Binary Skutterudites Research Articles

High-Throughput Prediction of the Thermal and Electronic Transport Properties of Large Physical and Chemical Spaces Accelerated by Machine Learning: Charting the ZT of Binary Skutterudites.

Thermal and electronic transport properties are the keys to many technological applications of materials. Thermoelectric, TE, materials can be considered a singular case in which not only one but three different transport properties are combined to describe their performance through their TE figure of merit, ZT. Despite the availability of high-throughput experimental techniques, synthesizing, characterizing, and measuring the properties of samples with numerous variables affecting ZT are not a cost- or time-efficient approach to lead this strategy. The significance of computational materials science in discovering new TE materials has been running in parallel to the development of new frameworks and methodologies to compute the electron and thermal transport properties linked to ZT. Nevertheless, the trade-off between computational cost and accuracy has hindered the reliable prediction of TE performance for large chemical spaces. In this work, we present for the first time the combination of new ab initio methodologies to predict transport properties with machine learning and a high-throughput framework to establish a solid foundation for the accurate prediction of thermal and electron transport properties. This strategy is applied to a whole family of materials, binary skutterudites, which are well-known as good TE candidates. Following this methodology, it is possible not only to connect ZT with the experimental synthetic (carrier concentration and grain size) and operando (temperature) variables but also to understand the physical and chemical phenomena that act as driving forces in the maximization of ZT for p-type and n-type binary skutterudites.

Influence of lattice strain on the mechanical properties of CoSb3 skutterudites

Elemental doping has been widely employed to manipulate the electrical and thermal transport properties of thermoelectric materials, without considering the influence to mechanical properties. In skutterudites, electronegative S-dopants have been found to reduce the lattice thermal conductivity. However, the effect of the covalent S-Sb bond on the mechanical properties of CoSb3 skutterudites remains unexplored. Compressive strength, in particular, exhibits a significant reduction in S-doped skutterudites, measuring only 249 MPa, representing a nearly 50% decrease compared to binary skutterudites. Additionally, there is a noticeable drop in the elastic, bulk and shear modulus. With molecular dynamics calculations, the shear strength of S-doped skutterudites also follows a sharp drop at a small ultimate strain. The evolutions of atomic configurations reveal that the lattice strain induced by S-dopants softens the Sb-Sb bond, ultimately leading to structure collapse under low shear stress conditions. These findings underscore the necessity for a comprehensive approach to elemental doping in thermoelectric community.

Enhanced Thermoelectric Performance in Ternary Skutterudite Co(Ge0.5Te0.5)3 via Band Engineering.

We report the phase evolution and thermoelectric properties of a series of Co(Ge0.5Te0.5)3-xSbx (x = 0-0.20) compositions synthesized by mechanical alloying. Pristine ternary Co(Ge0.5Te0.5)3 skutterudite crystallizes in the rhombohedral symmetry (R3̅), and Sb doping induces a structural transition to the cubic phase (ideal skutterudite, Im3̅). The Sb substitution increases the carrier concentration while maintaining a high thermopower even at higher doping levels owing to an increased effective mass. The exceptional electronic properties exhibited by Co(Ge0.5Te0.5)3 upon doping are attributed to the carrier transport from both the primary and secondary conduction bands, as shown by theoretical calculations. The enhanced electrical conductivity and high thermopower increase the power factor by more than 20 times. Because the dominant phonon propagation modes in binary skutterudites are associated with the vibrations of pnictogen rings, twisting the latter through the isoelectronic replacement of Sb4 rings with Ge2Te2 ones, as done in this study, can effectively reduce the thermal conductivity. This leads to an increase in the dimensionless figure-of-merit (zT) by a factor of 30, reaching 0.65 at 723 K for Co(Ge0.5Te0.5)2.9Sb0.1.

Nanosize La-filled CoSb3 skutterudite fabricated by electrospinning

Nanostructured binary skutterudites represented by MX3 are potential thermoelectric materials for high efficiency thermoelectric. In this work, we synthesized for the first-time high purity nanofibers of CoSb3 and rare earth filled LaCo4Sb12 with external diameters < 100 nm via electrospinning which provides a simple, easy to implement and versatile technique to develop novel nanostructured-based thermoelectric materials.

Effect of Pr-filling in binary skutterudites CoX3 (P, As and Sb) on structural, electronic, elastic and transport properties

ABSTRACTWe have studied the effect on the structural, electronic, elastic and transport properties of binary skutterudite CoX3 (X = P, As, Sb) after filling void spaces by Pr atoms. All the calculations are carried out using the full-potential linearised augmented plane wave (FP-LAPW) method within the generalised gradient approximation (GGA) viz, PBE, and PBEsol. In case of binary skutterudites, equilibrium lattice parameters obtained using PBEsol functional are in good agreement with the theoretical and experimental results. The Hubbard parameter (U) has been used with PBEsol-GGA functional to see its effect on the band structures of binary and ternary compounds. Filling of Pr atoms at void positions in binary skutterudites creates the gap between the valence and conduction band. The obtained values of the elastic constants show that CoX3, PrCo4X12 (X = P, As, Sb) are mechanically stable and brittle. Mechanically, PrCo4P12 and PrCo4As12 are anisotropic, but PrCo4Sb12 is isotropic. Obtained saturated Seeback coefficients are approximate −60 μV/K (PrCo4P12), −80 μV/K (PrCo4As12) and −68 μV/K (PrCo4Sb12) in the spin-up channel while in the spin-down channel corresponding values are −10, −50 and −120 μV/K at 800 K, respectively. These values are higher in magnitude than that in the corresponding binary compounds.

Theoretical investigations of thermoelectric phenomena in binary semiconducting skutterudites.

In this study, we explored the thermoelectric properties of the host thermoelectric materials (TM), namely, binary skutterudites, using a combination of simulations based on density functional theory and post-DFT Boltzmann's semiclassical theory. The calculations were performed close to the Fermi surface for the Seebeck coefficient and other thermoelectric parameters. Our results demonstrated that CoSb3 exhibited the highest Seebeck value at room temperature among all the compounds (CoP3, CoAs3, CoSb3, IrP3, IrAs3, IrSb3, RhAs3, and RhSb3), which confirmed that this compound is an ideal host material for thermoelectric applications. Furthermore, the calculated electrical conductivity values show that RhAs3 has the largest value of 3.736 × 105 Ω−1 m−1. However, at high temperatures, the Seebeck values for all of these compounds are almost constant due to the activation of the minority charge carriers.

Electronic structure and thermoelectric properties of pnictogen-substituted ASn1.5Te1.5 (A = Co, Rh, Ir) skutterudites

Substituting group 14 and 16 elements on the pnictogen site in the skutterudite structure yields a class of valence-precise ternary AX1.5Y1.5 compounds (A = Co, Rh, Ir, X = Sn, Ge, and Y = S, Se, Te), in which X and Y form an ordered sub-structure. Compared with unfilled binary skutterudites, pnictogen-substituted phases exhibit extremely low lattice thermal conductivity due to increased structural complexity. Here, we investigate the role of the transition metal species in determining the electronic structure and transport properties of ASn1.5Te1.5 compounds with A = Co, Rh, Ir. Density functional calculations using fully ordered structures reveal semiconducting behavior in all three compounds, with the band gap varying from 0.2 to 0.45 eV. In CoSn1.5Te1.5, the electronic density of states near the gap is significantly higher than for A = Ir or Rh, leading to higher effective masses and higher Seebeck coefficients. Experimentally, Ir and Rh samples exhibit relatively large p-type carrier concentrations and degenerate semiconducting behavior. In contrast, CoSn1.5Te1.5 shows mixed conduction, with n-type carriers dominating the Seebeck coefficient and light, high mobility holes dominating the Hall coefficient. zT values of up to 0.35 were obtained, and further improvement is expected upon optimization of the carrier concentration or with n-type doping.

Electronic band structures of binary skutterudites

The electronic properties of complex binary skutterudites, MX3 (M = Co, Rh, Ir; X = P, As, Sb) are explored, using various density functional theory (DFT) based theoretical approaches including Green's Function (GW) as well as regular and non-regular Tran Blaha modified Becke Jhonson (TB-mBJ) methods. The wide range of calculated bandgap values for each compound of this skutterudites family confirm that they are theoretically as challenging as their experimental studies. The computationally expensive GW method, which is generally assume to be efficient in the reproduction of the experimental bandgaps, is also not very successful in the calculation of bandgaps. In this article, the issue of the theoretical bandgaps of these compounds is resolved by reproducing the accurate experimental bandgaps, using the recently developed non-regular TB-mBJ approach, based on DFT. The effectiveness of this technique is due to the fact that a large volume of the binary skutterudite crystal is empty and hence quite large proportion of electrons lie outside of the atomic spheres, where unlike LDA and GGA which are poor in the treatment of these electrons, this technique properly treats these electrons and hence reproduces the clear electronic picture of these compounds.

BoltzWann: A code for the evaluation of thermoelectric and electronic transport properties with a maximally-localized Wannier functions basis

We present a new code to evaluate thermoelectric and electronic transport properties of extended systems with a maximally-localized Wannier function basis set. The semiclassical Boltzmann transport equations for the homogeneous infinite system are solved in the constant relaxation-time approximation and band energies and band derivatives are obtained via Wannier interpolations. Thanks to the exponential localization of the Wannier functions obtained, very high accuracy in the Brillouin zone integrals can be achieved with very moderate computational costs. Moreover, the analytical expression for the band derivatives in the Wannier basis resolves any issues that may occur when evaluating derivatives near band crossings. The code is tested on binary and ternary skutterudites CoSb3 and CoGe3/2S3/2. Program summaryProgram title: BoltzWannCatalogue identifier: AEQX_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEQX_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 710810No. of bytes in distributed program, including test data, etc.: 8337000Distribution format: tar.gzProgramming language: Fortran 90.Computer: Any architecture with a Fortran 90 compiler.Operating system: Linux, Windows, Solaris, AIX, Tru64 Unix, OSX.Has the code been vectorized or parallelized?: Yes.RAM: The example requires approximately 10 MB.Classification: 7.3, 7.9.External routines:BLAS and LAPACK (available on http://www.netlib.org/); MPI libraries (optional) for parallel executionNature of problem:Obtain electronic and thermoelectric transport properties for crystals.Solution method:The Boltzmann transport equations in the constant relaxation-time approximation are used. These equations require the integration of the band velocities over all the Brillouin zone; this is done numerically on a sufficiently dense k grid. Band energies and band derivatives are obtained by interpolation using the maximally-localized Wannier functions basis obtained with a preliminary run of the Wannier90 code.Unusual features:The maximally-localized Wannier functions interpolation scheme allows the use of analytical formulas (instead of finite-difference methods) to obtain the band derivatives.Additional comments:This is a package that is tightly integrated with the Wannier90 code (http://www.wannier.org). The Wannier90 code is included in the distribution package.Running time:The example runs (in its serial version) in less than 2 min.

Compression Behaviors of Binary Skutterudite CoP3 in Noble Gases up to 40 GPa at Room Temperature

The binary skutterudite CoP(3) has a large void at the body-centered site of each cubic unit cell and is, therefore, called a nonfilled skutterudite. We investigated its room-temperature compression behavior up to 40.4 GPa in helium and argon using a diamond-anvil cell. High-pressure in situ X-ray diffraction and Raman scattering measurements found no phase transition and a stable cubic structure up to the maximum pressure in both media. A fitting of the present pressure-volume data to the third-order Birch-Murnaghan equation of state yields a zero-pressure bulk modulus K(0) of 147(3) GPa [pressure derivative K(0)' of 4.4(2)] and 171(5) GPa [where K(0)' = 4.2(4)] in helium and argon, respectively. The Grüneisen parameter was determined to be 1.4 from the Raman scattering measurements. Thus, CoP(3) is stiffer than other binary skutterudites and could therefore be used as a host cage to accommodate large atoms under high pressure without structural collapse.

Ternary Skutterudites: Anion Ordering and Thermoelectric Properties

AbstractThe ternary skutterudites AB1.5Q1.5(A = Co, Rh, Ir, B =Ge, Sn and Q = S, Te) have been synthesized and structurally characterised. Powder diffraction data are consistent with the presence of anion ordering, which results in a lowering of the symmetry from cubic to rhombohedral. The ordered skutterudite structure contains rhomboidal four-membered B2Q2rings, instead of the homonuclear rectangular rings found in binary skutterudites. The electrical transport properties of the AB1.5Q1.5phases are consistent with semiconducting behaviour, and large values of the Seebeck coefficient have been observed for several of these materials. While their large electrical resistivities might make these materials unsuitable for thermoelectric applications unless significant improvements can be achieved by doping, these materials also exhibit significantly lower thermal conductivities than their binary counterparts.

Application of spark plasma sintering to the fabrication of binary and ternary skutterudites

Spark plasma sintering (SPS) synthesis of CoSb3 and LaFe4Sb12 from ball-milled elemental metal powders has been performed obtaining samples with high relative densities (∼99%). Fracture behaviour, the microstructure and mechanical properties were studied. From ultrasound and nano-indentation measurements, Young’s moduli of 148 GPa for CoSb3 and 141 GPa for LaFe4Sb12 were deduced. SPS has been also applied to compact alkali metal and alkaline-earth metal iron antimonides with filled skutterudite structure.

Investigations of Bonding in Skutterudites by Electron Energy-Loss Spectroscopy

AbstractThe local electronic structure of phosphorus in the binary skutterudites CoP3 and NiP3, and in the filled skutterudite LaFe4P12 are studied using a combination of electron energy-loss spectroscopy and ab initio calculations. Relative to CoP3 we observe a filling of phosphorus s and d states in NiP3, while for LaFe4P12 increased EELS intensity indicates more empty s and d states close to the Fermi-level.

Experimental and theoretical studies of plasma resonance and the electronic structure of binary skutterudites

ABSTRACTWe determine the plasmon energies of the skutterudites CoP3, CoAs3 and CoSb3 by electron energy loss spectroscopy, and compare with calculated values from the Drude model and density functional theory (DFT). For these compounds, whose doped versions have potential applications as thermoelectric materials, there is a relatively large discrepancy between experiment and theory based on the Drude model as well as the DFT-calculations. We also study the transitions from occupied to unoccupied states near the Fermi-level that show up at energies lower than the plasmon energies. The features observed are in general agreement with the DFT-calculations.

X-ray study with synchrotron radiation for P- and Sb-based skutterudite compounds at high pressures

Using synchrotron radiation, the x-ray diffraction of P- and Sb-based skutterudite compounds has systematically been studied at room temperature and high pressures. Bulk moduli of binary skutterudites CoX3 (X = P and Sb), filled skutterudites CeT4X12 (T = Fe, Ru and Os), LaRu4X12and PrRu4X12 are obtained from the volume versus pressure curves fitted by a Birch equation of state. Bulk moduli are 150–225 GPa for P-based skutterudites and 80–115 GPa for Sb-based skutterudites. The bulk modulus of the phosphides is about two times that of the corresponding antimonides. The antimonides are very compressible compared with the phosphides. The Grüneisen constants (γ) for skutterudite compounds are estimated from the compressibility and its volume derivative in the Debye approximation. The bulk modulus of CoX3 and CeT4X12 increases with increasing lattice constant, attaining a maximum for the Ru compounds. Bulk moduli for the Ru-based skutterudites are larger than those of the corresponding Fe and Os compounds. This may be closely related to the interesting physical properties in the Ru-based skutterudites. The bulk modulus of the binary skutterudites is smaller than that of the filled skutterudites. The cell volume for CeT4X12 (T = Fe, Ru and Os; X = P and Sb) decreases monotonically with increasing pressure. The valence state of Ce ion in these compounds does not change at high pressures.

Conditions for the Electrochemical Synthesis of the CoPn3 ( Pn = P , As , Sb ) Skutterudites

The binary pnictide skutterudites, have been successfully synthesized for the first time by molten salt electrolysis. The melt for depositing contained CoO and and graphite rods were used for both anode and cathode. All experiments were run in a flowing gas atmosphere. By adjusting the melt composition, single phase could be prepared. Attempts were also made to deposit using platinum and silver strip electrodes. Although powder was obtained in the crucible, no deposit was found on the Pt cathode, which was attacked by the phosphorus present in the melt and produced platinum phosphide A low deposition rate was observed for Ag cathodes, and as a result, a very small amount of material was deposited. This made it difficult to determine the type and number of phases formed. Using graphite electrodes, two other cobalt skutterudites were prepared. was synthesized from a melt and from a melt. While the skutterudite phases were formed in these systems, the optimal melt compositions and other experimental conditions necessary to prepare single phase deposits still need to be determined. © 2001 The Electrochemical Society. All rights reserved.

Excimer Pulsed Laser Ablated Molecular Beam Evaporation for the Deposition of Binary Skutterudites

AbstractThe family of materials bearing the skutterudite crystallographic structure have recently attracted interest because of their extra-ordinary electrical and thermal transport properties, as well as their peculiar magnetic qualities [1]. Particularly, the group of cobalt antimonide skutterudites has been referred to as electron crystals and phonon glasses. With the goal to prepare skutterudite thin films, laser ablation is unique in its capacity to transfer skutterudite target's complex composition untouched to substrates while providing an hype thermal beam that has a positive influence on thin film growth. We use a pulsed excimer laser at 193 nm to evaporate skutterudites targets in ultra-high vacuum and deposit thins films on silicon wafers and fused silica substrate. We have studied the composition and crystallographic structure of the deposits as a function of substrate temperature and target to substrate distance. Deposits are skutterudite policrystals of several hundreds nanometers. We investigated the electrical as well as the thermal transport properties of thin films having the desired phase and compare these with value reported for the bulk mono-crystals.

Raman spectroscopy investigation of lanthanide-filled and unfilled skutterudites

Raman-scattering spectra at room temperature and 10 K are studied for binary ``unfilled'' and lanthanide-filled skutterudite antimonides. In the binary skutterudites ${\mathrm{IrSb}}_{3}$ and ${\mathrm{RhSb}}_{3}$ all eight Raman-active vibrational modes have been identified while seven of the eight modes have been identified in ${\mathrm{CoSb}}_{3}.$ In the case of the filled skutterudites these vibrational modes are sensitive to the lanthanide ``filler'' ions which produce a shift and large broadening of the vibrational modes. Polarization measurements on these polycrystalline samples were used to identify the ${A}_{g}$ modes. The experimental data are compared to theoretical predictions.

熱電変換 （ＩＩＩ） スクッテルダイト系熱電材料の研究状況

A new class of thermoelectric materials, skutterudites and filled skutterudites, has become a topic because the materials have a dimensionless thermoelectric figure of merit ZT of 1 at 800K. Skutterudites are binary semiconducting compounds with the general formula MX3 (where M is Co, Rh, Ir; and X is P, As Sb). Within these materials, the three isothermal antimonides such as IrSb3, RhSb3, and CoSb3 exhibit excellent electronic transport properties, including a high value of the hole mobility (8000cm2/Vs). For example, p-type CoSb3 has a hole mobility value of 3000cm2/Vs at room temperature, and ZT of Pd/Pt-doped CoSb3 is 0.85.Filled skutterudite alloys with the general formula of RM′4X12, where R is La, Ce, Pr, Nd or Eu and M′ is Fe, Ru or Os, are particularly interesting, because formally they are isoelectric (that is, have the same number of valence electrons per unit cell) with the binary skutterudites such as CoSb3. Relative to CoSb3, the filled skutterudites exhibit a remarkable factor-of-ten decrease in the lattice thermal conductivity, which reaches a value at room temperature equal to that of vitreous silica (SiO2). This is believed to be due to the “rattling” effect of the rare earth atoms that scatters the heat-carrying phonons. For example, CeFe3CoSb12 or LaFe3CoSb12 has a low lattice thermal conductivity value of about 0.015W/cmK at near room temperature.

X-ray structure determination of pseudomerohedrally twinned CoGe1.5S1.5 and CoGe1.5Se1.5, absorption correction

Abstract The crystal structures of the mixed skutterudites CoGe1.5S1.5 and CoGe1.5Se1.5 were determined by X-ray diffraction of powder samples and pseudomerohedral multiple twins (space group R3, No. 146, Z = 24, CoGe1.5S1.5: a = 1133.63(10) pm, c = 1391.76(9) pm, R(F) = 2.2%, 4288 unique reflections; CoGe1.5Se1.5: a = 1173.00(8) pm, c = 1441.1(3) pm, R(F) = 2.8%, 828 unique reflections). The triple pseudomerohedral twins are connected by three twofold rotation axes, which are linked by a threefold rotation axis. An absorption correction was performed by an intensity and volume ratio weighted empirical method. The intensity data for CoGe1.5Se1.5 were collected in two different azimuth and θ positions in order to minimise the absorption effects. The structures obtained are compared with those of binary skutterudites with respect to symmetry relations of the space groups and bonding of the rhombic Ge2X2 units (X = S, Se), which partly reveal antiaromatic 4π electron behavior.