Stable compounds in the CaO-Al2O3 system at high pressures

TL;DR

Using evolutionary algorithms, the study predicts stable calcium aluminate compounds in the Earth's lower mantle at high pressures, identifying the CaFe2O4-type structure as dominant above 7 GPa.

Contribution

First computational prediction of stable calcium aluminate phases in the Earth's lower mantle using USPEX at high pressures.

Findings

CaAl2O4 is the only stable calcium aluminate at lower mantle pressures.

Identified new stable structures for CaO-Al2O3 compounds up to 7 GPa.

Discovered a metastable layered 'CA-III' phase with P21212 symmetry.

Abstract

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.