Half-Metallicity in Pb₂CoReO₆ Double Perovskite and High Magnetic Ordering Temperature in Pb₂CrReO₆ Perovskite

Pb₂MReO₆ (M = Co, Cr) perovskites were prepared by high pressure-high temperature method. Rietveld refinements of synchrotron powder X-ray diffraction show that the crystal structure of Pb₂CoReO₆ is trigonal (space group R-3) with almost complete ordering of Co and Re cations, while Pb₂CrReO₆ (space group Pm-3m) is a cubic perovskite with one single site for Cr and Re atoms. The difference between the symmetry and the degree of order was further clarified by X-ray absorption spectroscopy that establishes formal oxidation states in these phases as Pb₂Co²⁺Re⁶⁺O₆ and Pb₂Cr³⁺Re⁵⁺O₆. Pb₂CrReO₆ is a simple perovskite with a high magnetic ordering temperature of 643 K. Pb₂CoReO₆ is a double perovskite with 23% high field negative magnetoresistance at 10 K and 9 T. First-principles calculations of Pb₂CoReO₆ indicate a half metallic electronic structure.