Indium Doping Induced Lattice Constant Variation in Tin Pyrophosphate

SnP$_{\mathrm{2}}$O$_{\mathrm{7}}$ and Sn$_{\mathrm{1-x}}$In$_{\mathrm{x}}$P$_{\mathrm{2}}$O$_{\mathrm{7}}$ (x from 0 to 0.2), were synthesized and characterized by X-ray diffraction over a range of temperatures and pressures. XRD confirmed indium solubility limit to be x$=$0.12. LeBail and Rietveld refinements confirmed the room temperature structure of the undoped and doped material to be the Pa-3 space group and that doping induced an increase in the lattice constant a with temperature increase. a reaches its highest value at doping level x$=$0.1, consistent with its highest value of protonic conductivity as measured by Nagao et. al. XRD measurements under an evacuated atmosphere, and under 5 bar of He pressure, inhibited the lattice constant a increment, providing a hint for the mechanism of proton incorporation to the bulk and enhancement in conductivity.