Signature of Oxide-Ion Conduction in Alkaline-Earth-Metal-Doped YGaO.

We have studied alkaline-earth-metal-doped YGaO as a new family of oxide-ion conductor. Solid solutions of YGaO and 2% -Ca-, -Sr-, and -Ba-doped YGaO, i.e., YMGaO (M = Ca, Sr, and Ba), were prepared via a conventional solid-state reaction route. X-ray Rietveld refined diffractograms of all the compositions showed the formation of an orthorhombic structure having the 2 space group. Scanning electron microscopy (SEM) images revealed that the substitution of alkaline-earth metal ions promotes grain growth. Aliovalent doping of Ca, Sr, and Ba enhanced the conductivity by increasing the oxygen vacancy concentration. However, among all of the studied dopants, 2% Ca-doped YGaO was found to be more effective in increasing the ionic conductivity as ionic radii mismatch is minimum for Y/Ca. The total conductivity of 2% Ca-doped YGaO composition calculated using the complex impedance plot was found to be ∼0.14 × 10 S cm at 700 °C, which is comparable to many other reported solid electrolytes at the same temperature, making it a potential candidate for future electrolyte material for solid oxide fuel cells (SOFCs). Total electrical conductivity measurement as a function of oxygen partial pressure suggests dominating oxide-ion conduction in a wide range of oxygen partial pressure (ca. 10-10 atm). The oxygen-ion transport is attributed to the presence of oxygen vacancies that arise from doping and conducting oxide-ion layers of one, two-, or three-dimensional channels within the crystal structure. The oxide-ion migration pathways were analyzed by the bond valence site energy (BVSE)-based approach. Photoluminescence analysis, dilatometry, Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy studies were also performed to verify the experimental findings.