. The MI transition temperature (TMI) shifts to lower temperature with increasing La concentration. This change is qualitatively consistent with external pressure experiments. It could be due to a negative chemical pressure effect induced by La substitution. However, TMI also shifts to lower temperature with increasing Y concentration, which is expected for a positive chemical pressure effect. The results suggest that introducing a disorder into the periodicity of Sm affects sensitively the MI transition in this system.

Dependence of the superconducting transition temperature of the filled skutterudite compound PrPt4Ge12 on hydrostatic pressure

The temperature-dependent ac susceptibility of the filled skutterudite superconductor PrPt4Ge12 has been measured under hydrostatic He-gas pressure to 0.58GPa. The superconducting transition temperature Tc decreases linearly with pressure P from 7.91K at ambient pressure to 7.83K at 0.58GPa, giving the rate dTc/dP=−0.19±0.03K/GPa. Evidence is presented that suggests that the value of Tc in this compound is slightly reduced due to magnetic pair-breaking effects from the Pr3+ cations.

Influence of ZnO nano-inclusions on the transport properties of the CoSb3 skutterudite

Addition of nanoparticles into bulk materials is one of the strategies designed to synthesize thermoelectric materials with superior efficiency. The presence of nanoparticles is expected to act as a barrier of the heat transport due to an increased amount of nanoscopic interfaces and to enhance the electronic properties via quantum confinement effects. Here, this approach was applied to the skutterudite CoSb3 matrix in which different amounts of ZnO (2–12wt.%) nanoparticles were introduced. The samples were densified by spark plasma sintering and characterized by X-ray diffraction and scanning electron microscopy. The thermoelectric properties were investigated in a broad temperature range (2–800K) to better assess the influence of the ZnO nanoparticles. The nanostructured materials showed a significant enhancement in the thermopower values combined with a slight decrease in the thermal conductivity. A small amount of ZnO nanoparticles (2wt.%) has a beneficial influence on the dimensionless thermoelectric figure of merit ZT suggesting that introducing stable oxide nanoparticles may be an interesting way to improve the thermoelectric properties of skutterudite compounds.

Lower Thermal Conductivity and Higher Thermoelectric Performance of Fe-Substituted and Ce, Yb Double-Filled p-Type Skutterudites

Substituting Fe on Co sites is an effective way to produce p-type skutterudite compounds as well as to reduce the thermal conductivity of skutterudites. In this work, we investigated thermoelectric properties of Fe-substituted and Ce + Yb double-filled Ce x Yb y Fe z Co4−z Sb12 (x = y = 0.5, z = 2.0 to 3.25 nominal) skutterudite compounds by studying the Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient over a broad range of temperatures. All samples were prepared by using the traditional method of melting–annealing and spark plasma sintering. The signs of the Hall coefficient and Seebeck coefficient indicate that all samples are p-type conductors. Electrical conductivity increases with increasing Fe content. The temperature dependence of electrical conductivity indicates that a transition from the extrinsic to the intrinsic regime of conduction depends on the amount of Fe substituted for Co. The temperature dependence of mobility reflects the dominance of acoustic phonon scattering at temperatures above ambient. Except for Ce0.5Yb0.5Fe3.25Co0.75Sb12, the thermal conductivity increases with increasing Fe content, reaching the maximum value of 2.23 W/m K at room temperature for Ce0.5Yb0.5Fe3CoSb12. A high power factor (27 μW/K2 cm) combined with a rather low thermal conductivity for Ce0.5Yb0.5Fe3.25Co0.75Sb12 (nominal) lead to a dimensionless figure of merit ZT = 1.0 at 750 K for this compound, one of the highest ZT values achieved in p-type skutterudite compounds prepared by the traditional method of melting–annealing and spark plasma sintering.

Influence of Ni impurities on the thermoelectric properties of Ca-partially filled skutterudites CaxCo4Sb12

Polycrystalline samples of the skutterudite compounds CaxCo4Sb12 were synthesized and investigated by means of electrical resistivity, thermal conductivity, and thermopower in the 300–800 K temperature range. Additional Hall effect measurements were performed between 2 and 300 K. Both Ni-free and Ni-containing Co powders were used as starting materials to elucidate the impact of Ni impurities at the ppm level on the thermoelectric properties. The presence of minute amounts of Ni atoms in the crystal structure leads to enhanced thermopower values with respect to the Ni-free samples, likely associated with the development of additional electron pockets near the Fermi level. Ni impurities, thus, play a significant role on the transport properties and lead to a spectacular increase in the dimensionless thermoelectric figure of merit ZT of up to 150% at 800 K.

Quadrupole Susceptibility of Gd-Based Filled Skutterudite Compounds

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, the ground-state wavefunction is found to contain significant amount of the $j$-$j$ coupling component. From the evaluation of quadrupole susceptibility in a simple mean-field approximation, we point out a possibility to detect the softening of elastic constant in Gd-based filled skutterudites.