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Structure and oxygen mobility in mayenite (Ca12Al14O33): a high-temperature neutron powder diffraction study
1Department für Geo- und Umweltwissenschaften, Sektion Kristallographie, LMU München, Am Coulombwall 1, 85748 Garching, Germany. boysen@lmu.de
Acta Crystallographica. Section B, Structural Science
|September 18, 2007
Summary
High-temperature neutron diffraction reveals mayenite (Ca(12)Al(14)O(33)) has a stable aluminate framework. Free oxygen diffusion occurs via a jump-like process, confirmed by theoretical predictions.
Area of Science:
- Materials Science
- Solid-State Chemistry
- Crystallography
Background:
- Mayenite (Ca(12)Al(14)O(33)) is a calcium-aluminate with a framework structure containing 'free' oxygen.
- Previous models described 'free' oxygen occupying cages within the framework.
- Ambient temperature mayenite exhibits excess oxygen, attributed to radicals, becoming stoichiometric above 973 K under vacuum.
Purpose of the Study:
- To investigate the high-temperature crystal structure of mayenite using neutron powder diffraction.
- To elucidate the behavior and distribution of 'free' oxygen at elevated temperatures.
- To understand the mechanism of ionic conductivity in mayenite.
Main Methods:
- Neutron powder diffraction was employed to study mayenite structure up to 1323 K.
- Structural parameters were refined to determine the precise atomic arrangement.
- Analysis focused on the distribution and bonding of 'free' oxygen and calcium ions.
Main Results:
- The structure is characterized by a stable aluminate framework of AlO(4) tetrahedra with disordered Ca and 'free' O.
- At high temperatures, 'free' oxygen density spreads significantly, correlating with high ionic conductivity.
- Diffusion of 'free' oxygen is a jump-like process involving exchange with framework oxygen, not continuous movement.
Conclusions:
- Mayenite possesses a stable aluminate framework with mobile 'free' oxygen.
- High ionic conductivity is linked to the temperature-dependent distribution of 'free' oxygen.
- The diffusion mechanism involves discrete jumps, validating prior theoretical predictions.
