X-ray diffraction investigations of structural modifications in In-doped tin pyrophosphates

Abstract Laboratory and synchrotron x-ray powder diffraction were used to investigate the structural modifications that occur upon indium doping of tin pyrophosphate. The data collected under air, vacuum, and inert gas sample environments at temperatures (T) from 50 °C to 300 °C show that regardless of the In-doping level (0 ≤ x ≤ 0.18) all In x Sn 1-x P 2 O 7 samples are isomorphic (have the same P a -3 cubic crystal structure) at all temperatures and under all the conditions investigated. The cubic lattice parameter (a) increases linearly with T at all doping levels, but the “a vs. x| T “ isotherms exhibit a robust peak at x = 0.1 when data are collected on samples measured in air. On the other hand, Rietveld refinements against data collected on In x Sn 1-x P 2 O 7 samples yield values of O O bond lengths and P O P bond angles that show no major changes at x = 0.1 at any temperature. This is significant, as the Sn 0.9 In 0.1 P 2 O 7 (x = 0.1) compound is known to exhibit the highest proton conductivity within the series, but the microscopic details responsible for the increased proton conductivity are not understood. Finally, the peak observed in the “a vs. x| T “ curves vanishes if the measurements are taken on samples kept either under vacuum or in an inert gas environment. This is a remarkable behavior as it lends further support to our hypothesis that a key microscopic feature responsible for the large proton conductivity of the Sn 0.9 In 0.1 P 2 O 7 compound is the enlargement of the lattice constant at x = 0.1.