Zintl phases and related compounds are promising thermoelectric materials; for instance, high zT has been found in Yb14MnSb11, clathrates, and the filled skutterudites. The rich solid-state chemistry of Zintl phases enables numerous possibilities for chemical substitutions and structural modifications that allow the fundamental transport parameters (carrier concentration, mobility, effective mass, and lattice thermal conductivity) to be modified for improved thermoelectric performance. For example, free carrier concentration is determined by the valence imbalance using Zintl chemistry, thereby enabling the rational optimization of zT. The low thermal conductivity values obtained in Zintl thermoelectrics arise from a diverse range of sources, including point defect scattering and the low velocity of optical phonon modes. Despite their complex structures and chemistry, the transport properties of many modern thermoelectrics can be understood using traditional models for heavily doped semiconductors.

Zintl Chemistry for Designing High Efficiency Thermoelectric Materials

Thermoelectric clathrates hold significant promise for high temperature applications with zT values exceeding 1.3. The inorganic clathrates have been shown to be both chemically and thermally stable at high temperatures, and high performance can be obtained from both single crystals and processed powders. The clathrates also show excellent compatibility factors in segmented module applications. For a materials chemist it is furthermore of great importance that the clathrates exhibit a very rich chemistry with the ability for substitution of many different elements. This allows delicate tuning of both the crystal structure as well as the physical properties. With all these assets, it is not surprising that clathrates have been intensely investigated in the thermoelectric community during the past decade. The present perspective provides a review of the many studies concerned with the synthesis, crystal structure and thermoelectric properties of clathrates with emphasis on the type I clathrate.

Thermoelectric clathrates of type I

Mechanical properties of half-Heusler alloys

Type I clathrates have recently been identified as prospective thermoelectric materials for power generation purposes due to their very low lattice thermal conductivity values. The maximum thermoelectric figure of merit of almost all type I clathrates is, however, less than 1 and occurs at, or above, 1000 K, making them unfavorable especially for intermediate temperature applications. In this report, the Zintl-Klemm rule is demonstrated to be valid for Ni, Cu, and Zn transition metal substitution in the framework of type I clathrates and offers many degrees of freedom for material modification, design, and optimization. The cross-substitution of framework elements introduces ionized impurities and lattice defects into these materials, which optimize the scattering of charge carriers by the substitution-induced ionized impurities and the scattering of heat-carrying lattice phonons by point defects, respectively, leading to an enhanced power factor, reduced lattice thermal conductivity, and therefore improved thermoelectric figure of merit. Most importantly, the bandgap of these materials can be tuned between 0.1 and 0.5 eV by adjusting the cross-substitution ratio of framework elements, making it possible to design clathrates with excellent thermoelectric properties between 500 and 1000 K.

On the Design of High-Efficiency Thermoelectric Clathrates through a Systematic Cross-Substitution of Framework Elements

Single crystals of Ba8Au5.3Ge40.7 [space group Pm3n (No. 223), a = 10.79891(8) A] were prepared by a Bridgman technique. The crystal structure refinement based on single-crystal X-ray diffraction data does not reveal any vacancies in the Au/Ge framework or in the cages. In addition to the ionic bonding between Ba and the anionic framework, a direct interaction between Ba and Au atoms was identified in Ba8Au5.3Ge40.7 by applying the electron localizability indicator. As expected by the chemical-bonding picture, Ba8Au5.3Ge40.7 is a diamagnet and shows p-type electrical conductivity with a hole carrier concentration of 7.14 × 1019 cm−3 at 300 K and very low lattice thermal conductivity of ≈0.6 W m−1 K−1 at 500 K. The thermoelectric figure of merit ZT of single crystals of Ba8Au5.3Ge40.7 attains 0.3 at 511 K and reaches 0.9 at 680 K in a polycrystalline sample of closely similar composition. This opens up an opportunity for tuning of the thermoelectric properties of materials in the Ba−Au−Ge clathrate system...

Atomic Interactions in the p-Type Clathrate I Ba8Au5.3Ge40.7

Clathrate formation in the Ba-Pd-Ge system : Phase equilibria, crystal structure, and physical properties

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 semiconducting behaviour. The corresponding gap width was extracted from maxima in the Seebeck coefficient data as a function of temperature. Temperature dependent single crystal X-ray structure analyses (at 100 K, 200 K and 300 K) revealed the thermal expansion coefficients, Einstein and Debye temperatures for two selected samples Ba0.73Ni4Sb8.1Sn3.9 and Ba0.95Ni4Sb6.1Sn5.9. These data compare well with Debye temperatures from measurements of specific heat (4.4 K < T < 200 K). 
Several mechanical properties were measured and evaluated. Thermal expansion coefficients are 11.8.10-6 K-1 for Ni4Sb8.2Sn3.8 to 13.8.10-6 K-1 for Ba0.92Ni4Sb6.7Sn5.3. Room temperature Vicker's hardness values (up to a load of 24.5 mN) vary within the range of 2.6 GPa to 4.7 GPa. Severe plastic deformation (SPD) via high-pressure torsion (HPT) was used to introduce nanostructuring. Physical properties before and after HPT were compared, showing no significant effect on the material's thermoelectric behaviour.

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On Ni-Sb-Sn based skutterudites

系Zr-Ni-Snの構造と熱力学的モデリング【Powered by NICT】

Ternary YbPt2B crystallizes in the non-centrosymmetric hexagonal CePt2B-type structure (space group P6222). Electrical resistivity, specific heat and magnetic measurements reveal a magnetic instability at 5.6 K. Furthermore, a spin-reorientation of presumably a ferromagnetic type occurs around 1.5 K. The behaviour at low temperature is governed by a rather weak Kondo effect, TK ≤ 1 K, in the presence of strong crystalline electric field splitting, with a doublet ground state. Besides, the complex magnetic behaviour presumably results from a Dzyaloshinskii–Moriya interaction triggered by the absence of inversion symmetry in the crystal structure. Scaling according to the de Gennes factor traces back magnetic ordering in YbPt2B to the Rudermann–Kittel–Kasuya–Yoshida (RKKY) interaction and the smooth evolution of the lattice constants and the unit cell volume of REPt2B (RE = rare earths) refers to the 4f13 electronic configuration of Yb in YbPt2B.

https://iopscience.iop.org/article/10.1088/0953-8984/27/14/146001/pdf

G. Giester

Papers

Clathrate formation in the Ba-Pd-Ge system : Phase equilibria, crystal structure, and physical properties

On Ni-Sb-Sn based skutterudites

系Zr-Ni-Snの構造と熱力学的モデリング【Powered by NICT】

Thermal, magnetic and electronic properties of non-centrosymmetric YbPt2B

Editorial Note: Itinerant electron metamagnetism in LaCo 9 Si 4 [Phys. Rev. B 69 , 081404(R) (2004)]