Synthesis and Crystal Structure Determination of Ag₉FeS_4.1Te_1.9, the First Example of an Iron Containing Argyrodite

Abstract

Ag 9 FeS 4.1 Te 1.9 was prepared by solid state synthesis from stoichiometric amounts of the elements at 873 K. The compound forms gray crystals which are stable against air and moisture. The crystal structure was determined by X-ray diffraction from selected single crystals. Ag 9 FeS 4.1 Te 1.9 crystallizes in the space group F4̅ 3m, a = 11.0415(7) Å, V = 1346.1(1) Å 3 , and Z = 4 (powder data at 293 K). The compound shows a reversible phase transition upon cooling to the space group P2 1 3, a = 11.0213(1) Å, V = 1338.75(2) Å 3 , and Z = 4 (single crystal data at 200 K). The title compound is the first example of an iron containing argyroditetype material with Fe 3+ located in tetrahedral sites. Silver atoms are disordered at room temperature which was taken into account by nonharmonic refinement of the silver positions. The refinement converged to R 1 = 3.51% and wR 2 = 10.66% for the room temperature measurement and to R 1 = 1.55% and wR 2 = 5.23% for the 200 K data set (all data). Impedance measurements were performed in the temperature range from 323 to 473 K. Ionic conductivity values are 1.81 × 10 −2 S cm −1 at 323 K and 1.41 × 10 −1 S cm −1 at 468 K. The activation energy is 0.19 eV from 323 to 423 K and 0.06 eV from 393 to 473 K. DTA measurements reveal congruent melting at 907 K. A phase transition temperature of 232 K with an enthalpy of 7.9 kJ/mol was determined by DSC measurements. 57 Fe Mossbauer spectra show one signal at 298 K and a doublet at 78 K, indicating Fe 3+ and structural distortions upon cooling the samples. Hyperfine field splitting of iron is observed at 5 K. Measurements of the molar susceptibility revealed that the compound is paramagnetic down to a Neél temperature of T N = 22.1(5) K. Antiferromagnetic ordering is observed at lower temperatures. KEYWORDS: argyrodite, ion conductor, nonharmonic refinement, Mossbauer spectroscopy, magnetism ■ INTRODUCTIONThe mineral Argyrodite Ag 8 GeS 6 was the first representative of a large family of compounds which is nowadays called argyrodites. They have been investigated for a long time because of their interesting physical and chemical properties. Among these, ionic conductivity is the most prominent one. It is associated with argyrodites of Li, 1,2 Ag, 3,4 and Cu. 5,6 A huge number of argyrodites is known to date due to a manifold of substitution possibilities on the different sites. Kuhs et al. generalized the formula of argyrodites and found several substituted representatives with mixed anion substructure. 7 Today argyrodites consisting of mono-or divalent cations A (Cu + , Ag + , Li + , Cd 2+ , Hg 2+ ), and a tri-, tetra-, or pentavalent cation B forming a complex anion with a chalcogenide ion Q (O 2− , S 2− , Se 2− , Te 2− ) are known. The general formula is A m+ ((12−n−y)/m) B n+ Q 2− 6−y X − y . The multivalent cation B must have an appropriate size and must be capable of forming tetrahedra. Typical multivalent cations are the main group elements Al 3+ , Ga 3+ , Si 4+ , Ge 4+ , Sn 4+ , P 5+ , As 5+ , and Sb 5+ . A limited n...

Li4PS4I: A Li+ Superionic Conductor Synthesized by a Solvent-Based Soft Chemistry Approach

“…However, it might be the case that the conductivities measured at low temperatures are more influenced by grain boundaries (high E A ) than the values observed at higher temperatures where the inner-grain (bulk) contribution (low E A ), which is exclusively detected by NMR, dominates. Such a non-Arrhenius behavior with a flattening curve toward high temperatures was also observed for the argyrodite phase Ag 9 FeS 4.1 Te 1.9 …”

Stability and ionic mobility in argyrodite-related lithium-ion solid electrolytes

“…Critical reactions for structural stability are given in eqn ( 5), ( 11) and ( 12) for A = Li, Na or Cu, respectively: Na 7Àx PS 6Àx X x 2 xNaX + (2 À x)Na 2 S + Na 3 PS 4 (11) Cu 7Àx PS 6Àx X x 2 xCuX + (0.5 À 0.25x)Cu + (0.5 À 0.25x)Cu 7 S 4 + Cu 3 PS 4 (12) Fig. 8 Site energies and activation barriers between neighbouring sites in sulphide and selenide argyrodites.…”

“…11 Only a few argyrodites with trivalent M 3+ = Fe 3+ , Al 3+ , Ga 3+ have been reported. 12 A wider range of silver and copper argyrodites had been described 70 years ago 13 and subsequently studied more in depth by Kuhs et al 14 and Evain et al 15 Temperature-dependent order-disorder transitions and cationic disorder in the hightemperature phase yield moderately high ionic conductivity for the high-temperature polymorphs of quaternary Cu and Ag-containing argyrodites, 14,16 while the corresponding halidefree ternary argyrodites are generally found to be semiconductors, for which changes in the fundamental absorption-edge related to the polymorphic transitions were evidenced. 17 Lithium argyrodites first reported by the Deiseroth group 10 are promising candidates for solid electrolytes.…”

Festkörperchemie 2013

“…Eisenhaltig mögen es erstmals Argyrodite 20. Wie Pfitzner und Mitarbeiter mitteilten, befinden sich in Ag9FeS4,1Te1,9 isolierte Tetraeder FeIIIS4 (Abbildung 7).…”