Exploring novel functionalities in oxide ion conductors with excess oxygen

Abstract

Functional materials, particularly metal oxides, have been the focus of much attention in solid state chemistry for many years and impact every aspect of modern life. The approach adopted in this thesis to access desirable functionality for enhanced fundamental understanding is via modifying existing materials by deploying reducing synthetic procedures. This work spans several groups of inorganic crystalline materials, but is unified by the development of new properties within host compounds of particular relevance to solid oxide fuel cell technology, which allow interstitial oxide ion conduction at elevated temperatures. The Ca₁₂Al₁₄O₃₂e₂ electride was successfully synthesized by replacing the mobile extra-framework oxygen ions with electrons acting as anions. The high concentration of electrons in the C12A7 electride gives rise to an exceptionally high electronic conductivity of up to 245 S cm⁻¹ at room temperature. Making use of the high density of electrons in Ca₁₂Al₁₄O₃₂e₂ electride, the strong N-N bonds in N₂ was found to be broken when heating Ca₁₂Al₁₄O₃₂e₂ in a N₂ atmosphere. A reaction between silicate apatites and the titanium metal yielded another completely new electride material La₉.₀Sr₁.₀(SiO₄)₆O₂.₄e₀.₂ which was found to be a semiconductor. To fully understand the role of oxygen interstitials in silicate apatites, high-resolution transmission electron microscopy (HRTEM) was employed as the main technique in probing how the oxygen nonstoichiometry is accommodated at the atomic level. Atomic-resolution imaging of interstitial oxygen in La₉.₀Sr₁.₀(SiO₄)₆O₂.₅ proved to be a success in this thesis. Substitution of oxygen in 2a and interstitial sites with fluoride ions in La[subscript(8+y)]Sr[subscript(2- z)](SiO₄)₆O[subscript(2+(3y-2z)/2)] (0<y<2, 0<z<2) could be an approach to enriching the functionalities in the apatite structure. A wide range of fluoride substitution levels was tolerated in La[subscript(10-x)]Sr[subscript(x)](SiO₄)₆O[subscript(3-1.5x)]F[subscript(2x)] (x= 0.67, 1, 1.5, 2) and AC impedance measurements were found in support of a tentative conclusion that fluoride ions could be mobile in fluorinated apatites. The last part of this thesis was focused on a new class of fast oxide ion conductors based on Ge₅P₆O₂₅ whose performance was superior to both La₉.₀Sr₁.₀(SiO₄)₆O₂.₅ and Ca₁₂Al₁₄O₃₃ in the low temperature range.