59th Annual Report on Research 2014

During the first period of the project, we managed to synthesize high quality samples of Sn_1-xIn_xP₂O₇ for doping levels x between 0 and 0.18, confirmed the indium ion solubility as  ~ 0.12, and carried out LeBail and Rietveld refinements against diffraction data collected at different temperatures. This analysis yielded two important conclusions. First we found that doping-induced PO₄ tetrahedral distortions (i.e. P-O bond distance and O-P-O bond angle variations) are not large enough to play a major role in the proton conductivity enhancement observed for x=0.1. Second, we found that, at all temperatures, the lattice constant the cubic P a -3 phase, a, exhibits a well-defined dependence on the doping level, x, where the “a vs. x” curve displays a robust peak at x=0.1 (as shown in Figure 1).

Figure 1 – Typical temperature-resolved x-ray diffraction data collected on indium doped tin pyrophosphates (left panel); Variation of the lattice constant, a, with the indium doping level, x, obtained via full-profile fits to temperature-resolved x-ray data collected on samples exposed to air (middle panel); “a vs. x” dependence measured on indium doped pyrophosphates kept under vacuum (right panel).

Clearly, this is an important result, as it suggests that the larger unit cell volume observed at x=0.1 might be one of the structural elements contributing to the increase of the proton conductivity. All the data that led to this conclusion were collected on samples exposed to air during heating. More recently, we carried out similar types of measurements and analyses on samples contained in an evacuated or inert gas atmosphere. The right panel shows the values of the lattice constant at different temperatures for three different samples, SnP₂O₇ (x=0), Sn_0.9In_0.1P₂O₇ (x=0.1), and Sn_0.82In_0.18P₂O₇ (x=0.18) kept in vacuum during data collection. Remarkably, this inhibits the lattice constant increase (for x=0.1) at all investigated temperatures. We observed a similar behavior on samples kept under about 5 bar of inert gas (He) pressure. Corroborated with proton conductivity measurements by Nagao et al. (J. of Electrochem. Soc. 153 (8), A1604), and with the fact that Sn_1-xIn_xP₂O₇ samples do not nominally contain protons in the bulk, our findings provide an important hint for the microscopic origin of the proton conductivity enhancement in Sn_0.9In_0.1P₂O₇.

During the current reporting period, the project continued having a very positive impact on the career development of the PI and led to a significant improvement of our research capabilities and program. Indeed, based in part on our research on doped pyrophosphates sponsored by ACS-PRF we wrote a successful $495,000 grant proposal to the Department of Defense’s Army Research Office for the acquisition of a new powder x-ray diffractometer equipped with variable-temperature solid-gas reaction chamber and ultrafast detection capabilities. This instrument allows us to carry out new types of measurements (e.g. on samples contained under inert and reactive gas atmospheres). Our undergraduate students who participate in the program greatly benefit from this acquisition. They have the chance to perform experiments on one of the most advanced XRD systems, gather experience with scattering techniques before travelling to national synchrotron facilities, and, most importantly, have the opportunity to continue working on their projects upon their return to UT El Paso. In addition, our results obtained so far under the ACS-PRF sponsorship allowed us to submit another grant proposal requesting about $0.5M from that he Nuclear Regulatory Commission to establish a collaborative program between UT El Paso and Los Alamos National Laboratory (LANL), where teams of faculty and students from our minority serving institution will partner with LANL scientists on projects that are initiated and mostly carried out at UT El Paso, but have a well-defined component to be performed at the national laboratory.

So far, eight undergraduate students actively participated in the project and received extensive training with in-situ x-ray scattering techniques. Other significant results related to student participation include presentations at regional and national conferences, co-authorship in the two journal articles published this year that acknowledge this and previous ACS-PRF support, summer internships and collaborative work with other universities and national laboratories. One student received the Miner Hero Award, a major recognition UT El Paso bestows for enhancing the prestige of the University in undergraduate research.