Distorted Crystal Structure of the Inorganic Electride, Ca12Al14O32 Studied by Synchrotron X-ray and Neutron Diffractometries

The crystal structure of the inorganic electride with various deoxidised states from 0 to around 50% was investigated with the synchrotron-X-ray and neutron diffraction methods. The electride was prepared by extracting two O atoms from the twelve cages in the unit cell of C12A7(12CaO·7Al2O3). Each O atom distributes randomly among cages distorting the cage, in which it exists and the ones in the neighborhood. The distortion caused by the O atom, which is O3 in the text, is so large that it breaks one of the Al-O bonds and forms Al-O3 bond producing a new AlO4 tetrahedron. O3 also causes the statistical distribution or splitting up to four ‘split-atoms’ of the other atoms around O3. On the other hand, since O3 locates at the general position close to the centre of the cage with point group symmetry bar 4, the site occupancy is only 2/(4×12). Most of the split-atoms expected to exist were found on difference density maps in spite of the small site occupancies. Then, the distorted crystal structure corresponding to one of the four O3 atoms was reconstructed by selecting one of the split-atoms. The electrostatic potential around each split-atom was calculated to assess the selection. The distorted crystal structure of C12A7 determined is basically kept in the partially deoxidised electrides. Although the distorted crystal structure was almost solved, no indication of free electrons in the cage urges us more accurate measurements with neutron and synchrotron-X-ray diffraction in a near future.