Bi3Cr2.91O11: a ferromagnetic insulator from Cr(4+)/Cr(5+) mixing.

The search for materials with ferromagnetic and semiconducting/insulating properties has intensified recently because of their potential use in spintronics. However, the number of materials is rather limited because of conflicting requirements needed for the appearance of ferromagnetic and insulating properties. Here we show that Bi3Cr2.91O11 belongs to the scarce family of ferromagnetic insulators. Bi3Cr2.91O11 was synthesized at high pressure of 6 GPa and high temperature of 1570 K. Its crystal structure and properties were studied using single crystals. It crystallizes in the KSbO3-type structure with space group Pn3 and the lattice parameter a = 9.2181(2) Å. Bi3Cr2.91O11 has almost a 1:1 mixture of Cr(4+) and Cr(5+) ions distributed in one octahedral crystallographic site. Bi3Cr2.91O11 is a rare example of oxides having chromium ions in unusual oxidation states. The presence of Cr(4+) and Cr(5+) results in ferromagnetic properties with ferromagnetic Curie temperature TC = 220 K.