NEW MIXED-VALENT FERRITES (TL1.5HG0.5)SR3-XBAXFE2O9-DELTA 0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-2) WITH THE TL-2212-TYPE STRUCTURE

A new series of layered ferrites (Tl1.5Hg0.5)Sr3-xBaxFe2O9-delta has b een synthesized and characterized for the composition range 0 less tha n or equal to x less than or equal to 2. The structure has been refine d from powder XRD data and shown to be similar to that of the tetragon al Tl-2212-type superconducting copper oxides, Electron diffraction an d high-resolution electron microscopy studies reveal a regular stackin g of the [(Tl, Hg)O], [(Sr,Ba)O] and [FeO2] layers without defects or atomic ordering on both the (Tl, Hg) and (Sr, Ba) sites, Magnetic susc eptibility measurements and Fe-57-Mossbauer spectroscopy have been per formed for (Tl1.5Hg0.5) Sr2BaFe2O9-delta. They provide evidence for an tiferromagnetic ordering below 50 K with localized Fe3+ and Fe4+ speci es at 4.2 K, while intermediate valence states due to electron delocal ization between iron sites must be taken into account at 293 K. The ro le of mercury is discussed. (C) 1997 Academic Press.