The mineral argyrodite (Ag 8 GeS 6 ) was the first representative of a group of solids known as argyrodites [1,2] that are characterized, in general, by a high ionic conductivity and mobility of their Ag + ions. The structural and conductivity data of these compounds have been called on repeatedly to explain these physical properties in light of the complex argyrodite structure type. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] These studies have improved the general understanding of the diffusion paths of the mobile ions as a function of the temperature and structural properties. The Ag + ions in these compounds can be substituted by other cations, usually Cu + , and versatile substitution reactions of P and S whilst maintaining the topology of the argyrodite structure have also been reported.Considering the great interest in the mobility of Li + ions in solids it was surprising to us that there are only a few reports of experimental work with the corresponding crystalline Li analogues, and none of these appear to be aware of, or even mention, the argyrodite connection. One of these papers reports the synthesis of a black powder containing Li 7 PS 6 and Li 8 P 2 S 9 (minority phase), [19] and another gives detailed NMRspectroscopy-based information on the behavior of Li + ions in a series of pre-reacted amorphous and crystalline mixtures of Li 2 S and P 2 S 5 .[20] A few papers have been published on the application of glass ceramics obtained by high-energy milling of Li 2 S/P 2 S 5 mixtures and their application in secondary batteries in recent years [21][22][23] although, again the possible argyrodite connection, is not mentioned. Thus, well-established syntheses of crystalline single-phase Li argyrodites and reliable structural and physical data are not available.Although there is some controversy about the radii of the univalent cations of Group 11, [24] it can be assumed that the radii of Cu + and Li + are quite similar and different to Ag + (Cu + = 74 pm, Li + = 73 pm, Ag + = 114 pm; all coordination number(CN) = 4). This situation clearly favors a partial or full mutual substitution of Li + and Cu + ions in argyrodites and thus the existence of synthetic "Li-argyrodites". The enhanced reactivity of elemental Li and its compounds at higher temperatures compared to Ag and Cu, however, requires special precautions concerning appropriate container materials and may be the reason for this apparent knowledge gap.The crystal structures of the high-temperature phases of argyrodites are based on a tetrahedral close packing of the nonmetal atoms (chalcogen/halogen) following the topology of a cubic Laves phase (e.g., MgCu 2 ). The 24 chalcogen atoms in the unit cell of Ag 9 AlSe 6 (Z = 4[12] ) form 136 tetrahedral holes which are occupied by four Al 3+ (ordered) and 36 Ag + ions. These Ag + ions are dynamically and/or statically disordered, which explains the high Ag + (Cu + ) ionic conductivities of many argyrodites.Li 6 PS 5 X (X: Cl, Br, I) represents a series of argyrodites whose chemical formula i...

Fe3GeTe2 and Ni3GeTe2 are two new air‐stable, black‐metallic solids. They were characterized by single‐crystal X‐ray crystallography, high resolution transmission electron microscopy (HRTEM), and preliminary magnetic measurements. Both compounds crystallize in the hexagonal system [P63/mmc, Z = 2; Fe3GeTe2: a = 399.1(1) pm, c = 1633(3) pm;Ni3GeTe2: a = 391.1(1) pm, c = 1602.0(3) pm], and represent a new structure type with a pronounced macroscopic and microscopic layer character. They show close structural relationships to iron/nickel germanium alloys. Each layer in the title compounds represents a sandwich structure with two layers of tellurium atoms covering a triple‐layer Fe3Ge (Ni3Ge) substructure on both sides. Assuming full occupancies for the Fe and Ni sites, a mixed‐valence formulation for the transition‐metal atoms according to (M2+)(M3+)2(Ge4–)(Te2–)2 (M = Fe, Ni) may be concluded. A slightly reduced occupancy for one Fe/Ni position, however, indicates a more complicated local structural situation. This is confirmed by weak residual electron density in the van der Waals gap and by the results of detailed HRTEM and electron‐diffraction experiments for Ni3GeTe2. The latter results show variations in the arrangement of Ni atoms, as well as vacancies and a misfit of in‐plane disordered hexagonal layers. Fe3GeTe2 shows Curie–Weiss behavior above and ferromagnetism below 230 K, while Ni3GeTe2 exhibits temperature‐independent paramagnetism in the measured temperature range and a metallic behavior of the electrical resistance. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Crystal chemical data of high- (HT) and low-temperature (LT) modifications of lithium argyrodites with the compositions Li(7)PCh(6) (Ch=S, Se), Li(6)PCh(5)X (X=Cl, Br, I), Li(6)AsS(5)Br, and Li(6)AsCh(5)I (Ch=S, Se) based on single-crystal, powder X-ray (113 K