  Using evolutionary crystal structure prediction algorithm USPEX, we showed
that at pressures of the Earth's lower mantle CaAl2O4 is the only stable
calcium aluminate. At pressures above 7.0 GPa it has the CaFe2O4-type structure
and space group Pnma. This phase is one of prime candidate aluminous phases in
the lower mantle of the Earth. We show that at low pressures 5CaO * 3Al2O3
(C5A3) with space group Cmc21, CaAl4O7 (C2/c) and CaAl2O4 (P21/m) structures
are stable at pressures of up to 2.1, 1.8 and 7.0 GPa respectively. The
previously unknown structure of the orthorhombic 'CA-III' phase is also found
from our calculations. This phase is metastable and has a layered structure
with space group P21212.
