Topologically prone or cation compression restricted phase transition: An example of feldspar-related SrGe2B2O8

Abstract

The stability and phase transformation processes of SrGe2B2O8 were studied under extreme conditions (up to 960 ºC / 40 GPa) using in situ single-crystal and powder X-ray diffraction. The compound demonstrated stability up to 900 °C, when it starts to decompose to SrGe4O9 and Sr2B2Ge4O14. Under high-pressure (HP) conditions SrGe2B2O8 undergoes two phase transitions, accompanied by formation of B2O7 dimers and stepwise increasing of Ge coordination number from 4 to 6 through the 5-coordinated polyhedron. This unusual stepwise increase in the coordination number of the framework cation is consistent with the behavior of silicon in isostructural CaSi2B2O8, suggesting that certain topologies are prone to similar effects. We also assume that observed differences in HP behavior of isostructural paracelsian-like phases is caused by different compression capacities of tetrahedral cations (Si, Al, B).