Structural investigation of the oxide-ion electrolyte with SrMO 3 (M ¼ Si/Ge) structure R. Martinez-Coronado, a Preetam Singh, a J. Alonso-Alonso b and John B. Goodenough * a A neutron-diffraction study of the oxide-ion solid electrolytes Sr 1x Na x SiO 30.5x (x ¼ 0.2 and 0.4) and Sr 0.8 K 0.2 Ge 1y Si y O 2.9 (y ¼ 0.0 and 0.5) reveals that there are no interstitial oxygen atoms in the structures; the oxygen vacancies are more concentrated in the planar oxygen sites (O3 and O5) of corner-sharing tetrahedral units of the M 3 O 9 (M ¼ Si/Ge) complexes. From thermogravimetric analysis (TGA), the K-substituted samples lose weight above 100  C and are hygroscopic at room temperature; the oxygen vacancies are concentrated at the in-plane O3 and O5 sites and those in the terminal O2 or O4 are responsible for a 2D oxide-ion conductivity between the M 3 O 9 complexes. The Na-substituted samples lose little weight by 800  C and are not hygroscopic; the oxygen vacancies are located at all oxygen atom positions, being more pronounced at the in-plane O3 and O5; those in the terminal oxygen sites give an excellent oxide-ion conductivity. Moreover the high temperature NPD data for Sr 0.6 Na 0.4 SiO 2.8 disclose that the vacancies become more randomly dispersed above 400  C to give a smaller activation energy above 550  C for vacancy transfer between Si 3 O 90.5x complexes. 1. Introduction Oxide-ion solid electrolytes have, potentially, many technologi- cally important applications, including the electrolyte of a solid oxide fuel cell (SOFC); a high activation energy for oxide-ion motion has restricted their use to a high operating temperature (T op ). 1–3 Moreover, interfacial cation transfer between a perov- skite electrolyte and an isostructural mixed oxide-ion/electron conductor used as a catalytic cathode for the oxygen-reduction reaction (ORR) has made yttria-stabilized zirconia (YSZ) with the uorite structure the preferred electrolyte despite its restriction to a T op T 800  C. Recently a superior oxide-ion electrolyte family, Sr 3x A x M 3 O 90.5x ,A ¼ Na or K and M ¼ Si/Ge, has been discovered having a planar structure different from both the uorite and the perovskite structures. 4,5 The parent Sr 3 M 3 O 9 structure consists of close-packed sheets of Sr 2+ ions in trigonal- prismatic coordination separated by a layer of M 3 O 9 complexes consisting of three MO 4 tetrahedra sharing corners in a trian- gular planar array of two O3 and one O5 oxygen atoms parallel to the Sr 2+ planes; the two terminal O1 and O2 oxygen of the tetrahedra sharing a common O5 atom each have two terminal O1 and O2 oxygen coordinating the Sr 2+ ions; but the two O4 terminal oxygen of the tetrahedron sharing two O3 oxygen have Fig. 1 Schematic view of the crystal structure of Sr 0.6 Na 0.4 SiO 2.8 at room temperature showing a–b planes of Sr/Na atoms alternating with planes of M 3 O 9 units. a Texas Material Institute and Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX, 78712, USA. E-mail: jgoodenough@mail. utexas.edu; Tel: +1-512-471-1646 b Instituto de Ciencia de Materiales de Madrid (CSIC), Sor Juana In´ es de la Cruz, 3, Cantoblanco, 28049 Madrid, Spain Cite this: J. Mater. Chem. A, 2014, 2, 4355 Received 19th December 2013 Accepted 31st January 2014 DOI: 10.1039/c3ta15309f www.rsc.org/MaterialsA This journal is © The Royal Society of Chemistry 2014 J. Mater. Chem. A, 2014, 2, 4355–4360 | 4355 Journal of Materials Chemistry A PAPER Published on 31 January 2014. Downloaded by University of Texas Libraries on 08/03/2014 16:37:14. View Article Online View Journal | View Issue