A combined density functional theory and Fourier transform infrared spectroscopy study of the structure and specific site preference of protons and hydrides in the pyrochlore Sm1.92Ca0.08Sn2O7-delta is presented. Two protonic sites of particular high stability are identified, both located on O(1) oxygen atoms closely associated with a Ca dopant. Further, the unexpected presence of Ho hydride defects in undoped, oxygen deficient Sm2Sn2O7 is reported. Finally, the stretching frequencies and relative intensities for these and other sites are calculated. The main features of the Fourier transform infrared spectra are hereby resolved. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737786]

The electrical conductivity of the pyrochlore systems, Sm2Sn2O7, Sm1.92Ca0.08Sn2O7−δ and Sm2Sn1.92Y0.08O7−δ was studied using impedance spectroscopy under wet and dry gas (O2 and Ar) in the temperature range 150–1000°C. Enhancements of the bulk conductivity of all samples at temperatures up to ~550°C were observed for wet conditions consistent with significant levels of proton conduction. The presence of dissolved protons in the acceptor-doped materials, Sm1.92Ca0.08Sn2O7−δ and Sm2Sn1.92Y0.08O7−δ, is supported by infrared spectroscopy and thermogravimetric analysis. Proton conduction was confirmed by isotope effects under heavy water (O2/D2O and Ar/D2O). The A-site substituted sample Sm1.92Ca0.08Sn2O7−δ yielded the highest levels of proton conduction and displayed mixed ionic and electronic conduction under dry oxidising conditions. Electron hole conduction dominates in dry oxygen for Sm2Sn1.92Y0.08O7−δ and Sm2Sn2O7. For the A-site doped sample bulk and grain boundary conduction could be separated. The specific grain boundary conduction was calculated using the brick layer model and was found to be two orders of magnitude lower compared to the bulk conductivity. The unexpected increase in conductivity seen for the undoped sample under wet gas is discussed in the context of structural disorder and possible filling of the un-occupied anion site in the pyrochlore structure by OH-groups.

The proton conductivity in tin-based pyrochlores, Ln1.96Ca0.04Sn2O7-δ (Ln=La, Sm, Yb), has been investigated. Samples were prepared by conventional solid state sintering methods. Fractions of the powders were vacuum dried, protonated and deuterated so that infrared spectroscopy and thermogravimetric analysis could be carried out to study proton incorporation. Electrochemical impedance spectroscopy was used to characterise the conductivities of the sample. The bulk conductivity was higher in wet gas compared to the dry runs at temperatures below approximately 500°C for all samples. The level of proton conductivity was found to depend critically on the lanthanide size, and was highest for the largest La and mid-sized Sm ions, σH+≈4×10−7Scm−1 at 300°C, before falling sharply for the Yb-ion, σH+≈7×10−9Scm−1. Evidence from TGA and IR analyses indicates that the tendency for proton absorption was strongly influenced by the lanthanide size, being greatest for La1.96Ca0.04Sn2O7-δ. The results suggest that both proton concentration and proton mobility vary depending on the size of the A-site ion.

The results of the synthesis and characterisation of pyrochlore systems Sm1.92Ca0.08Ti2O7-delta and Sm2Ti1.92Y0.08O7-delta are reported. The electrical conductivity of the materials was studied using impedance spectroscopy under wet and dry oxygen and argon. Enhancements of the bulk conductivity at temperatures up to 500 degrees C were observed for wet conditions indicative of significant proton conductivity. The presence of dissolved protons in the materials is supported by thermogravimetric analysis and infrared spectroscopy. Proton conduction was confirmed by measurements in O-2/D2O and Ar/D2O. The results reveal the importance of the correct choice of dopant site for the pyrochlore structure, with A-site substitution yielding the highest levels of proton, as well as oxide ion, conduction. For both samples bulk, rather than grain boundary, conduction is found to be dominant. (C) 2009 Elsevier BM. All rights reserved.