BaRuO3 chemically belongs to ARuO3 family. It has unique hexagonal structure rather than an orthorhombic GdFeO3-type structure of other members, such as SrRuO3 and CaRuO3. The structure of ARuO3 compounds can be described on the basis of the close-packed stacking of AO3 layers in which the center of oxygen octahedra is occupied by ruthenium. A number of polymorph structures exist by mixing the cubic and hexagonal close-packed stacking of AO3 layers.
Up to date, only three polymorph structures have been observed in BaRuO3 bulk materials, which are denoted by 4H, 6H and 9R structure. The 9R structure (Fig. 1 (a)) is most stable at 1100 °C at atmosphere pressure. As pressure is increased, the 9R structure will transform through metastable 4H structure (Fig. 1(b)) to metastable 6H structure. The physical properties of BaRu03 polymorphs differ significantly with the variation of the structure. Hence, it is desirable to form phase pure polymorphs of BaRuO3 to examine the structure-property relationship.