Synthesis, crystal structure and magnetic properties of an Os-containing pillared perovskite La5Os3MnO16

A new Os-containing, pillared perovskite, La5Os3MnO16, has been synthesized by solid state reaction in sealed quartz tubes. This extends the crystal chemistry of these materials which had been known only for Mo and Re, previously. The crystal structure has been characterized by X-ray and neutron powder diffraction and is described in space group C-1 with parameters a=7.9648(9)Å; b=8.062(1)Å; c=10.156(2)Å, α=90.25(1)°, β=95.5(1)°; γ=89.95(2)°, for La5Os3MnO16. The compound is isostructural with the corresponding La5Re3MnO16 phase. A very short Os–Os distance of 2.50(1)Å was found in the dimeric pillaring unit, Os2O10, suggestive of a triple bond as demanded by electron counting. Nearly spin only values for the effective moment for Os5+ (S=32) and Mn2+ (S=52) were derived from magnetic susceptibility data. Evidence for magnetic transitions was seen near ∼180 and 80K. Neutron diffraction data indicate that Tc is 170(5)K. The magnetic structure of La5Os3MnO16 at 7K was solved revealing that Os5+ and Mn2+ form ferrimagnetically coupled layers with antiferromagnetic interlayer ordering. The ordered moments are 4.2(4)μB for Mn2+ and 1.5(4)μB for Os5+, which are reduced from the respective spin only values of 5.0 and 3.0μB. The observation of net ferrimagnetic (antiparallel) intraplanar coupling between Os5+(t2g3) and Mn2+(t2g3eg2) is interesting as it appears to contradict the Goodenough-Kanamori rules for 180° superexchange.