Synthesis and Structural Determination of the Disordered Bixbyite Cu_3-x Sb_1+x O_5.5+3x/2 with Spin-Glass Behaviour

The ternary copper antimony oxide Cu_3-x Sb_1+x O_5.5+3x/2 (x=0.23) has been synthesized under 0.8-1.3 MPa pO₂ at 1022-1082 °C. Rietveld refinements of X-ray and neutron powder diffraction patterns concluded that the oxide adopts a bixbyite type structure, crystallising in the cubic space group Ia-3 with the unit cell parameter a=9.61164(4) Å at room temperature from powder neutron diffraction data. The cationic 8b and 24d sites were found to be occupationally disordered where both Cu and Sb could be found on both sites. This is supported by X-ray absorption spectroscopy experiments showing more than one possible Cu environment. There was a significant net deficiency of oxygen in the compound which was first inferred from observations of a thermochromic-like phenomena and also seen from in situ high temperature neutron diffraction experiments. Magnetic susceptibility and magnetization measurements show paramagnetic behaviour with spin-glass like transition below 6 K.