Revisiting the cubic crystal structures of Sr4Nb2O9 and Sr5Nb2O10

Abstract

We have synthesized polycrystalline and single crystal samples of Sr4Nb2O9 and Sr5Nb2O10 and revisited the crystal structure of the high-temperature cubic phase. By careful analysis of single-crystal X-ray diffraction (SXRD), powder synchrotron X-ray diffraction (Syn-PXRD) and powder neutron diffraction (PND) data, we arrive at a structure model in space group 𝐹4#3𝑚 (216), a subgroup of the reported 𝐹𝑚3#𝑚 (225) model. The 𝐹4#3𝑚 model gives a better fit to the diffraction data, especially PND. We observed an interstitial oxide ion O3 on the 48h site near O1, which gives a tetrahedral Nb1−O polyhedron rather than an octahedral one as found in the 𝐹𝑚3#𝑚 (225) model. The temperature-dependent conductivities of Sr4Nb2O9 and Sr5Nb2O10 in dried O2 were studied using impedance spectroscopy. The activation energies of Sr4Nb2O9 and Sr5Nb2O10 were estimated to be 1.18(1) eV and 1.17(4) eV respectively. This crystallographic arrangement of O1 and O3 (spread over split sites) is likely a key structural factor behind oxide ionic migration in Sr4Nb2O9 and Sr5Nb2O10.