Ruthenium( V) Oxides from Low-Temperature Hydrothermal Synthesis**

Low-temperature (200 degrees C) hydrothermal synthesis of the ruthenium oxides Ca1.5Ru2O7, SrRu2O6, and Ba2Ru3O9(OH) is reported. Ca1.5Ru2O7 is a defective pyrochlore containing Ru-V/VI; SrRu2O6 is a layered Ru-V oxide with a PbSb2O6 structure, whilst Ba2Ru3O9(OH) has a previously unreported structure type with orthorhombic symmetry solved from synchrotron X-ray and neutron powder diffraction. SrRu2O6 exhibits unusually high-temperature magnetic order, with antiferromagnetism persisting to at least 500K, and refinement using room temperature neutron powder diffraction data provides the magnetic structure. All three ruthenates are metastable and readily collapse to mixtures of other oxides upon heating in air at temperatures around 300-500 degrees C, suggesting they would be difficult, if not impossible, to isolate under conventional high-temperature solid-state synthesis conditions.