The interaction of defects in a mayenite structure.

Calcium aluminate Ca12Al14O33 initially known as cement compound after a series of structural refinements presented a complex cubic structure. It demonstrates a surprising feature consisting of the occurrence of spherical cavities named as cages. After this, mayenite is regarded as a kind of anti-zeolite. An alternative representation of such compounds formulated as highly non-stoichiometric garnets is highlighted in the present study. Mayenite is described as non-stoichiometric garnet {Ca3-x□x}1[Ca, Al]2Al3O12-x using the Raman spectroscopy. The dependence of phase diagram Al2O3-CaO in the region near the mayenite composition on the oxygen activity is summarized regarding its structural deficiency. It is pointed that cage is the result of defects association whereas structural defects tend to interact with each other. The main consequence of such interaction is a spontaneous formation of a core-shell structure. It is evidenced by the presence of a near-surface layer of small thickness over a compact oxide. The local symmetry of the mayenite structure at room temperature in oxidized and reduced (electride) state was studied by Raman spectroscopy which revealed the presence of luminescence without the rare-earths' doping.