Extension of the "1201" Family to Strontium-Rich Chromite and Ferrite, Bi0.4Sr2.5Cr1.1O4.9 and Bi0.4Sr2.5Fe1.1O5

Two new Sr-rich "1201"-type oxides, Bi0.4Sr2.5Cr1.1O4.9 and Bi0.4Sr2.5Fe1.1O5 have been synthesized. These compounds, intergrowths of double rock-salt layers with single perovskite layers, show a 1:1 ordering between (Bi,M) and Sr species within the intermediate rock-salt layer [Bi0.4M0.1Sr0.5O]. The XANES study shows that bismuth is mainly trivalent, whereas iron is mixed valent containing 50% Fe3+ and 50% Fe4+ (also confirmed by M?ssbauer), and chromium could be a mixture of Cr3+ and Cr6+ sitting in the perovskite and rock-salt-type sites, respectively. Both compounds exhibit antiferromagnetic interactions. The Cr-phase is a strong insulator, whereas the Fe-phase exhibits a semi-conductor-like resistivity whose value at room temperature is close to that of isotypic cobaltite.