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 the PND data. We observed an interstitial oxide ion (O3) on the 48h site near the typical perovskite 24e site (O1), which gives a Td Nb–O symmetry rather than an Oh one as found in the Fm3¯m 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 disordered crystallographic arrangement of the O1 and O3 anions is likely a key structural factor behind oxide ionic migration in Sr4Nb2O9 and Sr5Nb2O10.