“Ba6Nb4RuO18” and “LaBa4Nb3RuO15” – The structural consequences of substituting paramagnetic cations into A B−1O3 cation-deficient perovskite oxides

he B-cation deficient perovskite phases Ba6Nb4RuO18 and LaBa4Nb3RuO15 were prepared by ceramic synthesis. Neutron powder diffraction analysis indicates that rather than the 6-layer and 5-layer cation-deficient perovskite structures expected for these phases (by analogy to the known structures of Ba6Nb4TiO18 and LaBa4Nb3TiO15) they adopt 5-layer and 4-layer B-cation deficient perovskite structures respectively, and are better described as Ba5Nb3.33Ru0.81O15 and Ba3.16La0.84Nb2.36Ru0.72O12. The factors that lead to the compositionally analogous Nb/Ru and Nb/Ti phases adopting different structures are discussed on the basis of the difference between d0 and non-d0 transition metal cations.