Signature of Oxide-Ion Conduction in Alkaline-Earth-Metal-Doped Y3GaO6

Abstract

We have studied alkaline-earth-metal-doped Y₃GaO₆ as a new family of oxide-ion conductor. Solid solutions of Y₃GaO₆ and 2% -Ca²⁺-, -Sr²⁺-, and -Ba²⁺-doped Y₃GaO₆, i.e., Y_(3-0.06)M_0.06GaO_6-δ (M = Ca²⁺, Sr²⁺, and Ba²⁺), 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 Cmc2₁ space group. Scanning electron microscopy (SEM) images revealed that the substitution of alkaline-earth metal ions promotes grain growth. Aliovalent doping of Ca²⁺, Sr²⁺, and Ba²⁺ enhanced the conductivity by increasing the oxygen vacancy concentration. However, among all of the studied dopants, 2% Ca²⁺-doped Y₃GaO₆ was found to be more effective in increasing the ionic conductivity as ionic radii mismatch is minimum for Y³⁺/Ca²⁺. The total conductivity of 2% Ca-doped Y₃GaO₆ composition calculated using the complex impedance plot was found to be ∼0.14 × 10^-3 S cm^-1 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^-20-10^-4 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.