A new high-pressure CaGe₂O₅ polymorph with 5- and 6-coordinated germanium

We discovered a new CaGe₂O₅ polymorph in high-pressure experiments (above 8 GPa). The phase is orthorhombic, space group Pbam, with a = 7.306(2), b = 8.268(2), c = 5.714(1) Å, V = 345.2(1) ų, and Z = 4. The new phase, which we call post-titanite CaGe₂O₅, is the high-pressure polymorph of titanite CaGe₂O₅. The structure of this new polymorph is based on a network of 5- and 6-coordinated Ge polyhedra and 8-coordinated Ca atoms. Following the germanate analog to silicate, post-titanite CaSi₂O₅ could be expected to form at high-pressure conditions and thus might exist in Earth's mantle.