Main group oxide goes metallic (4/16/2008)

The highly-renowned journal 'Nature Materials' reported on April 6, 2008 that scientists of RWTH Aachen University and the universities of Giessen, Braunschweig and Darmstadt succeeded for the first time in producing an insulator-metal transition in an oxide of a main group element. This phenomenon is of great interest not only for fundamental research but also for technological applications, e.g. in data storage. The team at RWTH Aachen University is headed by Professor Manfred Martin.

Insulator-metal transitions in which an electrical insulator becomes metallic belong to the most fascinating phenomena in solid state research. They are investigated world wide. Potential causes for the transition are electron interactions, that means the electrons try to avoid each other, and structural disorder, e.g. in amorphous solids without long range order. Most examples of insulator-metal transitions concern transition metal compounds since transition metals change their valence state easily.

Now, Professor Martin and his colleagues have produced an insulator-metal transition in an oxide of a main group element, with a change in electrical conductivity of 7 orders, by combining structural and chemical disorder in an amorphous gallium oxide with strong gallium excess. During annealing of this material, crystallization of Ga2O3 takes place. The excess gallium atoms are released into the amorphous matrix where they reduce the band gap, until at a critical gallium excess the insulator-metal transition occurs. This novel mechanism to induce an insulator-metal transition is not restricted to gallium oxide and opens up a promising route to achieve metallic behaviour in oxides that are only known as classical insulators.

The experimental results and the deduced model of the insulator-metal transition were confirmed by theoretical calculations of the electronic properties of non-stoichiometric gallium oxide. Furthermore, the researchers could demonstrate a close correlation between the structural and electronic properties, the electrical conductivity, and the optical properties of the new material.

The project was supported by the German Research Foundation (DFG) within the Priority Program 1136 'Substitution effects in ionic solids'.

The detailed research results can be found on the website of the journal 'Nature Materials' (http://www.nature.com/nmat/journal/vaop/ncurrent/abs/nmat2164.html). They will be published also in the upcoming print version of the magazine.

Note: This story has been adapted from a news release issued by the RWTHAACHEN University

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