MIXED-VALENT IRON-OXIDES WITH THE 0201-1201 INTERGROWTH STRUCTURE - (PB1-XTLX)SR4FE2O9 0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1)

A new series of iron oxides (Pb1-xTlx)Sr4Fe2O9 has been isolated for t he composition range (0 less-than-or-equal-to x less-than-or-equal-to 1). The powder X-ray diffraction and the microscopy studies of these p hases show that they consist of an intergrowth of the 0201 (K2NiF4-typ e) and 1201 (TlSr2CuO5-type) structures. Mossbauer spectroscopy and ma gnetic susceptibility measurements give evidence for iron of the mixed valence Fe(III)-Fe(IV). The main striking features of these oxides de al with i) the existence of several distorted octahedral oxygen enviro nments of iron with a tendancy for a pyramidal (5 + 1) coordination, i i) the complexity of the mixed [Tl, Pb)O]infinity layers characterized by a splitting of the metallic and oxygen site which suggests a tetra hedral coordination of the cations in agreement with their 5d10 electr onic configuration.