ИСТИНА

Semi-empirical and ab-initio theoretical investigation of crystal structure geometry, interatomic distances, phase densities and elastic properties for some phases with simple CaAl2O4 composition was performed at pressures up to 200 GPa. Two independent simulation methods predict the appearance of unknown yet super-dense modification of CaAl2O4 chemical composition. In this structure the coordination polyhedron of Al may be described as distorted 7-vertex one. Ca atoms are situated in 10-vertex polyhedra with Ca-O distances from 1.96 to 2.49 Å. It becomes the densest modification at pressures of 170 GPa (DFT prediction) or 150 GPa (semi-empirical prediction). Both approaches indicate that this super-dense CaAl2O4 modification with a «stuffed α-PbO2» type structure could be a probable candidate for mutual accumulation of Ca and Al in the lower mantle. The existence of this phase can be verified experimentally at high pressures.