60th Annual Report on Research 2015

This project is aimed at investigating microscopic structures and mechanisms responsible for the enhanced proton transport in doped pyrophosphates Sn_1-xM_xP₂O₇(M = In, Sc, Al) at intermediate-temperatures, and to uncover how certain doping-induced microscopic features lead to the enhancement of the proton conductivity in these compounds. In addition, it is our goal to involve undergraduate students in research carried out in our X-ray Scattering Laboratory (XRSLab) at The University of Texas at El Paso (UTEP), as well as at national user facilities, such as the National Synchrotron Light Source at Brookhaven. Finally, this ACS-PRF grant is expected to contribute to the professional development of the PI by fostering his research program through new student mentoring, collaborative opportunities, and preliminary data for major external funding proposals.

Scientifically, we have achieved several specific objectives related to the main goal of the project. We have successfully synthesized the parent compound SnP₂O₇ as well as several members of the Sn_1-xIn_xP₂O₇ series (0<x<0.18). We confirmed the solubility limit of the In atoms to be x=0.12, and carried out crystal structure refinements from high-resolution temperature-resolved x-ray diffraction data for all values of x. In a different set of experiments, we performed similar measurements and analyses on samples contained under an evacuated or inert gas atmosphere. Our first finding was that all samples are isomorphic (have the same P a -3 cubic crystal structure) at all temperatures and under all the conditions investigated. This clearly raises the important question of "what is special" at the microscopic level about the Sn_0.9In_0.1P₂O₇ (x=0.1) sample, which exhibits the highest proton conductivity among the series. Our first observation in this regard was that that the cubic lattice constant (a) has a linear temperature dependence for all values of the doping level (x), but the "a vs. x" dependence exhibits a robust peak exactly at x=0.1. We also investigated selected O-O bond lengths and P-O-P bond angles for different values of x, but found nothing particular for x=0.1. Finally, we found that the peak observed in the "a vs. x" dependence vanishes if the measurements are taken on samples contained either in vacuum or in an inert gas. 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_0.9In_0.1P₂O₇compound is the enhancement of the lattice constant at x=0.1.

Student involvement has been particularly strong, especially at the undergraduate level. More than ten undergraduate students from different Departments and Colleges have participated in projects related to this ACS-PRF sponsorship. They were offered opportunities to carry out research both in our XRSLab at UTEP and at national user facilities where they interacted and worked with some of the best researchers in the field. Four such students graduated and either continued to graduate or professional school or found employment in STEM fields. Moreover, the success of the project attracted other, more advanced, students at the M.S. and Ph.D. level. They now work with us on structural properties of ionic conductors under different sponsorships.

The professional development of the PI has tremendously benefited from this grant. During the time the grant was active the number of students in our research group increased from three (including two undergraduate students) to ten (including one postdoc, three Ph.D. two M.S., and four undergraduates). We have forged new collaborations with researchers at world-renowned laboratories (Los Alamos, Brookhaven, Sandia) and, most importantly, used preliminary data obtained under this ACS-PRF grant to apply and secure further sponsorship from the Department of Defense (DoD) -in the amount of ~$1M- both for instrumentation and for research related to this and other projects on ionic conductors. Clearly, none of this would have been possible without the initial sponsorship from the ACS-PRF.