Competition between Anion-Deficient Oxide and Oxyhydride Phases during the Topochemical Reduction of LaSrCoRuO6

Binary metal hydrides can act as low-temperature reducing agents for complex oxides in the solid state, facilitating the synthesis of anion-deficient oxide or oxyhydride phases. The reaction of LaSrCoRuO₆, with CaH₂ in a sealed tube yields the face-centered cubic phase LaSrCoRuO_3.2H_1.9. The reaction with LiH under similar conditions converts LaSrCoRuO₆ to a mixture of tetragonal LaSrCoRuO_4.8H_1.2 and cubic LaSrCoRuO_3.3H_2.13. The formation of the LaSrCoRuO_xH_y oxyhydride phases proceeds directly from the parent oxide, with no evidence for anion-deficient LaSrCoRuO_6–x intermediates, in contrast with many other topochemically synthesized transition-metal oxyhydrides. However, the reaction between LaSrCoRuO₆ and LiH under flowing argon yields a mixture of LaSrCoRuO₅ and the infinite layer phase LaSrCoRuO₄. The change to all-oxide products when reactions are performed under flowing argon is attributed to the lower hydrogen partial pressure under these conditions. The implications for the reaction mechanism of these topochemical transformations is discussed along with the role of the hydrogen partial pressure in oxyhydride synthesis. Magnetization measurements indicate the LaSrCoRuO_xH_y phases exhibit local moments on Co and Ru centers, which are coupled antiferromagnetically. In contrast, LaSrCoRuO₄ exhibits ferromagnetic behavior with a Curie temperature above 350 K, which can be rationalized on the basis of superexchange coupling between the Co¹⁺ and Ru²⁺ centers.

Reaction between LaSrCoRuO₆ and LiH or CaH₂ yields LaSrCoRuO_xH_y oxyhydride phases if the P(H₂) in the system is sufficient to stabilize these products with respect to hydrogen loss. Conversely, reaction between LaSrCoRuO₆ and LiH under low P(H₂) conditions yields the infinite layer phase LaSrCoRuO₄, indicating that the P(H₂) of the system can direct the outcome of topochemical reductions.