Structural Chemistry and Electronic Properties of the Hexagonal Perovskites BaIr1−xCoxO3−δ (x=0.5, 0.7, 0.8)

Abstract

The room temperature crystal structures of three compositions in the system BaIr1−xCoxO3−δ (x=0.5, 0.7, 0.8) have been determined from X-ray and neutron powder diffraction data. BaIr0.5Co0.5O3.02(1), BaIr0.3Co0.7O2.84(1), and BaIr0.2Co0.8O2.83(1) adopt 12R, 10H, and 5H perovskite structures, respectively; the distribution of Co and Ir cations over corner-sharing and face-sharing sites has been determined, and the Co/Ir–O bond lengths have been used to assign the cation oxidation states as Ir5+ and Co3+/Co4+. Arguments based on bond lengths and magnetic susceptibility data have been used to assign spin states; at room temperature BaIr0.5Co0.5O3−δ contains both high-spin and low-spin Co3+. The distribution of anion vacancies has been determined for x=0.7, 0.8; in the latter case there is evidence for the replacement of some BaO3 layers in the pseudo-hcp structure by BaO2 layers, thus introducing tetrahedral transition metal sites which are found to be occupied by Co. Neutron diffraction experiments carried out at 5 K revealed a monoclinic distortion in the x=0.5 sample, with the environment of the high-spin Co3+ cation undergoing a Jahn–Teller distortion. The structures of the x=0.7, 0.8 phases did not change significantly on cooling. Measurements of magnetization as a function of applied field suggest that spin glass or cluster glass behavior is present in x=0.5, 0.7 below 50 K. In the case of x=0.8, magnetization and neutron diffraction data suggest the formation of a weak ferromagnet with a saturation magnetization of 0.60 μB per Co cation at 5 K.