Eu3F4S2: Synthesis, crystal structure, and magnetic properties of the mixed-valent europium(II,III) fluoride sulfide EuF2·(EuFS)2

Using the method to synthesize rare-earth metal(III) fluoride sulfides MFS (M=Y, La, Ce–Lu), in some cases we were able to obtain mixed-valent compounds such as Yb3F4S2 instead. With Eu3F4S2 another isotypic representative has now been synthesized. Eu3F4S2 (tetragonal, I4/mmm, a=400.34(2), c=1928.17(9) pm, Z=2) is obtained from the reaction of metallic europium, elemental sulfur, and europium trifluoride in a molar ratio of 5:6:4 within seven days at 850 °C in silica-jacketed gas-tightly sealed platinum ampoules. The single-phase product consists of black plate-shaped single crystals with a square cross section, which can be obtained from a flux using equimolar amounts of NaCl as fluxing agent. The crystal structure is best described as an intergrowth structure, in which one layer of CaF2-type EuF2 is followed by two layers of PbFCl-type EuFS when sheeted parallel to the (001) plane. Accordingly there are two chemically and crystallographically different europium cations present. One of them (Eu2+) is coordinated by eight fluoride anions in a cubic fashion, the other one (Eu3+) exhibits a monocapped square antiprismatic coordination sphere with four F− and five S2− anions. Although the structural ordering of the different charged europium cations is plausible, a certain amount of charge delocalization with some polaron activity has to take place, which is suggested by the black color of the title compound. Temperature dependent magnetic susceptibility measurements of Eu3F4S2 show Curie–Weiss behavior with an experimental magnetic moment of 8.19(5) μB per formula unit and a paramagnetic Curie temperature of 0.3(2) K. No magnetic ordering is observed down to 4.2 K. In accordance with an ionic formula splitting like (EuII)(EuIII)2F4S2 only one third of the europium centers in Eu3F4S2 carry permanent magnetic moments. 151Eu-Mössbauer spectroscopic experiments at 4.2 K show one signal at an isomer shift of −12.4(1) mm/s and a second one at 0.42(4) mm/s. These signals occur in a ratio of 1:2 and correspond to Eu2+ and Eu3+, respectively. The spectra at 78 and 298 K are similar, thus no change in the Eu2+/Eu3+ fraction can be detected.