# Stepwise Reduction of Ruthenium and Growth of Magnetic Structure During Hydrothermal Crystallisation of SrRu2O6 from KRuO4

## TL;DR

Scientists used a new reactor to track how SrRu2O6 forms from KRuO4, observing changes in ruthenium oxidation states and magnetic structure during crystallization.

## Contribution

The study introduces a novel hydrothermal reactor design enabling in situ neutron diffraction to track both atomic and magnetic structure evolution during crystallization.

## Key findings

- The Ru(VII) precursor KRuO4 decays to form a Ru(VI) oxyhydroxide intermediate before SrRu2O6.
- Magnetic Bragg peak intensities evolve alongside crystal structure growth below the Néel temperature.
- The magnetic moment per Ru in SrRu2O6 matches ex situ studies at the same temperatures.

## Abstract

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.

In situ neutron powder diffraction data collected while heating the Ru(VII) reagent KRuO4 in the presence of aqueous Sr2+ reveal the crystallisation of a Ru(VI) oxyhydroxide before the expected Ru(V) oxide; at the same time evolution of antiferromagnetism in the final SrRu2O6 product follows the growth of its crystal structure.