NEW MIXED-VALENT IRON-OXIDES WITH THE 1212-TYPE STRUCTURE - TL1-XPBXSR3FE2O8

A new series of thallium and lead-based iron oxides (Tl1-xPbx)Sr3Fe2O8 has been synthesized for 0 less than or equal to x less than or equal to 0.5. The structure has been refined from X-ray diffraction data an d shown to be similar to that of the parent 1212-type copper oxides (T l1-xPbx)Sr2CaCu2O7. Magnetic susceptibility measurements and Mossbauer spectroscopy have been performed for TlSr3Fe2O8 in particular. Mossba uer spectroscopy provides valuable information about the crucial role of the oxygens located on both sides of the [Tl-O](infinity) plane in modifying the octahedral oxygen environment of iron. This is discussed in connection with electric field gradient calculations which have be en carried out for high spin Fe3+ with various types of oxygen environ ment.