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...

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

Possible three-dimensional diffusion pathways of lithium ions in crystalline lithium argyrodites are discussed based on earlier studies of local dynamics and site preferences. The specific Li-ionic conductivities of the lithium argyrodites Li 7 PS 6 and Li 6 PS 5 X (X: Cl, Br, I) and their temperature dependences are measured by impedance spectroscopy using different electron-blocking and ion-blocking electrode systems. Measurements were carried out between 160 K and 550 K depending on the respective sample. Bulk and grain boundary contributions and the influence of sample preparation are discussed.