The topochemical reduction of ru-containing double perovskite oxides
Keywords: solid state chemistry
oxides
crystallography
magnetism
perovskite

This work focuses on the topochemical reduction of some ruthenium-containing double perovskite phases using various reducing agents (Zr, LiH, CaH2) to form novel metastable reduced phases and anion-substituted phases.
The progress of the reduction of LaSrNiRuO6 with CaH2 to yield LaSrNiRuO4 is related to the synthesis conditions of the parent oxide LaSrNiRuO6 phase, where quenched samples of LaSrNiRuO6 react much faster than the slow-cooled samples. Detailed structural and microstructural characterisation of quenched and slow-cooled samples of LaSrNiRuO6 show that both samples are structurally indistinguishable, with differences in reactivity arising from their microstructures: quenched samples of LaSrNiRuO6 have small/strained crystalline domains while slow-cooled samples have large/less strained crystalline domains. The difference in the reactivity is attributed to the microstructure of LaSrNiRuO6, where small crystalline domains have greater plasticity and lower the activation energy of the reduction reaction. In addition, it is likely that enhanced microstructural strain in the quenched sample act to destabilise the starting material, which further lowers the activation energy of this reduction reaction. These observations have demonstrated that the microstructure of a starting material plays an important role in topochemical reactions.
Similarly, the reactivity of the analogous Ru-containing double perovskite phase LaSrCoRuO6 can be enhanced through rapid cooling. The quenched LaSrCoRuO6 is then reduced with various reducing agents and several novel metastable reduced phases are successfully synthesised.
Reduction of LaSrCoRuO6 with a Zr getter yields a novel oxygen deficient phase LaSrCoRuO5. LaSrCoRuO5 adopts a complex anion-vacancy ordered structure which can be described in the space group P1121. The structure of LaSrCoRuO5 consists of corner sharing CoO6 octahedra, RuO5 square-based pyramidal units and CoO4 square-planar units. BVS calculations indicate the Co2+ centres in this compound disproportionate into a 1:1 ratio of Co+ and Co3+ centres, with Co+ centres residing at the square-planar sites and Co3+ centres locating at the octahedral sites. The magnetisation data collected from the sample show a ferromagnetic state at 5 K while neutron powder diffraction data collected from the sample at 5 K show no evidence of long-range magnetic ordering.
The more metastable LaSrCoRuO4 phase can be prepared via the reduction of LaSrCoRuO6 with LiH under flowing argon. This compound adopts an infinite layer structure, containing corner sharing CoO4 and RuO4 square-planar units. Magnetisation data collected from LaSrCoRuO4 show that this sample is ferromagnetic up to room temperature.
The disproportionation of Co2+ centres observed in LaSrCoRuO5 is highly unusual. Therefore, doping of LaSrCoRuO5 was investigated by varying the ratio of La and Sr at the A-site. When the system is slightly hole/electron doped, the resulting doped reduced phases are isostructural with the undoped LaSrCoRuO5 phase. However, when the system becomes heavily hole doped, a different anion-vacancy ordered structure is observed. This observation suggests that the oxidation states of the transition metals are coupled with the complicated anion-ordered structure adopted LaSrCoRuO5.
Finally, reduction of LaSrCoRuO6 with CaH2 and LiH in sealed ampoules yields novel oxyhydride phases with a range of oxide/hydride contents. Short-ranged anion ordering is observed in one of the oxyhydride phases. The partial anion ordering can be explained via the different preferences of the cis and trans MO4H2 arrangements.