Synthesis and crystal structure determination of Ag9FeS4.1Te1.9, the first example of an iron containing argyrodite

Ag9FeS4.1Te1.9 was prepared by solid state synthesis from stoichiometric amounts of the elements at 873 K. The compound forms grey crystals which are stable against air and moisture. The crystal structure was determined by X-ray diffraction from selected single crystals. Ag9FeS4.1Te1.9 crystallizes in the space group F4¯3m, a = 11.0415(7) Å, V = 1346.1(1) Å3, and Z = 4 (powder data at 293 K). The compound shows a reversible phase transition upon cooling to the space group P213, a = 11.0213(1) Å, V = 1338.75(2) Å3, and Z = 4 (single crystal data at 200 K). The title compound is the first example of an iron containing argyrodite-type material with Fe3+ located in tetrahedral sites. Silver atoms are disordered at room temperature which was taken into account by non-harmonic refinement of the silver positions. The refinement converged to R1 = 3.51% and wR2 = 10.66% for the room temperature measurement and to R1 = 1.55% and wR2= 5.23% for the 200 K data set (all data). Impedance measurements were performed in the temperature range from 323 K to 473 K. Ionic conductivity values are 1.81*10-2 Scm-1 at 323 K and 1.41*10-1 Scm-1 at 468 K. The activation energy is 0.19 eV from 323 K to 423 K. and 0.06 eV from 393 K 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/mole was determined by DSC measurements. 57Fe Mössbauer spectra show one signal at 298 K and a doublet at 78 K, indicating Fe3+ and structural distor-tions upon cooling the samples. Hyperfine field splitting of iron is observed at 5 K. Measurements of the molar susceptibility re-vealed that the compound is paramagnetic down to a Néel temperature of TN = 22.1(5) K. Antiferromagnetic ordering is observed at lower temperatures