Non-Stoichiometric Sodium-Containing Late Metal Oxides

Abstract

Alkali-containing metal oxide compounds often form structures with interesting properties such as magnetic and ion conducting behavior; the mobility of these alkali cations plays an important role in tuning their properties. In this work, sodium mobility is investigated in two crystal structure types: the cubic hyperkagomé and the layered α -NaFeO\(_{2}\) structures. Sodium mobility is important for the realization of their properties, and its eff ects are probed by careful syntheses that manipulate the Na:M ratio and by x-ray diff raction analysis. In the cubic hyperkagom é crystal structure (Na\(_{4}\)M\(_{3}\)O\(_{8}\), M = Ir, Pt), sodium cations are contained in 3D channels. Here, the synthesis of Na4Ir3O8 is optimized for sodium stoichiometry and reaction conditions. Cation mobility is probed by exposing samples to common gases (air, CO\(_{2}\), H\(_{2}\)O) and oxidative solutions (CH\(_{3}\)CN/Br\(_{2}\)). Changes in the unit cell and magnetic properties were used to confirm bromine's ability to remove sodium from the structure, creating a mixture of Ir\(^{4+}\) and Ir\(^{5+}\). Next, sodium mobility and nonstoichiometry were investigated in the α -NaFeO\(_{2}\) crystal structure, which contains 2D layers of sodium cations. An Na\(_{2-x}\)IrO\(_{3}\) crystal synthesis was attempted under reaction conditions different from those used for Na4Ir3O8, and it was found that copper from the furnace was incorporated into the structure. An additional α -NaFeO\(_{2}\) type phase was synthesized, forming the novel solid solution Na\(_{3-x}\)NaSn\(_{2-x}\)Sb\(_{x}\)O\(_{6}\), and optimized for 0 < x 1.1. It was found that the sodium vacancies created by the introduction of antimony results in high sodium ionic conductivity, which thus allows this material to act as a solid electrolyte for a sodium ion battery. Finally, a new variant of α-NaFeO\(_{2}\) type compounds Na\(_{2}\)Sn\(_{2}\)MO\(_{6}\) (M = Ni, Co, and Mg) was synthesized and characterized; these novel phases may also have interesting cathodic and ionic conduction properties. It is found that varying the stoichiometry of sodium in several sodium metal oxide compounds leads to a plethora of novel materials and interesting magnetic and conductive properties which should be investigated further.