A study of novel electrolyte materials with interstitial oxides as mobile species

Abstract

This thesis investigates the interstitial oxide ion accommodation, induced local deformation around interstitial defects and increase in ionic conductivity in doped La3Ga5GeO14 materials with a langasite structure. The choice of langasites as a target system for oxide interstitial doping was motivated by their structural resemblance with apatite and melilite materials, which by the accommodation of up to 3.7 and 4.6 % of extra oxygen respectively increased the ionic conductivity up to 5.6 ⨯ 10-3 S∙cm-1 in La9.75Sr0.25(SiO4)6O2.875 and 4.1 ⨯ 10-2 S∙cm-1 in La1.54Sr0.46Ga3O7.27 at 700 °C. The good ionic conducting properties shown by these materials lights the way for the development of novel oxide ionic conducting materials toward the doping of tunnelled structures with large voids for oxide interstitials and away from the more traditional approach based on the generation of oxygen vacancies by aliovalent doping of close-packed systems such as fluorite (.. YSZ, GDC) and perovskite (.. LSGM) type materials. Here we show that the La3Ga5GeO14 langasite is able to accommodate 5.36 % of extra oxygen in La3Ga3.75Ge2.5O14.75 by a carefully developed Pechini route. Elemental analysis in various La3Ga5-xGe1+xO14+x/2 compositions presented in this work demonstrated the successfully substitution of Ga for Ge at the desired ratio with the consequent incorporation of extra oxygen. AC conductivity measurements were carried out on dense pellets prepared by conventional and fast Spark Plasma-assisted (SPS) sintering methods revealing an increase in the conductivity of two orders of magnitude (~4 × 10-3 S∙cm-1 at 700 °C in La3Ga5GeO14.25) when compared to the un-doped langasite (La3Ga5GeO14, ~1 × 10˗5 S∙cm-1). The location of the extra oxygen and the induced local deformation was studied by 71Ga and 17O Solid-State NMR and high-resolution neutron powder diffraction (NPD) techniques. The extra oxygen incorporated by doping was found to form a (Ga/Ge)2O8 unit predicted by DFT calculations. Two neighbouring tetrahedra sites bridged by oxygen are relaxed into two pseudo squarebased pyramid like polyhedral sharing one edge. This unit shows a high resemblance to the Ge2O8 unit found in La3GaGe5O16. Two additional doping mechanisms were studied involving the partial and complete substitution of La/Ga in La3Ga5-xGe1+xO14+x/2 for isovalent cations: La3˗yLnyGa5˗xGe1+xO14+x/2 where Ln = Pr, Nd, Sm and Gd and La3Al5-y-xGayGe1+xO14+x/2. Neutron powder diffraction studies carried out in these compositions revealed a (Ge/M)2O8 environment for the extra oxygen analogous to the one determined for La3Ga3.5Ge2.5O14.75 and La3Ga4Ge2O14.5. The conductivity analysis revealed a decrease in the bulk conductivity at 500 °C with the decrease in size of the large lanthanide atoms from ~2.44 × 10-5 S∙cm-1 in La3Ga4.5Ge1.5O14.25 to 4.51 × 10-6 S∙cm-1 in La1.5Nd1.5Ga4.5Ge1.5O14.25. The isovalent substitution of Ga3+ for Al3+ causes a decrease in conductivity by one order of magnitude, where the highest conductivity value of 2.94 × 10-6 S∙cm-1 is reached by La3Al4.8Ge1.2O14.1 at 500 °C.