Stepwise Reduction of Ruthenium and Growth of Magnetic Structure During Hydrothermal Crystallisation of SrRu ₂ O ₆ from KRuO ₄

Using a new design of hydrothermal reactor constructed from a null-scattering TiZr alloy in situ powder neutron diffraction data are collected to follow the crystallisation of the Ru(V)-containing high temperature antiferromagnet SrRu₂O₆. Full pattern fitting allows quantitative analysis of the evolution of crystalline phases during reaction, revealing the decay of the Ru(VII) precursor KRuO₄, the transient presence of the Ru(VI) oxyhydroxide SrRuO₃(OD)₂ and the eventual growth of SrRu₂O₆. At 140 °C crystallisation is retarded, prolonging the lifetime of SrRuO₃(OD)_2, while at 180 °C complete formation of SrRu₂O₆ is observed. At the same time, evolution of the magnetic Bragg peak intensities resulting from the antiferromagnetic ordering of Ru(V) moments in SrRu₂O₆ is seen. This unique observation is possible since crystallisation occurs below the Néel temperature of the material. The magnetic structure is found to grow concurrently with the atomic structure, with calculated magnetic moment per Ru consistent with ex situ studies at the same temperatures. The reported experiment design will be applicable for the study of many other families of inorganic materials that crystallise under solvothermal conditions.