In cerium oxide has been elucidated. Many environmentally friendly technologies, such as catalytic converters and solid-oxide fuel cells, exploit an amazing property of solid CeO2, also known as ceria. Under oxygen-poor conditions, ceria can release oxygen, transforming itself into Ce2O3. The Ce2O3, in turn, easily takes up oxygen under oxygen-rich conditions and changes back to ceria. Now, physicists from several universities in Sweden offer a detailed quantum-mechanical description of how these reactions occur. The researchers showed that the pivotal transition from CeO2 to Ce2O3 results from the formation of an oxygen vacancy, in which the oxygen leaves behind two electrons that become localized on two nearby cerium ions. The charge on that pair of ions then changes from +4 to +3, and a series of reduced compounds form, ending with Ce2O3. The...

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