Mixed valency and anionogenic magnetism in alkali metal oxides

PhD ceremony: Mr. G.P. Giriyapura Parameshwarappa, 11.00 uur, Academiegebouw, Broerstraat 5, Groningen

Dissertation: Mixed valency and anionogenic magnetism in alkali metal oxides

Promotor(s): prof. T.T.M. Palstra

Faculty: Mathematics and Natural Sciences

This thesis investigates the magnetic properties of alkali metal oxides. These are unusual magnetic materials because the magnetism arises from oxygen rather than the metal atoms, as is usually the case.

It is shown that oxygen-based magnetism shares many characteristics with much more widely studied transition metal-based materials. The main focus of the thesis is on the synthesis and properties of novel alkali oxides containing mixed superoxide (O2-) and peroxide (O22-) anions. This difference in charge is known as mixed valency and is a topic that until now has not been studied for magnetic oxygen. However, in transition metal materials mixed valency is often responsible for important physical phenomena, such as colossal magnetoresistance and superconductivity.

Novel compounds with mixed oxygen valency have been synthesized by using mixed cations with different charges, such as Ba2+ and K+ in Ba1-xKxO2, or by varying the oxygen content in AO2-x (A = alkali metal). The coupling between neighboring superoxide and peroxide anions in some of these new compounds is significant and involves dynamic transfer of charge. This charge transfer can be enhanced by external stimuli such as light. The type of magnetism and degree of charge transfer in alkali oxides is closely linked to the crystal structure and can thus be varied via the type of alkali cation and the oxygen content.

This work opens new possibilities for the synthesis of further novel magnetic oxygen compounds with as rich a variety of magnetic properties as transition metal materials.