Type-I clathrate compounds have attracted a great deal of interest in connection with the search for efficient thermoelectric materials. These compounds constitute networked cages consisting of nano-scale tetrakaidecahedrons (14 hedrons) and dodecahedrons (12 hedrons), in which the group 1 or 2 elements in the periodic table are encaged as the so-called rattling guest atom. It is remarkable that, though these compounds have crystalline cubic-structure, they exhibit glass-like phonon thermal conductivity over the whole temperature range depending on the states of rattling guest atoms in the tetrakaidecahedron. In addition, these compounds show unusual glass-like specific heats and THz-frequency phonon dynamics, providing a remarkable broad peak almost identical to those observed in topologically disordered amorphous materials or structural glasses, the so-called Boson peak. An efficient thermoelectric effect is realized in compounds showing these glass-like characteristics. This decade, a number of experimental works dealing with type-I clathrate compounds have been published. These are diffraction experiments, thermal and spectroscopic experiments in addition to those based on heat and electronic transport. These form the raw materials for this article based on advances this decade. The subject of this article involves interesting phenomena from the viewpoint of not only physics but also from the view point of the practical problem of elaborating efficient thermoelectric materials. This review presents a survey of a wide range of experimental investigations of type-I clathrate compounds, together with a review of theoretical interpretations of the peculiar thermal and dynamic properties observed in these materials.

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Phonon-glass electron-crystal thermoelectric clathrates : Experiments and theory

The chemistry of ternary nitrides is reviewed with special focus on developments of the last two years (1996 and 1997). In particular, structures and properties of ternary and higher transition metal nitrides, main group nitrides, subnitrides, and nitride halides are compared, and a section on thermodynamics of ternary nitrides is included. Finally, methods for the preparation of gallium nitride single crystals are summarized.

Recent developments in nitride chemistry

Silicon is a promising anode material for Li-ion batteries due to its high theoretical specific capacity. From previous work, silicon nanowires (SiNWs) are known to undergo amorphorization during lithiation, and no crystalline Li-Si product has been observed. In this work, we use an X-ray transparent battery cell to perform in situ synchrotron X-ray diffraction on SiNWs in real time during electrochemical cycling. At deep lithiation voltages the known metastable Li(15)Si(4) phase forms, and we show that avoiding the formation of this phase, by modifying the SiNW growth temperature, improves the cycling performance of SiNW anodes. Our results provide insight on the (de)lithiation mechanism and a correlation between phase evolution and electrochemical performance for SiNW anodes.

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In Situ X-ray Diffraction Studies of (De)lithiation Mechanism in Silicon Nanowire Anodes

The review surveys different aspects of chemical studies of semiconducting clathrates belonging to a unique family of inclusion compounds. The classification, crystal and electronic structures, properties and methods for the synthesis of semiconducting clathrates are considered on the basis of analysis of recent publications. The prospects for fundamental research and the construction of new-generation materials based on semiconducting clathrates are discussed. The bibliography includes 161 references.

https://iopscience.iop.org/article/10.1070/RC2004v073n09ABEH000916/pdf

Semiconducting clathrates: synthesis, structure and properties

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

Ba6Ge25, cubic, P4i32 (No. 213), a = 14.5564(2) A, V = 3075.8 A, Ζ = 4, pm = 5.698 g -cm", R g t ( F ) = 0.089, wR(F) = 0.086, Γ =293 Κ. Source of material The compound was prepared from the elements by melting in an open glassy carbon crucible (HF furnace, argon atmosphere) and annealing at 923 Κ (12 d). Ba6Ge25 is silvery metallic, brittle, stable in air and moisture. Discussion BaaGe25 belongs to the chiral clathrate structure family cP 124, being the second binary prototype of the BaoIn4Ge2i type [1-4]. The structure is characterized by a chiral 3D framework of condensed Ge2o pentagondodecahedra (pdod) embedded in a 3D channel labyrinth. Each pdod ist centered by Bal and the rest Ba atoms occupy the cavities in the zeolite-like labyrinth. The rather large elongation of the displacement ellipsoid of Ba2 (M2) is typical for the structure family. The Ge—Ge bond lengths vary between 2.47 A and 2.56 A. Per formula unit, there are seventeen fourfold bonded (4b) Ge° atoms and eight Ge are threefold bonded (3b)Ge : (Ba)6((3b)Ge")8((4b)Ge°)i7(4el. There are no indications of vacancies in the Ge framework. The compound is a Zintl phase with few conduction electrons. Below 180 K, the compound is a semiconducting Zintl phase with 4e in localized antibonding states of the Ge framework and it becomes metallic above 250 Κ [1], Table 1. Data collection and handling. Crystal: silvery block, size 0.10 χ 0.08 χ 0.02 mm Wavelength: Mo Ka radiation (0.71073 A) μ·· 315.23 cm" Diffractometer, scan mode: Nicolet R3m/V, ω 20max: 65° N(hkl)„,easured, N(hkl)u„ique: 1750, 1405 Criterion for F0bs, N(hkl)gC. Fobs > 4 o(Fobs), 630 N(param)Kiinea'· 49 Programs: SHELXTL-plus [5], ATOMS [6] Table 2. Atomic coordinates and displacement parameters (in A). Atom Site χ y ζ U11 U22 C/33 U12 U13 i/23 Ba(l) 8c 0.0607(2) X X 0.017(1) U11 Uu 0.002(1) U12 U12 Ba(2) 12d 1/8 0.1887(3) y+1/4 0.107(6) 0.031(2) U21 0.276(3) 0.005(3) -U13 Ba(3) 4 a 3/8 3/8 3/8 0.025(2) Uu Uu 0.010(2) Uu U12 Ge(l) 24e 0.2929(4) 0.9538(4) 0.7501(4) 0.016(3) 0.015(2) 0.017(3) 0.002(2) 0.003(2) 0.001(2) Ge(2) 8c 0.9251(4) X X 0.015(2) Uu υ η 0.002(2) U\2 U12 Ge(3) 8c 0.2182(5) X X 0.027(2) V11 Uu 0.005(3) Un Un Ge(4) 12d 1/8 0.8309(4) y+1/4 0.010(3) 0.017(2) U22 0.156(2) -0.004(2) U n Ge(5) 24e 0.9166(4) 0.0847(4) 0.8532(4) 0.015(3) 0.020(3) 0.015(2) 0.001(2) 0.001(2) 0.000(2) Ge(6) 24e 0.1853(4) 0.9899(3) 0.8767(4) 0.015(2) 0.013(2) 0.017(3) -0.004(2) -0.003(2) 0.003(2) * Correspondence author (e-mail: carrillo@cpfs.mpg.de) 10.1515/ncrs-2000-0208 Downloaded from PubFactory at 09/02/2016 06:03:31AM via free access 208 Hexabarium pentacosagermanide

Crystal structure of hexabarium pentacosagermanide, Ba6Ge25

Crystal structure of lithium tetracalcium tris(dinitridoborate), LiCa4(BN2)3

BagGe43, cubic, Pmln (No. 223), a = 10.6565(2)Ä,V= 1210.2 Ä, Z= 1, R&l(F) = 0.036, wRrei(F) = 0.055, T= 293 K. Source of material BasGe43 was prepared as single phase by melting the elements in an open glassy carbon crucible (HF furnace, argon atmosphere) and annealing at 1068 Κ (6 d). The single crystals were obtained from the as-cast sample. The substance is silvery metallic, brittle, relatively stable in air and moisture. EDX analysis of bulk samples resulted in the chemical composition Ba8.o(i)Ge42.9(i). Experimental details The lattice parameters were determined from the least-squares refinement of the 2Θ values of 54 reflections (18° < 2Θ < 100°, powder data). The X-ray powder data were obtained on the sample with nominal composition BasGe43 (λ(ΟιΚαι) = 1.540598 Ä; LaB6 standard, a = 4.15695(6) Ä). Discussion Ba8Ge43 was first obtained as a by-product of the decomposition of Ba3Ge4C2 [ 1 ] at 1123 Κ (1 h) under high pressure (ca. 40 kb). The BagGe43 structure (upper part of the figure) belongs to the clathrate-I type with Pearson symbol cP(54-3). Refined occupancies of Ge( 1), Ge(31) and Ge(32) positions resulted in the chemical composition BagGe42.93(5). Only the Ge(l) site is deficient in germanium. Consequently, the neighbouring Ge(3) position splits into the two near sites Ge(31) and Ge(32) (Table 2). The latter occurrs only if Ge(l) is empty. Similar results were obtained for the A8Sn44a2 clathrates, A = K, Rb, Cs [2,3]. A short range ordered model of the vacancies (•) in the germanium environment of Ba(2) atoms is shown in the lower part of the figure (atomic distances in Ä). The investigation of the possible long range ordering in long time annealed samples is in progress. Per formula unit, 31 germanium atoms are fourfold bonded (4b) and 12 germanium atoms in the Ge(31) sites are threefold bonded (3b). The compound may be formulated as (Ba)8((3b)Ge-)i2((4b)Ge°)3i(4e-). According to this, BagGc43D3 is a Zintl phase with a few electrons in not localized antibonding states of the network or in conduction bands. Semimetallic or metallic properties are expected [4, 5], Table 1. Data collection and handling. Crystal: silvery metallic, platelike, size 0 . 0 2 x 0 . 1 5 x 0 . 2 5 mm Wavelength: Mo Ka radiation (0.71073 Ä) μ: 326.16 cm\" Diffractometer, scan mode: Nicolet R3m/V, ω 2Gmax: 55.04° N(hkl)measured, N(hkl)unique: 1892,273 Criterion for /0bs, N(hkl)gL. /obs > 2 <7(/obsJ, 209 N(param)Kiined'· 19 Programs: SHELXL-97 [6], ATOMS [7] Table 2. Atomic coordinates and displacement parameters (in Ä). Atom Site Occ. χ y ζ fiso Ge(31) 24k 0.505(4) 0 Ge(32) 24* 0.495 0 0.3073(3) 0.1124(3) 0.0133(8) 0.3332(3) 0.1314(3) 0.0190(9) * Correspondence author (e-mail: carrillo@cpfs.mpg.de)

J. Curda

Papers

Crystal structure of hexabarium pentacosagermanide, Ba6Ge25

Crystal structure of lithium tetracalcium tris(dinitridoborate), LiCa4(BN2)3

Crystal structure of the defect clathrate-I, Ba8Ge43

Crystal structure of lithium tetraeuropium tris(dinitridoborate), LiEu4(BN2)3

Crystal structure of lithium magnesium dinitridoborate, LiMgBN2