Synthesis and characterization of Bi31Cr5O61.5, a new bismuth chromium oxide, potential mixed-ionic–electronic conductor for solid oxide fuel cells

Single crystals, as well as pure powder of the new bismuth chromate, Bi31Cr5O61.5 were synthesized by slow cooling of a mixture of Bi2O3–Cr2O3 and then fully structurally characterized using X-ray diffraction. It crystallizes in the monoclinic space group P21/n with unit cell parameters a = 23.5794(8), b = 11.6189(4), c = 24.3629(8) Å and β = 108.02(1)°, Z = 4. The final conventional agreement factors converged to R = 0.0540 and wR = 0.0561 for 27520 independent reflections. The +VI chromium oxidation state was proved by thermogravimetric and magnetic measurements. The structure can be described by the association of isolated CrO4 tetrahedra surrounded by 11 or 12 Bi atoms forming truncated or not cuboctahedra. The stability of both pure powder sample and pressed sintered pellets was studied at elevated temperature by X-ray diffraction and by dilatometry previously to ionic conductivity measurements done by impedance spectroscopy.