Synthesis, Crystal Structure, and Physical Properties of the Type-I Clathrate Ba8−δNix□ySi46–x–y

Crystal Chemistry and Thermoelectric Properties of Type-I Clathrate Ba₈Ni∼3.8SixGe42.2-x (x = 0, 10, 20, 42.2)

Thermoelectric materials are actively considered for waste heat recovery applications. To improve the heat to electricity conversion efficiency, fundamental understanding on composition, crystal structure, and interrelation with the thermoelectric properties is necessary. Here, we report the chemical and thermoelectric properties of type-I clathrates Ba 8 Ni 3.8 Si x Ge 42.2 - x (x = 0, 10, 20, 42.2), to show that the Si substitution can retain the low lattice thermal conductivity as in pure Ge-based clathrates by adding defects (cage distortion) scattering and/or alloying effect, and the charge carrier concentration can be optimized and thus the electronic properties can be improved by tailoring the vacancy content. We demonstrate the vacancies in the pure Ge-based compound by Rietveld refinement, and possible vacancies in the quaternary compound by transport property measurements. We also show that, for intrinsic property studies in these compounds with such a complex crystal structure, a heat treatment for as cast alloys is necessary for phase purity and composition homogeneity. The highest Z T value of 0.19 at 550 ° C is reached in the compound with x = 10 .

Anodes for Lithium-Ion Batteries Based on Type I Silicon Clathrate Ba8Al16Si30 - Role of Processing on Surface Properties and Electrochemical Behavior

Type I silicon clathrates based on Ba₈Al_ySi_46-y (8 < y < 12) have been studied as potential anodes for lithium-ion batteries and display electrochemical properties that are distinct from those found in conventional silicon anodes. Processing steps such as ball-milling (typically used to reduce the particle size) and acid/base treatment (used to remove nonclathrate impurities) may modify the clathrate surface structure or introduce defects, which could affect the observed electrochemical properties. In this work, we perform a systematic investigation of Ba₈Al_ySi_46-y clathrates with y ≈ 16, i.e, having a composition near Ba₈Al₁₆Si₃₀, which perfectly satisfies the Zintl condition. The roles of ball-milling and acid/base treatment were investigated using electrochemical, X-ray diffraction, electron microscopy, X-ray photoelectron and Raman spectroscopy analysis. The results showed that acid/base treatment removed impurities from the synthesis, but also led to formation of a surface oxide layer that inhibited lithiation. Ball-milling could remove the surface oxide and result in the formation of an amorphous surface layer, with the observed charge storage capacity correlated with the thickness of this amorphous layer. According to the XRD and electrochemical analysis, all lithiation/delithiation processes are proposed to occur in single phase reactions at the surface with no discernible changes to the crystal structure in the bulk. Electrochemical impedance spectroscopy results suggest that the mechanism of lithiation is through surface-dominated, Faradaic processes. This suggests that for off-stoichiometric clathrates, as we studied in our previous work, Li⁺ insertion at defects or vacancies on the framework may be the origin of reversible Li cycling. However, for clathrates Ba₈Al_ySi_46-y with y ≈ 16, Li insertion in the structure is unfavorable and low capacities are observed unless amorphous surface layers are introduced by ball-milling.

Type I Clathrates as Novel Silicon Anodes: An Electrochemical and Structural Investigation

Silicon clathrates contain cage-like structures that can encapsulate various guest atoms or molecules. An electrochemical evaluation of type I silicon clathrates based on Ba8Al y Si46-y as the anode material for lithium-ion batteries is presented here. Postcycling characterization with nuclear magnetic resonance and X-ray diffraction shows no discernible structural or volume changes even after electrochemical insertion of 44 Li (≈1 Li/Si) into the clathrate structure. The observed properties are in stark contrast with lithiation of other silicon anodes, which become amorphous and suffer from large volume changes. The electrochemical reactions are proposed to occur as single phase reactions at approximately 0.2 and 0.4 V versus Li/Li+ during lithiation and delithiation, respectively, distinct from diamond cubic or amorphous silicon anodes. Reversible capacities as high as 499 mAh g-1 at a 5 mA g-1 rate were observed for silicon clathrate with composition Ba8Al8.54Si37.46, corresponding to ≈1.18 Li/Si. These results show that silicon clathrates could be promising durable anodes for lithium-ion batteries.

Electronic band structure and low-temperature transport properties of the type-I clathrate Ba8Ni(x)Ge(46-x-y□y)

We present the evolution of the low-temperature thermodynamic, galvanomagnetic and thermoelectric properties of the type-I clathrate Ba8Ni(x)Ge(46-x-y□y) with the Ni concentration studied on polycrystalline samples with 0.0 ≤ x ≤ 6.0 by means of specific heat, Hall effect, electrical resistivity, thermopower and thermal conductivity measurements in the 2-350 K temperature range and supported by first-principles calculations. The experimental results evidence a 2a × 2a × 2a supercell described in the space group Ia3d for x ≤ 1.0 and a primitive unit cell a × a × a (space group Pm3n) above this Ni content. This concentration also marks the limit between a regime where both electrons and holes contribute to the electrical conduction (x ≤ 1.0) and a conventional, single-carrier regime (x > 1.0). This evolution is traced by the variations in the thermopower and Hall effect with x. In agreement with band structure calculations, increasing the Ni content drives the system from a nearly-compensated semimetallic state (x = 0.0) towards a narrow-band-gap semiconducting state (x = 4.0). A crossover from an n-type to a p-type conduction occurs when crossing the x = 4.0 concentration i.e. for x = 4.1. The solid solution Ba8Ni(x)Ge(46-x-y□y) therefore provides an excellent experimental platform to probe the evolution of the peculiar properties of the parent type-I clathrate Ba8Ge43□3 upon Ge/Ni substitution and filling up of the vacancies, which might be universal among the ternary systems at low substitution levels.

Rapid crystal growth of type-II clathrates A8Na16Si136 (A = K, Rb, Cs) by spark plasma sintering

Single crystals of clathrate-II A₈Na₁₆Si₁₃₆ (A = K, Rb, Cs) were synthesized by spark plasma sintering by simultaneous electrochemical redox and ion-exchange reactions.